1.5 Repetition

Control Structures Part 2 - Repetition

Each line of your code is an instruction for Python to execute.

Python will execute the lines in order, from line 1 to the end of your file.

This is called sequence. It works for simple programs, but there are other ways to control the flow of your program.

Selection allows you to make decisions based on a test. Repetition allows you to run a section of code over and over.

Let's look at these ideas separately.

Repetition / Loops

When you have a series of commands that you want to repeat, you're going to want to use a loop. Each time a loop runs, we call it an iteration.

There are two types of loops.

1. Loops that repeat based on a number - for loop
2. Loops that repeat based on logical truth - while loop

For Loop

The for loop is used when you need to repeat your code based on a certain number of times. This is pseudo code:

for counter in range(start, stop):

    cool stuff happens here, as many times as the loop runs

    look, a 2nd line of cool stuff

and finally, a line of code OUTSIDE the for loop

What do you notice?

• The for loop has a counter variable - in this case it's called counter
• The for loop has a range - in this case, the range() will create a list of numbers from the value in the variable start, to the value in the variable stop
• The for loop has an indent - EVERYTHING that lines up with that indent is part of the for loop

Problem: Print the string Hello 5 times

for loopcounter in range(0, 5):

    print('Hello')

What happened in the loop? The for loop needed a counter, that variable was called loopcounter. The for loop ran from the numeric range, 0 to 5. Each time it looped, the print() executed.

Check, Do, Math

Check: to see if the counter variable is in the range

Do: do each indented line of code in the loop

Math: typically the counter variable goes up by one.

Problem: Print the loopcounter variable so that we can see the loop run.

for loopcounter in range(0, 5):

    print(loopcounter)

What happened in the loop? The for loop ran five times. The amount of loops was controlled by the range(). Each time the loop ran, the variable loopcounter was printed with print(). Did you notice where the number started and left off? Most people, mistakenly, expect numbers to start at 1, not 0.

Let's look at helping those folks out in the next example.

Problem: Print the loopcounter variable so that it starts at 1, and ends at 5.

for loopcounter in range(1, 6):

    print(loopcounter)

That output looks better, doesn't it? But what is it telling us about numeric ranges?

Problem: Print the 5 'times table', from 1 to 10.

for counter in range(1, 11):

    print(counter * 5)

Problem: for loop without range()

The data structure that I'm going to use in the next example is called a list. Don't worry about lists for now. Let's just use it.

Our first for loop examples have Python iterating across a range of numbers, using range(). Python can also loop across a set of data. It does all the counting for you:

caf_specials = [ 'breakfast sandwich', 'milk', 'rice', 'pizza', 'salad', 'fish and chips' ]

for i in caf_specials:

    print(i)

What happened? Python took each item in the list, and held it in the variable i.

As the loop, looped, the print() printed whatever was in the variable i.

The loop ran for as many times as there were things in the list.

Python will do the same with a string.

my_string = "This is the most interesting sentence I could think of."

for i in my_string:

    print(i)

The range()

At this point in the course, for loops depend on the range(). It stands to reason that we should explore this built-in function.

#range([start], stop[, step])

#start: Starting number of the sequence.

#stop: Generate numbers up to, but not including this number.

#step: Difference between each number in the sequence.

for i in range(5):      # only stop value provided

    print(i)

print('')

for i in range(0, 5):    # start and stop value provided

    print(i)

print('')

for i in range(5, 0, -1):    # start and stop and negative step value provided

    print(i)

print('')

for i in range(0, 6, 2):    # start and stop and step value provided

    print(i)

Looking at Loops another way: TURTLE

import turtle #import the turtle module

bob = turtle.Turtle() #create a turtle named bob

#Let's draw a square

bob.forward(100)        #forward movement 100 px

bob.left(90)            #turn left 90 degrees

bob.forward(100)        #forward movement 100 px

bob.left(90)            #turn left 90 degrees

bob.forward(100)        #forward movement 100 px

bob.left(90)            #turn left 90 degrees

bob.forward(100)        #forward movement 100 px

bob.left(90)            #turn left 90 degrees

# Is there a way to draw the square using a loop?

#Let's move bob away from his first square

bob.penup()

bob.right(90)

bob.forward(100)

bob.pendown()

for x in range(4):      #loop instruction set 

  bob.forward(100)        #forward movement 100 px

  bob.left(90)            #turn left 90 degrees

Which square solution looks more efficient? The one with the 8 commands, or the one with 2 commands in a loop?

Nested loops

A loop can be placed in another loop. This is called a nested loop.

for outer_loop_counter in range(0,4):

    print('OUT')

    for inner_loop_counter in range(0,4):

        print('In')

What do you notice?

The outer loop seems to run once, then the inner loop runs several times, and then the outer loop runs, then the inner loop runs several times.

Let's look at at that previous example, but modify the print(), so that you can see what's happening to the counters.

for outer_loop_counter in range(0,4):

    print('OUT', outer_loop_counter)

    for inner_loop_counter in range(0,4):

        print('IN', inner_loop_counter)

Notice that the inner loop is reset?

What would a student's school week look like in a nested loop?

for day in range(0,5):

    print('A School Day Begins')

    for period in range(0,5):

        print('A School Period')

    print('A School Day Ends')

Looking at Nested Loops another way: TURTLE

import turtle #import the turtle module

bob = turtle.Turtle() #create a turtle named bob

for outerLoop in range(4):      #OUTER loop

  bob.pencolor('red')     #outer loop draw everything in red

  bob.forward(100)        #forward movement 100 px

  bob.left(90)            #turn left 90 degrees

  for innerLoop in range(2):  #INNER loop

    bob.pencolor('black') #INNER loop, draw everyting in black

    bob.right(90)         #turn right 90 degrees

    bob.circle(25)        #draw circle with 25 px radius

In the previous code, we can see the OUTER loop drawing the sides of a square.

When we get to a corner, the INNER loop draws 2 circles.

Then the INNER loop stops once the counter is out of the range.

Then OUTER loop continues.

These steps happen for as many times as the OUTER loop counter is in its range.

While Loops

While loops are also known as conditional loops. While require a condition to be true. This condition is also known as a test.

Before we start learning about while loops, I'm going to show you how to stop an infinite loop. CTRL-c remember that.

x = 5

while x == 5:

    print(x)

Well, that happened? That's a lot of 5's... Hope you remembered CTRL-c!

The while loop was set to run as long as x == 5.

If the operator == is confusing you, please take a moment to look at the operators note.

It turns out that x was set to 5 before the the loop, and x will remain 5 until, forever.

Let's try that again

x = 5

while x > 0:

    print(x)

    x -= 1

In this example, x was set to 5 before the loop.

If the operators > or -= are confusing you, please take a moment to look at the operator note.

The while loop was set to run as long as x was greater than 0.

But, if you look in the loop, we are subtracting 1 from x on each rotation. Eventually, we get to the situation where 0 > 0 and the loop stops because the condition is false.

Flags

Sometime when I am talking about while loops, you'll hear me reference a flag. What I mean is a boolean that controls the while loop. Here's an example:

flag = True

while flag == True:

    print('Hello')

    userinput = input('Please input the word STOP, to stop this loop: ')

    if userinput == 'STOP':

        flag = False

    else:

        print('I guess this is not going to stop')

Looking at While Loops another way: TURTLE

import turtle #import the turtle module

bob = turtle.Turtle() #create a turtle named bob

loopControl = True

while loopControl == True:

  bob.circle(50)

  bob.left(45)

  bob.forward(10) 

# Before you run this, ask yourself, will this ever end?

User Input and Loops

As we have already discovered, for loops work well when your loops are based on counting, and while loops are very effective when you have a boolean expression (True or False).

How can we manage our loops based on other input?

userInput = input('Please enter a number: ')

userInput = int(userInput)

for counter in range(0, userInput):

    print(userInput)

z = True

while z:

    print('Hi')

    userInput = input('Please type Q to quit ')

    if userInput == "Q":

        z = False

w = input('Please type in a number: ')

w = int(w)

for w in range(0, w):

    print(w)

ICS3C Python Program Template for a Menu Driven Program

# Author: Mr. Liconti, based on Jack M.'s original code

# Date: 2/9/2018

# File Name: rect_area_MENU.py

# Description: making a program to calculate the area of a rectangle. Added a Menu

#import statments here

#global variables here

flag = True #loop control

print('This program will give you the area of a rectangle.')

while flag == True:

                        #Display menu options 

    print('   MENU')

    print('==========')

    print('S to start')

    print('H for help')

    print('Q to quit')

    print('==========')

    print(' ')

    choice = input('Option?: ')     #get user's choice

    if choice == 'H':

        print('This program will give you the area of a rectangle.')

        print('Please provide the height and width when asked.')

        print('You can quit this program from the main menu by pressing Q')

    elif choice == 'Q':

        flag = False

    elif choice == 'S':

        height = input('height: ')        #this is where you input the height of the rectangle

        width = input('width: ')          #this is where you input the width of the rectangle

        height = int(height)            #changing height into a int because it will be a number

        width = int(width)              #changing the width to a int because it will be a number

        area = height * width           #area is equal to height times width

        area = str(area)                #area is changing to a string because it was a int

        print('The answer is: ' + area + ' meters') #the area is in meters so you would

                                                #add meters to the end

    else:

        print('Not a valid choice. Only S, Q and H are valid choices')

print('Program ending.')