Last updated: Monday 1st March 2021, 11:46 PT, AD

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And now for something
completely different . . .

Part 5 Python 3 - Repetition and String Formatting

5.1 The while loop

5.2 The for loop

5.3 String formatting

5.4 Nested loops - loops within loops

The while statement

Like the if statement, the while statement makes use of a Boolean expression...

x = 1

while x < 5:

print ('Hi spam')

x = x + 1

print ('done')

The Boolean expression (condition) in this example is:

x < 5

The expression has a value of true or false (1 or 0).

If the condition is true,

the indented statements are executed,

otherwise, the statements are skipped.

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The break statement

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The continue statement

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A Sentinel-controlled while loop

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#  File:       05-07.py

#  Purpose:    Example: 'sentinel-controlled' while loop

#              Calculates average score of a class

#  Programmer: Anne Dawson

#  Course:     CSCI120A

#  Date:       Tuesday 5th October 2004, 6:31 PT

# initialization phase

totalScore = 0     # sum of scores

numberScores = 0   # number of scores entered

# processing phase

score = input( "Enter score, (Enter -9 to end): " )   # get one score

score = int( score )   # convert string to an integer

while score != -9: # -9 is used as a sentinel ( a lookout or sentry value )

    totalScore = totalScore + score

    numberScores = numberScores + 1

    score = input( "Enter score, (Enter -9 to end): " )

    score = int( score )

# termination phase

if numberScores != 0: # division by zero would be a run-time error

   average = float( totalScore ) / numberScores

   print ("Class average is", average)

else:

   print ("No scores were entered")

Counter-controlled repetition with the for loop

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Using continue with the for loop

for c in range (1,6):

if c == 3:

continue

print (c)

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Using break with the for loop

for c in range (1,6):

if c == 3:

break

print (c)

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Printing strings and numbers

d = 10

c = 75

print ('Total is: ', d, 'dollars and', c, ' cents')

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Numbers can be printed from within a single string

by using a special method known as string formatting

An overloaded operator behaves differently

depending on the context.

In the following example we see the % operator

being used to specify how a string should be printed

(i.e. string formatting).

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Example program 05-14.py illustrates

how to repeat a program at the user's request:

# File: 05-14.py

# Purpose: Example: how to repeat a program at the user's request

# Programmer: Anne Dawson

# Course: CSCI120A, CSCI165

# Date: Thursday 19th October 2006, 7:58 PT

print ("This is the start of the program")

answer = 'y'

while (answer == 'y' or answer == 'Y'):

print ("This is a statement from within the while loop")

print ("This is another statement from within the while loop")

answer = input("Do you want to run this program again? y/n")

print ("Goodbye!")

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Example program 05-15.py - 05-18.py

illustrate how to use loops inside loops:

#  File:       05-17.py

#  Purpose:    Example: how to use a loop within a loop

#              a nested for loop

#  Programmer: Anne Dawson

#  Course:     CSCI120A, CSCI165

#  Date:       Wednesday 27th June 2007, 9:45 PT

print("This is the start of the program")

for i in range(1,6):

    for j in range(1,6):

        print ("i: " + str(i) + " j: " + str(j) )

    print()

'''

Notice that with a loop repeating 5 times,

***within*** a loop that repeats 5 times

means that you can control 25 processes.

'''

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The output after the program runs:

This is the start of the program

i: 1 j: 1

i: 1 j: 2

i: 1 j: 3

i: 1 j: 4

i: 1 j: 5

i: 2 j: 1

i: 2 j: 2

i: 2 j: 3

i: 2 j: 4

i: 2 j: 5

i: 3 j: 1

i: 3 j: 2

i: 3 j: 3

i: 3 j: 4

i: 3 j: 5

i: 4 j: 1

i: 4 j: 2

i: 4 j: 3

i: 4 j: 4

i: 4 j: 5

i: 5 j: 1

i: 5 j: 2

i: 5 j: 3

i: 5 j: 4

i: 5 j: 5

This presentation uses the following program files:

Last updated: Monday 1st March 2021, 11:46 PT, AD

And now for something completely different . . .

Part 5

Python 3 - Repetition and String Formatting

5.1 The while loop

5.2 The for loop

5.3 String formatting

5.4 Nested loops - loops within loops

The while statement

x = 1

while x < 5:

print ('Hi spam')

x = x + 1

print ('done')

The break statement

The continue statement

A Sentinel-controlled while loop

Counter-controlled repetition with the for loop

Using continue with the for loop

for c in range (1,6):

if c == 3:

continue

print (c)

Using break with the for loop

for c in range (1,6):

if c == 3:

break

print (c)

Printing strings and numbers

d = 10

c = 75

print ('Total is: ', d, 'dollars and', c, ' cents')

Numbers can be printed from within a single string

by using a special method known as string formatting

This presentation uses the following program files:

05-01.py

05-02.py

05-03.py

05-04.py

05-05.py

05-06.py

05-07.py

05-08.py

05-09.py

05-10.py

05-11.py

05-12.py

05-13.py

05-14.py

05-15.py

05-16.py

05-17.py

05-18.py

And now for something
completely different . . .