While loops

Overview

Teaching: 20 min
Exercises: 40 min

Questions

• What other constructs are available for changing the flow of a program?

Objectives

• Change program flow using available language constructs

• Demonstrate how to execute a section of code until a condition is met

We have seen that for loops provide a powerful form of repetition - the ability to repeat the same actions for every unit of data in a collection.

while loops facilitate another form of repetition; in this case, repeating a set of commands while a condition is True.

num_moons = 3
while num_moons > 0:
    print(num_moons)
    num_moons = num_moons - 1

While loops:

• start with the keyword while
• are followed by an expressions that evaluates to True or False
  • here, num_moons has be set, or initialised to 3 before the loop starts, so the condition is initially True
  • we change the value of num_moons within the loop, so that eventually the condition becomes False
• the while statement (first line of a while loop) ends with a colon :
• code within the while loop is indented. This code is performed at each iteration through the loop
• end when the condition becomes False

Assignment operators

In the while loop above, we updated the value of the variable num_moons while the loop was running, by typing the following code:

num_moons = num_moons - 1

This pattern, where modify the value of a variable using an operator and reassign the new value back to the same variable, is so common that Python has special short-hand assignment operators for this task. In the example above, the equivalent assignment operator looks like this:

num_moons -= 1

while loops can run zero times

num_moons = -3
while num_moons > 0:
    print(num_moons)
    num_moons -= 1

or they may run infinitely…

forever = 'the infinite while loop'
while forever:
    pass

To interrupt your code, press ‘I’ twice quickly. Note: pass is a valid statement within the body of a while loop (or a for loop, or an if statement). It means ‘go onto the next step’.

interrupting execution

Try out the following code:

counts = 1
while counts:
    counts += 1

What is happening here?

Interrupt your code. Now try:

print(counts)

cumulative sum

Calculating the sum of a series of numbers is a common use for a while loop.>

n = 10
current_sum = 0
i = 1
while  i <= n :
   current_sum += i
   i = i + 1
print("The sum of the numbers from 1 to", n, "is ", current_sum)

The sum of the numbers from 1 to 10 is 55

1. How many times is the body of the while loop executed?
2. Write out, on paper, a table that describes the values of n, current_sum, and i at each step through the loop.
3. Why didn’t we call our variable ‘sum’ instead of the longer, more cumbersome ‘current_sum’?

Exercise - Things that can go wrong with while loops

In the examples below, without running them try to decide why we will not get the required answer. Run each, one at a time, and then correct them. Remember that when the input next to a notebook cell is [*] your Python interpreter is still working.

# while loop - summing the numbers 1 to 10
n = 10
cur_sum = 0
i = 0
while  i <= n :
    i += 1
    cur_sum += i
    
print("The sum of the numbers from 1 to", n, "is ", cur_sum)

# while loop - summing the numbers 1 to 10
n = 10
cur_sum = 0
boolvalue = False
i = 0
while  i <= n and boolvalue:
    cur_sum = cur_sum + i
    i = i + 1
    
print("The sum of the numbers from 1 to", n, "is ", cur_sum)

# while loop - summing the numbers 1 to 10
n = 10
cur_sum = 0
i = 0
while  i != n :
    cur_sum = cur_sum + i
    i = i + 1

print("The sum of the numbers from 1 to", n, "is ", cur_sum)

# while loop - summing the numbers 1.1 to 9.9 in steps of 1.1
n = 9.9
cur_sum = 0
i = 0
while  i != n :
    cur_sum = cur_sum + i
    i = i + 1.1
    print(i)
    
print("The sum of the numbers from 1.1 to", n, "is ", sum)

Solution

1. Because i is incremented before the sum, you are summing 1 to 11.
2. The Boolean value is set to False the loop will never be executed.
3. When i does equal 10 the expression is False and the loop does not execute so we have only summed 1 to 9
4. Because you cannot guarantee the internal representation of Float, you should never try to compare them for equality. In this particular case the i never ‘equals’ n and so the loop never ends!

combining control structures: while and if

1. Fill in the blanks in the following code, so that it prints all the odd numbers from 0 to 10

   num = 0
   while num <= __:
    if (num % 2) __ 1:
        print(__)
        ___ += 1
   

1. Now, edit this code so it prints only the even numbers from 0 to 20.
2. Can you find a simpler way to code this, using just a while loop? (i.e. no if statement)

Key Points

• Most programs will require ‘Loops’ and ‘Branching’ constructs.

• while statements, in addition to for, allow for looping through sections of code

• The programmer must provide a condition to end a while loop.