Flow Control (Repetition Statements)

Chapter 5

In chapter 4, we saw that we can control the flow of a program using selection statements - i.e. with the keywords if, elif and else. The next mode of flow control will be using repetition.

The While Statement#

The while statement allows us to repeatedly execute a block of code as long as a condition remains valid.

x = 3

while(x > 0):
    print("Looping")
    x -= 1  # equivalent to writing x = x - 1

The above code will print Looping for a total of 3 times before terminating. As you would have noticed, in each loop we decrement the value of x by one. The first time we execute the loop, x is 3. The second time it would be 2 and so on, until in the final loop when it becomes 0. Now, the next time we check if x > 0, the condition evaluates to be false and hence, the loop terminates.

If we try to represent the above code block in terms of a simple flowchart, it will look like this.

Repetition Statements

The For Loop#

We can also execute a code block repeatedly with the for loop.

# prints 0, 1, 2
for i in range(3):
    print(i)

# prints 2, 3, 4, 5, 6
for i in range(2, 7):
    print(i)

# prints 0, 2, 4, 6, 8
for i in range(0, 10, 2):
    print(i)

The above three examples show how we can use for loops with the range operator.

In the first example, variable i gets assigned to 0 first. range(3) means that the loop is executed as long as i < 3. In every loop, i is incremented by 1.

Similarly, in the second example, range(2, 7), i gets a starting value of 2 and the loop is executed as long as i < 7. In every loop, i is incremented by 1.

In the third example, range(0 ,10, 2), i gets a starting value of 0 and the loop is executed as long as i < 10. While the first two example incremented i by 1 in each loop, here i is incremented by 2. This explains why the output is from 0 to 10 in multiples of 2.

The Break Statement#

Both while and for loops provide great utility in almost every program you write. Another useful statement is break which is used to exit out of a loop.

i = 0
while True:
    print(i)
    i += 1
    if(i > 3):
        break

In this example, we enter an infinite loop by saying while True. To break out of this infinite loop, we check for a specific condition if i > 3 in every loop. If that condition evaluates to be true, we break out of the loop - i.e. loop is terminated.

The Continue Statement#

Sometimes we would also want to skip a specific iteration of the loop. We can do this using the continue statement. Previously, we saw that we can print all even numbers from 0 to 10 using range(0, 10, 2). Lets try re-writing this, making use of the continue statement now.

for i in range(0, 10):
    if(i % 2 != 0):
        continue
    print(i)

Here, in each iteration/loop, we check if i is odd by using the condition i % 2 != 0. This reads as "if the remainder when i is divided by 2 is not 0", we continue onto the next iteration without executing any of the following statements.

For example, when i is equivalent to 3, since (3 % 2 = 1), the condition i % 2 != 0 evaluates to false. We continue onto the next iteration without executing line 4 - i.e. nothing is printed.