Problem Solving with Algorithms and Data Structures
lecture notes on Algorithms and Data Structures and problem solving with algorithms and data structures using python pdf free download
Problem Solving with Algorithms and
Brad Miller, David Ranum
September 22, 2013CHAPTER
• To review the ideas of computer science, programming, and problem-solving.
• To understand abstraction and the role it plays in the problem-solving process.
• To understand and implement the notion of an abstract data type.
• To review the Python programming language.
1.2 Getting Started
The way we think about programming has undergone many changes in the years since the ﬁrst
electronic computers required patch cables and switches to convey instructions from human
to machine. As is the case with many aspects of society, changes in computing technology
provide computer scientists with a growing number of tools and platforms on which to practice
their craft. Advances such as faster processors, high-speed networks, and large memory ca-
pacities have created a spiral of complexity through which computer scientists must navigate.
Throughout all of this rapid evolution, a number of basic principles have remained constant.
The science of computing is concerned with using computers to solve problems.
You have no doubt spent considerable time learning the basics of problem-solving and hope-
fully feel conﬁdent in your ability to take a problem statement and develop a solution. You have
also learned that writing computer programs is often hard. The complexity of large problems
and the corresponding complexity of the solutions can tend to overshadow the fundamental
ideas related to the problem-solving process.
This chapter emphasizes two important areas for the rest of the text. First, it reviews the frame-
work within which computer science and the study of algorithms and data structures must ﬁt,
in particular, the reasons why we need to study these topics and how understanding these top-
ics helps us to become better problem solvers. Second, we review the Python programming
language. Although we cannot provide a detailed, exhaustive reference, we will give examples
and explanations for the basic constructs and ideas that will occur throughout the remaining
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1.3 What Is Computer Science?
Computer science is often difﬁcult to deﬁne. This is probably due to the unfortunate use of
the word “computer” in the name. As you are perhaps aware, computer science is not simply
the study of computers. Although computers play an important supporting role as a tool in the
discipline, they are just that – tools.
Computer science is the study of problems, problem-solving, and the solutions that come out
of the problem-solving process. Given a problem, a computer scientist’s goal is to develop an
algorithm, a step-by-step list of instructions for solving any instance of the problem that might
arise. Algorithms are ﬁnite processes that if followed will solve the problem. Algorithms are
Computer science can be thought of as the study of algorithms. However, we must be careful to
include the fact that some problems may not have a solution. Although proving this statement
is beyond the scope of this text, the fact that some problems cannot be solved is important for
those who study computer science. We can fully deﬁne computer science, then, by including
both types of problems and stating that computer science is the study of solutions to problems
as well as the study of problems with no solutions.
It is also very common to include the word computable when describing problems and solu-
tions. We say that a problem is computable if an algorithm exists for solving it. An alternative
deﬁnition for computer science, then, is to say that computer science is the study of problems
that are and that are not computable, the study of the existence and the nonexistence of algo-
rithms. In any case, you will note that the word “computer” did not come up at all. Solutions
are considered independent from the machine.
Computer science, as it pertains to the problem-solving process itself, is also the study of
abstraction. Abstraction allows us to view the problem and solution in such a way as to
separate the so-called logical and physical perspectives. The basic idea is familiar to us in a
Consider the automobile that you may have driven to school or work today. As a driver, a user
of the car, you have certain interactions that take place in order to utilize the car for its intended
purpose. You get in, insert the key, start the car, shift, brake, accelerate, and steer in order to
drive. From an abstraction point of view, we can say that you are seeing the logical perspective
of the automobile. You are using the functions provided by the car designers for the purpose of
transporting you from one location to another. These functions are sometimes also referred to
as the interface.
On the other hand, the mechanic who must repair your automobile takes a very different point
of view. She not only knows how to drive but must know all of the details necessary to carry
out all the functions that we take for granted. She needs to understand how the engine works,
how the transmission shifts gears, how temperature is controlled, and so on. This is known as
the physical perspective, the details that take place “under the hood.”
The same thing happens when we use computers. Most people use computers to write docu-
ments, send and receive email, surf the web, play music, store images, and play games without
any knowledge of the details that take place to allow those types of applications to work. They
view computers from a logical or user perspective. Computer scientists, programmers, technol-
ogy support staff, and system administrators take a very different view of the computer. They
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Figure 1.1: Procedural Abstraction
must know the details of how operating systems work, how network protocols are conﬁgured,
and how to code various scripts that control function. They must be able to control the low-level
details that a user simply assumes.
The common point for both of these examples is that the user of the abstraction, sometimes
also called the client, does not need to know the details as long as the user is aware of the way
the interface works. This interface is the way we as users communicate with the underlying
complexities of the implementation. As another example of abstraction, consider the Python
math module. Once we import the module, we can perform computations such as
This is an example of procedural abstraction. We do not necessarily know how the square
root is being calculated, but we know what the function is called and how to use it. If we
perform the import correctly, we can assume that the function will provide us with the correct
results. We know that someone implemented a solution to the square root problem but we only
need to know how to use it. This is sometimes referred to as a “black box” view of a process.
We simply describe the interface: the name of the function, what is needed (the parameters),
and what will be returned. The details are hidden inside (see Figure 1.1).
1.3.1 What Is Programming?
Programming is the process of taking an algorithm and encoding it into a notation, a pro-
gramming language, so that it can be executed by a computer. Although many programming
languages and many different types of computers exist, the important ﬁrst step is the need to
have the solution. Without an algorithm there can be no program.
Computer science is not the study of programming. Programming, however, is an important
part of what a computer scientist does. Programming is often the way that we create a repre-
sentation for our solutions. Therefore, this language representation and the process of creating
it becomes a fundamental part of the discipline.
Algorithms describe the solution to a problem in terms of the data needed to represent the
problem instance and the set of steps necessary to produce the intended result. Programming
languages must provide a notational way to represent both the process and the data. To this
end, languages provide control constructs and data types.
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Control constructs allow algorithmic steps to be represented in a convenient yet unambiguous
way. At a minimum, algorithms require constructs that perform sequential processing, selection
for decision-making, and iteration for repetitive control. As long as the language provides these
basic statements, it can be used for algorithm representation.
All data items in the computer are represented as strings of binary digits. In order to give these
strings meaning, we need to have data types. Data types provide an interpretation for this
binary data so that we can think about the data in terms that make sense with respect to the
problem being solved. These low-level, built-in data types (sometimes called the primitive data
types) provide the building blocks for algorithm development.
For example, most programming languages provide a data type for integers. Strings of binary
digits in the computer’s memory can be interpreted as integers and given the typical meanings
that we commonly associate with integers (e.g. 23, 654, and− 19). In addition, a data type also
provides a description of the operations that the data items can participate in. With integers,
operations such as addition, subtraction, and multiplication are common. We have come to
expect that numeric types of data can participate in these arithmetic operations.
The difﬁculty that often arises for us is the fact that problems and their solutions are very
complex. These simple, language-provided constructs and data types, although certainly suf-
ﬁcient to represent complex solutions, are typically at a disadvantage as we work through the
problem-solving process. We need ways to control this complexity and assist with the creation
1.3.2 Why Study Data Structures and Abstract Data Types?
To manage the complexity of problems and the problem-solving process, computer scientists
use abstractions to allow them to focus on the “big picture” without getting lost in the details.
By creating models of the problem domain, we are able to utilize a better and more efﬁcient
problem-solving process. These models allow us to describe the data that our algorithms will
manipulate in a much more consistent way with respect to the problem itself.
Earlier, we referred to procedural abstraction as a process that hides the details of a particular
function to allow the user or client to view it at a very high level. We now turn our attention to a
similar idea, that of data abstraction. An abstract data type, sometimes called an ADT, is a
logical description of how we view the data and the operations that are allowed without regard
to how they will be implemented. This means that we are concerned only with what the data
is representing and not with how it will eventually be constructed. By providing this level of
abstraction, we are creating an encapsulation around the data. The idea is that by encapsulating
the details of the implementation, we are hiding them from the user’s view. This is called
Figure 1.2 shows a picture of what an abstract data type is and how it operates. The user
interacts with the interface, using the operations that have been speciﬁed by the abstract data
type. The abstract data type is the shell that the user interacts with. The implementation is
hidden one level deeper. The user is not concerned with the details of the implementation.
The implementation of an abstract data type, often referred to as a data structure, will require
that we provide a physical view of the data using some collection of programming constructs
and primitive data types. As we discussed earlier, the separation of these two perspectives will
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Figure 1.2: Abstract Data Type
allow us to deﬁne the complex data models for our problems without giving any indication
as to the details of how the model will actually be built. This provides an implementation-
independent view of the data. Since there will usually be many different ways to implement
an abstract data type, this implementation independence allows the programmer to switch the
details of the implementation without changing the way the user of the data interacts with it.
The user can remain focused on the problem-solving process.
1.3.3 Why Study Algorithms?
Computer scientists learn by experience. We learn by seeing others solve problems and by
solving problems by ourselves. Being exposed to different problem-solving techniques and
seeing how different algorithms are designed helps us to take on the next challenging problem
that we are given. By considering a number of different algorithms, we can begin to develop
pattern recognition so that the next time a similar problem arises, we are better able to solve it.
Algorithms are often quite different from one another. Consider the example of sqrt seen
earlier. It is entirely possible that there are many different ways to implement the details to
compute the square root function. One algorithm may use many fewer resources than another.
One algorithm might take 10 times as long to return the result as the other. We would like to
have some way to compare these two solutions. Even though they both work, one is perhaps
“better” than the other. We might suggest that one is more efﬁcient or that one simply works
faster or uses less memory. As we study algorithms, we can learn analysis techniques that
allow us to compare and contrast solutions based solely on their own characteristics, not the
characteristics of the program or computer used to implement them.
In the worst case scenario, we may have a problem that is intractable, meaning that there is no
algorithm that can solve the problem in a realistic amount of time. It is important to be able
to distinguish between those problems that have solutions, those that do not, and those where
solutions exist but require too much time or other resources to work reasonably.
There will often be trade-offs that we will need to identify and decide upon. As computer
scientists, in addition to our ability to solve problems, we will also need to know and understand
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solution evaluation techniques. In the end, there are often many ways to solve a problem.
Finding a solution and then deciding whether it is a good one are tasks that we will do over and
1.4 Review of Basic Python
In this section, we will review the programming language Python and also provide some more
detailed examples of the ideas from the previous section. If you are new to Python or ﬁnd that
you need more information about any of the topics presented, we recommend that you consult
a resource such as the Python Language Reference or a Python Tutorial. Our goal here is to
reacquaint you with the language and also reinforce some of the concepts that will be central
to later chapters.
Python is a modern, easy-to-learn, object-oriented programming language. It has a powerful
set of built-in data types and easy-to-use control constructs. Since Python is an interpreted
language, it is most easily reviewed by simply looking at and describing interactive sessions.
You should recall that the interpreter displays the familiar prompt and then evaluates the
Python construct that you provide. For example,
print("Algorithms and Data Structures")
Algorithms and Data Structures
shows the prompt, theprint function, the result, and the next prompt.
1.4.1 Getting Started with Data
We stated above that Python supports the object-oriented programming paradigm. This means
that Python considers data to be the focal point of the problem-solving process. In Python, as
well as in any other object-oriented programming language, we deﬁne a class to be a description
of what the data look like (the state) and what the data can do (the behavior). Classes are
analogous to abstract data types because a user of a class only sees the state and behavior of
a data item. Data items are called objects in the object-oriented paradigm. An object is an
instance of a class.
Built-in Atomic Data Types
We will begin our review by considering the atomic data types. Python has two main built-in
numeric classes that implement the integer and ﬂoating point data types. These Python classes
are calledint andfloat. The standard arithmetic operations, +,− , ,/, and (exponentia-
tion), can be used with parentheses forcing the order of operations away from normal operator
precedence. Other very useful operations are the remainder (modulo) operator, %, and integer
division, //. Note that when two integers are divided, the result is a ﬂoating point. The inte-
ger division operator returns the integer portion of the quotient by truncating any fractional part.
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Operation Name Operator Explanation
less than Less than operator
greater than Greater than operator
less than or equal = Less than or equal to operator
greater than or equal = Greater than or equal to operator
equal == Equality operator
not equal = Not equal operator
logical and and Both operands True for result to be True
logical or or Either operand True for result to be True
logical not not Negates the truth value: False becomes
True, True becomes False
Table 1.1: Relational and Logical Operators
print(2+3 4) 14
print((2+3) 4) 20
print(2 10) 1024
print(2 100) 1267650600228229401496703205376
The boolean data type, implemented as the Python bool class, will be quite useful for
representing truth values. The possible state values for a boolean object are True and False
with the standard boolean operators,and,or, andnot.
False or True
not (False or True)
True and True
Boolean data objects are also used as results for comparison operators such as equality (==)
and greater than (). In addition, relational operators and logical operators can be combined
together to form complex logical questions. Table 1.1 shows the relational and logical
operators with examples shown in the session that follows.
print(5 == 10)
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Figure 1.3: Variables Hold References to Data Objects
Figure 1.4: Assignment changes the Reference
print((5 = 1) and (5 = 10))
Identiﬁers are used in programming languages as names. In Python, identiﬁers start with a
letter or an underscore (_), are case sensitive, and can be of any length. Remember that it is
always a good idea to use names that convey meaning so that your program code is easier to
read and understand.
A Python variable is created when a name is used for the ﬁrst time on the left-hand side of
an assignment statement. Assignment statements provide a way to associate a name with a
value. The variable will hold a reference to a piece of data and not the data itself. Consider the
the_sum = 0
the_sum = the_sum + 1
the_sum = True
The assignment statementthe_sum = 0 creates a variable calledthe_sum and lets it hold the
reference to the data object 0 (see Figure 1.3). In general, the right-hand side of the assignment
statement is evaluated and a reference to the resulting data object is “assigned” to the name on
the left-hand side. At this point in our example, the type of the variable is integer as that is
the type of the data currently being referred to by “the_sum.” If the type of the data changes
(see Figure 1.4), as shown above with the boolean value True, so does the type of the variable
(the_sum is now of the type boolean). The assignment statement changes the reference being
held by the variable. This is a dynamic characteristic of Python. The same variable can refer to
many different types of data.
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Operation Name Operator Explanation
indexing Access an element of a sequence
concatenation + Combine sequences together
repetition Concatenate a repeated number of times
membership in Ask whether an item is in a sequence
length len Ask the number of items in the sequence
slicing : Extract a part of a sequence
Table 1.2: Operations on Any Sequence in Python
Built-in Collection Data Types
In addition to the numeric and boolean classes, Python has a number of very powerful built-
in collection classes. Lists, strings, and tuples are ordered collections that are very similar in
general structure but have speciﬁc differences that must be understood for them to be used
properly. Sets and dictionaries are unordered collections.
A list is an ordered collection of zero or more references to Python data objects. Lists are
written as comma-delimited values enclosed in square brackets. The empty list is simply .
Lists are heterogeneous, meaning that the data objects need not all be from the same class and
the collection can be assigned to a variable as below. The following fragment shows a variety
of Python data objects in a list.
1, 3, True, 6.5
my_list = 1,3,True,6.5
1, 3, True, 6.5
Note that when Python evaluates a list, the list itself is returned. However, in order to remember
the list for later processing, its reference needs to be assigned to a variable.
Since lists are considered to be sequentially ordered, they support a number of operations that
can be applied to any Python sequence. Table 1.2 reviews these operations and the following
session gives examples of their use.
Note that the indices for lists (sequences) start counting with 0. The slice operation, my_list1 :
3, returns a list of items starting with the item indexed by 1 up to but not including the item
indexed by 3.
Sometimes, you will want to initialize a list. This can quickly be accomplished by using
repetition. For example,
my_list = 0 6
0, 0, 0, 0, 0, 0
One very important aside relating to the repetition operator is that the result is a repetition
of references to the data objects in the sequence. This can best be seen by considering the
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Method Name Use Explanation
append a_list.append(item) Adds a new item to the end of a list
insert a_list.insert(i,item) Inserts an item at the𝑖 position in a list
pop a_list.pop() Removes and returns the last item in a list
pop a_list.pop(i) Removes and returns the𝑖 item in a list
sort a_list.sort() Modiﬁes a list to be sorted
reverse a_list.reverse() Modiﬁes a list to be in reverse order
del del a_listi Deletes the item in the𝑖 position
index a_list.index(item) Returns the index of the ﬁrst occurrence of item
count a_list.count(item) Returns the number of occurrences of item
remove a_list.remove(item) Removes the ﬁrst occurrence of item
Table 1.3: Methods Provided by Lists in Python
my_list = 1,2,3,4
A = my_list 3
The variableA holds a collection of three references to the original list calledmy_list. Note
that a change to one element ofmy_list shows up in all three occurrences inA.
Lists support a number of methods that will be used to build data structures. Table 1.3 provides
a summary. Examples of their use follow.
my_list = 1024, 3, True, 6.5
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You can see that some of the methods, such as pop, return a value and also modify the list.
Others, such as reverse, simply modify the list with no return value. pop will default to
the end of the list but can also remove and return a speciﬁc item. The index range starting
from 0 is again used for these methods. You should also notice the familiar “dot” notation
for asking an object to invoke a method. my_list.append(False) can be read as “ask the
object my_list to perform its append method and send it the value False.” Even simple
data objects such as integers can invoke methods in this way.
In this fragment we are asking the integer object54 to execute itsadd method (called__add__
in Python) and passing it21 as the value to add. The result is the sum,75. Of course, we usually
write this as54 + 21. We will say much more about these methods later in this section.
One common Python function that is often discussed in conjunction with lists is the range
function. range produces a range object that represents a sequence of values. By using the
list function, it is possible to see the value of the range object as a list. This is illustrated
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
5, 6, 7, 8, 9
5, 7, 9
10, 9, 8, 7, 6, 5, 4, 3, 2
The range object represents a sequence of integers. By default, it will start with 0. If you
provide more parameters, it will start and end at particular points and can even skip items. In
our ﬁrst example, range(10), the sequence starts with 0 and goes up to but does not include
10. In our second example, range(5, 10) starts at 5 and goes up to but not including 10.
range(5, 10, 2) performs similarly but skips by twos (again, 10 is not included).
Strings are sequential collections of zero or more letters, numbers and other symbols. We call
these letters, numbers and other symbols characters. Literal string values are differentiated
from identiﬁers by using quotation marks (either single or double).
my_name = "David"
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Method Name Use Explanation
center a_string.center(w) Returns a string centered in a ﬁeld of size𝑤
count a_string.count(item) Returns the number of occurrences of item
in the string
ljust a_string.ljust(w) Returns a string left-justiﬁed in a ﬁeld of size
lower a_string.lower() Returns a string in all lowercase
rjust a_string.rjust(w) Returns a string right-justiﬁed in a ﬁeld of
find a_string.find(item) Returns the index of the ﬁrst occurrence of
split a_string.split(s_char) Splits a string into substrings ats_char
Table 1.4: Methods Provided by Strings in Python
Since strings are sequences, all of the sequence operations described above work as you would
expect. In addition, strings have a number of methods, some of which are shown in Table 1.4.
' David '
Of these, split will be very useful for processing data. split will take a string and return
a list of strings using the split character as a division point. In the example, v is the division
point. If no division is speciﬁed, the split method looks for whitespace characters such as tab,
newline and space.
A major difference between lists and strings is that lists can be modiﬁed while strings cannot.
This is referred to as mutability. Lists are mutable; strings are immutable. For example, you
can change an item in a list by using indexing and assignment. With a string that change is not
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1, 3, True, 6.5
1024, 3, True, 6.5
Traceback (most recent call last):
File "pyshell84", line 1, in module
TypeError: 'str' object does not support item assignment
Note that the error (or traceback) message displayed above is obtained on a Mac OS X machine.
If you are running the above code snippet on a Windows machine, your error output will more
likely be as follows.
Traceback (most recent call last):
File "pyshell84", line 1, in -toplevel-
TypeError: object doesn't support item assignment
Depending on your operating system, or version of Python, the output may slightly vary. How-
ever it will still indicate where and what the error is. You may want to experiment for yourself
and get acquainted with the error message for easier and faster debugging. For the remainder
of this work, we will only display the Mac OS X error messages.
Tuples are very similar to lists in that they are heterogeneous sequences of data. The difference
is that a tuple is immutable, like a string. A tuple cannot be changed. Tuples are written as
comma-delimited values enclosed in parentheses. As sequences, they can use any operation
described above. For example,
my_tuple = (2,True,4.96)
(2, True, 4.96)
(2, True, 4.96, 2, True, 4.96, 2, True, 4.96)
However, if you try to change an item in a tuple, you will get an error. Note that the error
message provides location and reason for the problem.
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Operator Use Explanation
in x.in(set) Set membership
len len(set) Returns the cardinality (i.e. the length) of the set
set1 set2 Returns a new set with all elements from both sets
& set1 & set2 Returns a new set with only the elements common to both sets
- set1 - set2 Returns a new set with all items from the ﬁrst set not in second
= set1 = set2 Asks whether all elements of the ﬁrst set are in the second
Table 1.5: Operations on a Set in Python
Traceback (most recent call last):
File "pyshell137", line 1, in module
TypeError: 'tuple' object does not support item assignment
A set is an unordered collection of zero or more immutable Python data objects. Sets do not
allow duplicates and are written as comma-delimited values enclosed in curly braces. The
empty set is represented byset(). Sets are heterogeneous, and the collection can be assigned
to a variable as below.
False, 4.5, 3, 6, 'cat'
my_set = 3,6,"cat",4.5,False
False, 3, 4.5, 6, 'cat'
Even though sets are not considered to be sequential, they do support a few of the familiar
operations presented earlier. Table 1.5 reviews these operations and the following session gives
examples of their use.
False, 3, 4.5, 6, 'cat'
False in my_set
"dog" in my_set
Sets support a number of methods that should be familiar to those who have worked with them
in a mathematics setting. Table 1.6 provides a summary. Examples of their use follow. Note
that union, intersection, issubset, and difference all have operators that can be
used as well.
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Method Name Use Explanation
union set1.union(set2) Returns a new set with all elements from
intersection set1.intersection(set2) Returns a new set with only the elements
common to both sets
difference set1.difference(set2) Returns a new set with all items from ﬁrst set
not in second
issubset set1.issubset(set2) Asks whether all elements of one set are in
add set.add(item) Adds item to the set
remove set.remove(item) Removes item from the set
pop set.pop() Removes an arbitrary element from the set
clear set.clear() Removes all elements from the set
Table 1.6: Methods Provided by Sets in Python
False, 3, 4.5, 6, 'cat'
your_set = 99,3,100
False, 3, 4.5, 6, 99, 'cat', 100
False, 3, 4.5, 6, 99, 'cat', 100
my_set & your_set
False, 4.5, 6, 'cat'
my_set - your_set
False, 4.5, 6, 'cat'
3,100 = your_set
False, 3, 4.5, 6, 'house', 'cat'
False, 3, 6, 'house', 'cat'
3, 6, 'house', 'cat'
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Operator Use Explanation
my_dictk Returns the value associated with𝑘, otherwise its an error
in key in my_dict Returns True if key is in the dictionary, False otherwise
del del my_dictkey Removes the entry from the dictionary
Table 1.7: Operators Provided by Dictionaries in Python
Our ﬁnal Python collection is an unordered structure called a dictionary. Dictionaries are
collections of associated pairs of items where each pair consists of a key and a value. This
key-value pair is typically written askey:value. Dictionaries are written as comma-delimited
key:value pairs enclosed in curly braces. For example,
capitals = 'Iowa':'DesMoines','Wisconsin':'Madison'
'Wisconsin': 'Madison', 'Iowa': 'DesMoines'
We can manipulate a dictionary by accessing a value via its key or by adding another key-value
pair. The syntax for access looks much like a sequence access except that instead of using the
index of the item we use the key value. To add a new value is similar.
capitals = 'Iowa':'DesMoines','Wisconsin':'Madison'
for k in capitals:
print(capitalsk," is the capital of ", k)
It is important to note that the dictionary is maintained in no particular order with respect to the
keys. The ﬁrst pair added ('Utah': 'SaltLakeCity') was placed ﬁrst in the dictionary
and the second pair added ('California': 'Sacramento') was placed last. The place-
ment of a key is dependent on the idea of “hashing,” which will be explained in more detail
in Chapter 4. We also show the length function performing the same role as with previous
Dictionaries have both methods and operators. Table 1.7 and Table 1.8 describe them, and the
session shows them in action. The keys, values, and items methods all return objects that
contain the values of interest. You can use the list function to convert them to lists. You
will also see that there are two variations on the get method. If the key is not present in the
dictionary, get will return None. However, a second, optional parameter can specify a return
'brad': 1137, 'david': 1410
phone_ext.keys() Returns the keys of the dictionary phone_ext
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Method Name Use Explanation
keys my_dict.keys() Returns the keys of the dictionary in adict_keys
values my_dict.values() Returns the values of the dictionary in a
items my_dict.items() Returns the key-value pairs in a dict_items ob-
get my_dict.get(k) Returns the value associated with 𝑘, None other-
get my_dict.get(k,alt) Returns the value associated with𝑘,𝑎𝑙𝑡 otherwise
Table 1.8: Methods Provided by Dictionaries in Python
"brad" in phone_ext
1137 in phone_ext
False 1137 is not a key in phone_ext
phone_ext.values() Returns the values of the dictionary
dict_items(('brad', 1137), ('david', 1410))
('brad', 1137), ('david', 1410)
1.4.2 Input and Output
We often have a need to interact with users, either to get data or to provide some sort of result.
Most programs today use a dialog box as a way of asking the user to provide some type of
input. While Python does have a way to create dialog boxes, there is a much simpler function
that we can use. Python provides us with a function that allows us to ask a user to enter some
data and returns a reference to the data in the form of a string. The function is calledinput.
Python’s input function takes a single parameter that is a string. This string is often called
the prompt because it contains some helpful text prompting the user to enter something. For
example, you might call input as follows:
1.4. Review of Basic Python 19Problem Solving with Algorithms and Data Structures, Release 3.0
user_name = input('Please enter your name: ')
Now whatever the user types after the prompt will be stored in the user_name variable.
Using the input function, we can easily write instructions that will prompt the user to enter
data and then incorporate that data into further processing. For example, in the following two
statements, the ﬁrst asks the user for their name and the second prints the result of some simple
processing based on the string that is provided.
user_name = input("Please enter your name ")
print("Your name in all capitals is",user_name.upper(),
"and has length", len(user_name))
It is important to note that the value returned from the input function will be a string
representing the exact characters that were entered after the prompt. If you want this string
interpreted as another type, you must provide the type conversion explicitly. In the statements
below, the string that is entered by the user is converted to a ﬂoat so that it can be used in
further arithmetic processing.
user_radius = input("Please enter the radius of the circle ")
radius = float(user_radius)
diameter = 2 radius
We have already seen that the print function provides a very simple way to output values
from a Python program. print takes zero or more parameters and displays them using a single
blank as the default separator. It is possible to change the separator character by setting the
sep argument. In addition, each print ends with a newline character by default. This behavior
can be changed by setting the end argument. These variations are shown in the following
print("Hello","World", sep=" ")
print("Hello","World", end=" ")
print("Hello", end=" "); print("World")
It is often useful to have more control over the look of your output. Fortunately, Python
provides us with an alternative called formatted strings. A formatted string is a template in
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Character Output Format
u Unsigned Integer
f Floating point as m.ddddd
e Floating point as m.ddddde+/-xx
E Floating point as m.dddddE+/-xx
g Use%e for exponents less than− 4 or greater than +5, otherwise us%f
c Single character
s String, or any Python data object that can be converted to a string by using thestr
% Insert a literal % character
Table 1.9: String Formatting Conversion Characters
which words or spaces that will remain constant are combined with placeholders for variables
that will be inserted into the string. For example, the statement
print(name, "is", age, "years old.")
contains the words is and years old, but the name and the age will change depending on
the variable values at the time of execution. Using a formatted string, we write the previous
print("%s is %d years old." % (name, age))
This simple example illustrates a new string expression. The % operator is a string operator
called the format operator. The left side of the expression holds the template or format string,
and the right side holds a collection of values that will be substituted into the format string.
Note that the number of values in the collection on the right side corresponds with the number
of % characters in the format string. Values are taken in order, left to right from the collection
and inserted into the format string.
Let’s look at both sides of this formatting expression in more detail. The format string may
contain one or more conversion speciﬁcations. A conversion character tells the format opera-
tor what type of value is going to be inserted into that position in the string. In the example
above, the %s speciﬁes a string, while the %d speciﬁes an integer. Other possible type spec-
iﬁcations include i, u, f, e, g, c, or %. Table 1.9 summarizes all of the various type
In addition to the format character, you can also include a format modiﬁer between the % and
the format character. Format modiﬁers may be used to left-justify or right-justify the value
with a speciﬁed ﬁeld width. Modiﬁers can also be used to specify the ﬁeld width along with a
number of digits after the decimal point. Table 1.10 explains these format modiﬁers.
The right side of the format operator is a collection of values that will be inserted into the
format string. The collection will be either a tuple or a dictionary. If the collection is a tuple,
the values are inserted in order of position. That is, the ﬁrst element in the tuple corresponds
to the ﬁrst format character in the format string. If the collection is a dictionary, the values
are inserted according to their keys. In this case all format characters must use the (name)
1.4. Review of Basic Python 21