ceng4315636732530_1245235week_06_0.7.pdf

1
Programming with Python for Engineers
Lecture-6
Lecture : Week-6
Subject : Repetitive Execution
- while & for statements
- continue & break
- List comprehension
Lecturer: Hüseyin SAYIN
E-Mail : hsayin@metu.edu.tr
Coordinating Assistant : Alperen Dalkıran
Support E-Mail : 2xx@ceng.metu.edu.tr

2
Objectives
• In this chapter, you will learn:
– Repetitive execution in Python:
* while & for statements; continue & break;
* List comprehension;

3
Repetitive Execution
(iteration or loop)
iteration/loop: Repeating a sequence of
instructions over-and-over is a programming
ingredient
• iteration/loop is used for;
- to systematically deal with all possible cases
or all elements of a collection.
- to jump from one case to another as a
function of the previous case.

4
(iteration or loop)
Python provides two statements for iteration.
Namely, while and for. whereas.
• for is used mostly for (Type I) iterations.
• while is used for both types.

5
(iteration or loop)
The repetition of a sequence of instructions is
carried out either for a known number of times
or until a criterion is met
• Computing letter grades of a class (Type I)
for all students.
• Finding the shortest path from city A to city B in
a map (Type II).
• Root finding by Newton-Raphson method
(Type II).
• Finding darkest and brightest point(s) in an
image (Type I, for all pixels).

6
(iteration or loop)
The repetition of a sequence of instructions is
carried out either for a known number of times or
until a criterion is met
• Computing a function value using Taylor’s
expansion (Type I).
• Computing the next move in a chess game
(Type II).
• Obtaining the letter frequency of a text (Type I)
for all letters.

7
(iteration or loop)
Python provides two statements for iteration.
Namely, while and for. whereas.
• for is used mostly for (Type I) iterations.
• while is used for both types.

The syntax of while resembles the syntax of the if
statement:
• Certainly possible to have a statement group
subject to the while.
• while statement execution is;
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The while Statement

Since a while statement is a statement, it can be
part of another while statement (or any other
compound statement):
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The Nested Loops

The "inner loop" will be executed one time for
each iteration of the "outer loop".
Example: Print each adjective for every fruit.
Python Code: Output:
10
The while Statement

11
Structured-Programming Summary

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• Infinite Loops
…
while 1: # while true
print(“Infinite Loop”)
…
while -9:
print (“Infinite Loop”)
…
The while Repetition Statement

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• Infinite Loops
…
while true: # while true
print(“Infinite Loop”)
…
while -9:
print (“Infinite Loop”)
…

14
• Infinite Loops
…
vCounter = 1
while vCounter:
print (“{ounter}n”)
vCounter += 1

15
• Null Loops
…
while 0:
print(“Null Loop”)
…
…
while false:
print(“Null Loop”)
…

Example 1: Finding Average of Numbers in a
List:
• Finding a list of numbers average.
• The mathematical definition:
• Li is the ith number in the list which contains N
numbers
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Examples with while Statement

List:
• The algorithm before implementing a solution in
Python.
• Pseudocode
17

List:
Python Code
Program Output: 18

Example 2: Factorial of a number
n! can be defined formally as.
• The mathematical definition:
19

n! can be defined formally as.
Pseudocode:
20

n! can be defined formally as:
Python Code:
Program Output: 21

The syntax of for resembles the syntax of the if
statement:
• An iterator is an object that provides a
countable number of values on demand..
• It has an internal mechanism; that responds to
three requests:
1. Reset (initialize) it to the start.
2. Respond to the “Are we at the end?” question.
3. Provide the next value in line.
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for Statement

The semantics of the for statement is;
23
for Statement

What iterators do we have?
1. All containers are also iterators (strings, lists,
dictionaries, sets).
2. The built-in function range returns an iterator
that generates a sequence of integers,
starting from 0 (default), and increments by 1
(default), and stops before a given number.
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for Statement

Example 1: Using the start parameter.
Python Code:
Output:
25
Examples with for and range() Statement

Python Code:
Output:
26

Python Code:
Output:
27

Python Code:
Output:
28

The "inner loop" will be executed one time for
each iteration of the "outer loop".
Example: Print each adjective for every fruit.
29
The Nested Loops

Example 1: split a list of words into two lists:
those that start with a vowel and those that start
with a consonant.
Python Code:
30
Examples with for Statement

To alter the flow of execution in a while or for
repetition:
• almost always used in combination with statement
grouping (a group of statements is subject to the while
or for),
• executing a sequence of statements by means of
statement grouping,
• Somewhere in the process, without waiting for the
terminating condition to become False in a while
statement or exhausting all items in the iterator in a for
statement, you decide to terminate looping.
31
continue and break Statements

To alter the flow of execution in a while or for
repetition:
• The break statement does exactly serve this purpose:
The while (or for) is stopped immediately and the
execution continues with the statement after the while
(or for).
• Stops the loop altogether, and goes on with
the next instruction after the loop end.
• Somewhere in the process you decide not to execute
the rest of the statements in the grouping and straight
away continue with the test. This is achieved by a
continue statement usage.
• stops the current iteration and tells Python to execute
the next one. 32

How continue and break statements change
execution in a while statement:
33

How continue and break statements change
execution in a for statement:
34

With the continue statement we can stop the
current iteration, and continue with the next.
35

This notation has three parts (from left-to-right):
or as something more elaborate:
36

37

A well-known and extensively-used notation to
describe a set in mathematics is the so-called
‘set-builder notation’. This is also known as set
comprehension. List comprehensions are a way
to create lists in a very concise way.
1. an expression in terms of a variable,
2. a colon or vertical bar separator, and
3. a logical predicate.
38
set and list Comprehension

Something like this:
which defines the set:
39

Python Code:
40

which defines the set:
41

Python Code:
42

Example-3:
Python Code:
Output:
43

Example-3:
Python Code:
print( [[0 for i in range(3)] for j in range(4)] )
Output:
[[0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0] ]
44

Example-3:
Python Code:
print( [[0 for i in range(3)] for j in range(4)] )
Output:
[
[0, 0, 0],
[0, 0, 0],
[0, 0, 0],
[0, 0, 0]
]
45

Example-4:
Python Code:
print( [[1 if i==j else 0 for i in range(3)] for j
in range(3)] )
Output:
[[1, 0, 0], [0, 1, 0], [0, 0, 1]]
46

Example-4:
Python Code:
print( [[1 if i==j else 0 for i in range(3)] for j
in range(3)] )
Output:
[[1, 0, 0],
[0, 1, 0],
[0, 0, 1]]
47

Example-5:
Python Code:
print( [[(i,j) for i in range(3)] for j
in range(4)] )
Output:
[[(0, 0),(1, 0),(2, 0)],[(0, 1),(1, 1),(2, 1)],
[(0, 2),(1, 2),(2, 2)],[(0, 3),(1, 3),(2, 3)]]
48

Example-5:
Python Code:
print( [[(i,j) for i in range(3)] for j
in range(4)] )
Output:
[
[(0, 0), (1, 0), (2, 0)],
[(0, 1), (1, 1), (2, 1)],
[(0, 2), (1, 2), (2, 2)],
[(0, 3), (1, 3), (2, 3)]
]
49

Example-6:
Python Code:
print( [[(i+j)%2 for i in range(3)] for j
in range(4)] )
Output:
[[0, 1, 0], [1, 0, 1], [0, 1, 0], [1, 0, 1]]
50

Example-6:
Python Code:
print( [[(i+j)%2 for i in range(3)] for j
in range(4)] )
Output:
[[0, 1, 0],
[1, 0, 1],
[0, 1, 0],
[1, 0, 1]]
51

Example-7:
Python Code:
print( [[i+3*j for i in range(3)] for j
in range(4)] )
Output:
[[0, 1, 2], [3, 4, 5], [6, 7, 8], [9, 10, 11]]
52

Example-7:
Python Code:
print( [[i+3*j for i in range(3)] for j
in range(4)] )
Output:
[[0, 1, 2],
[3, 4, 5],
[6, 7, 8],
[9,10,11]]
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54
Chapter Objectives
In this chapter, you will learn:
• Repetitive execution in Python
– Repetitive execution with while and for statements.
– Changing the flow of iterations with break and
continue statements,
– Set and list comprehension as a form of iterative
computation in an expression.

56
The break and continue Statements
• break
– Causes immediate exit from a while or for statement
– Program execution continues with the first statement after
the structure
– Common uses of the break statement
• Escape early from a loop

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