Loop Control

An introduction to the use of Iteration in Python

Loop Control

Using for loops and while loops in Python allow you to automate and repeat tasks in an efficient manner.

But sometimes, an external factor may influence the way your program runs. When this occurs, you may want your program to exit a loop completely, skip part of a loop before continuing, or ignore that external factor. You can do these actions with break, continue, and pass statements.

Introduction

In Python, the break statement provides you with the opportunity to exit out of a loop when an external condition is triggered. You’ll put the break statement within the block of code under your loop statement, usually after a conditional if statement.

Let’s look at an example that uses the break statement in a for loop:

number = 0

for number in range(10):
   number = number + 1

   if number == 5:
      break    # break here

   print('Number is ' + str(number))

print('Out of loop')

In this small program, the variable number is initialised at 0. Then a for statement constructs the loop as long as the variable number is less than 10.

Within the for loop, the number increases incrementally by 1 with each pass because of the line number = number + 1.

Then, there is an if statement that presents the condition that if the variable number is equivalent to the integer 5, then the loop will break.

Within the loop is also a print() statement that will execute with each iteration of the for loop until the loop breaks, since it is after the break statement.

To see when we are out of the loop, we have included a final print() statement outside of the for loop.

When we run this code, our output will be the following:

Number is 1
Number is 2
Number is 3
Number is 4
Out of loop

This shows that once the integer number is evaluated as equivalent to 5, the loop breaks, as the program is told to do so with the break statement.

The break statement causes a program to break out of a loop.

Break Statement

The continue statement gives you the option to skip over the part of a loop where an external condition is triggered, but to go on to complete the rest of the loop. That is, the current iteration of the loop will be disrupted, but the program will return to the top of the loop.

The continue statement will be within the block of code under the loop statement, usually after a conditional if statement.

Using the same for loop program as in the Break Statement section above, we’ll use a continue statement rather than a break statement:

number = 0

for number in range(10):
   number = number + 1

   if number == 5:
      continue    # continue here

   print('Number is ' + str(number))

print('Out of loop')

The difference in using the continue statement rather than a break statement is that our code will continue despite the disruption when the variable number is evaluated as equivalent to 5. Let’s look at our output:

Number is 1
Number is 2
Number is 3
Number is 4
Number is 6
Number is 7
Number is 8
Number is 9
Number is 10
Out of loop

Here we see that the line Number is 5 never occurs in the output, but the loop continues after that point to print lines for the numbers 6-10 before leaving the loop.

You can use the continue statement to avoid deeply nested conditional code, or to optimize a loop by eliminating frequently occurring cases that you would like to reject.

The continue statement causes a program to skip certain factors that come up within a loop, but then continue through the rest of the loop.

Continue Statement

When an external condition is triggered, the pass statement allows you to handle the condition without the loop being impacted in any way; all of the code will continue to be read unless a break or other statement occurs.

As with the other statements, the pass statement will be within the block of code under the loop statement, typically after a conditional if statement.

Using the same code block as above, let’s replace the break or continue statement with a pass statement:

number = 0

for number in range(10):
   number = number + 1

   if number == 5:
      pass    # pass here

   print('Number is ' + str(number))

print('Out of loop')

The pass statement occurring after the if conditional statement is telling the program to continue to run the loop and ignore the fact that the variable number evaluates as equivalent to 5 during one of its iterations.

We’ll run the program and take a look at the output:

Number is 1
Number is 2
Number is 3
Number is 4
Number is 5
Number is 6
Number is 7
Number is 8
Number is 9
Number is 10
Out of loop

By using the pass statement in this program, we notice that the program runs exactly as it would if there were no conditional statement in the program. The pass statement tells the program to disregard that condition and continue to run the program as usual.

The pass statement can create minimal classes, or act as a placeholder when working on new code and thinking on an algorithmic level before hammering out details.

Pass Statement

Write a program that demonstrates BREAK.

Write a program that demonstrates BREAK and CONTINUE

Write a program that demonstrates BREAK, CONTINUE and PASS.

10.5 Loop Control
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