7C: Loops

Lesson 7 has three parts A, B, C which can be completed in any order.

So far we have learned basic programming commands (e.g. assigning values, printing) and, a way to control which statements are executed using the if statement. In this lesson, we introduce loops (sometimes called repetition or iteration): a way to make a computer do the same thing (or similar things) over and over. For example, spell-checking every word in a document would be done with a loop. We will describe the two kinds of Python loops in this lesson: while loops and for loops.

while Loops

A while statement repeats a section of code over and over again as long as some condition is true. Here is an example:

Here is the general structure:

  • The first line is while «condition»: where «condition» is an expression which returns True or False (a Boolean expression, like with if statements).
  • Afterwards, we put an indented block (again, like an if statement) consisting of the statements which we want to be repeated over and over. This is called the body.
  • When you run the program, the following is repeated:
    • The condition is tested; if the condition is True then the body is executed and afterwards we repeat from the top; once the condition is evaluated to False, the loop stops.

So in the example above, we keep repeating the loop body until timeLeft is not greater than 0.

Coding Exercise: Countup
Modify the example above to give a program where we count up starting from 1 and going up until 10, and then print Blastoff!
You may enter input for the program in the box below.

With loops, it is easy to write a program that runs forever in an infinite loop.
Infinite loop
You get the error "Time Limit Exceeded" because the CS Circles web server enforces a time limit; after 1 second the program is terminated. If you ran the program at home, it would run forever (until you yourself force it to terminate, usually by pressing Ctrl-C). 

for Loops

There is another kind of loop in Python called a for loop. In many situations either kind of loop (for/while) can be used but one is simpler than another, so it is useful to know how to use both. A for loop is built in order to easily loop through a range of numbers (or as we will see in a later lesson, any list of data).

Here is an example of a for loop.

The general structure of a numerical for loop is

for «variableName» in range(«startValue», «tailValue»):
  «indented block of commands, called the loop "body"»
As usual, the body block can be multiple lines long, as long as all of those lines are indented by the same amount. First the loop body is executed with variableName set to startValue. Then it repeats with variableName set to startValue+1, then again with startValue+2, et cetera. This continues until variableName has value tailValue-1, and afterwards the loop stops.

The loop ends with tailValue-1, and not tailValue!

Here is an example of a for loop inside another for loop.

This code prints a 5×5 square of ones.  
Note: when we multiply a number X by ten and add one, we're essentially putting an extra 1 digit at the end of X. For example, (1867*10)+1=18671.

Coding Exercise: One Triangle
Modify the previous program in two ways. First, instead of a square, make it draw a triangle shaped like this: ◤. Second, instead of always having 5 lines, it should take the desired size as input from input(). For example, if the input is 3, then the output should be

Click here for a hint. 
You may enter input for the program in the box below.

The break and continue Statements

The break statement is like an emergency escape for a while or for loop: break causes an immediate jump to the commands after the end of the loop body. Here is an example using break: it reads all lines of input until it finds one that says "END".

Looping through all lines of input

The continue statement makes you skip the rest of a loop, then repeat the body from the next round (usually called the next "iteration").


Here is a visualized example that combines break and continue. Can you predict what it will output?


Coding Exercise: Square Census
The square numbers are the integers of the form K × K, e.g. 9 is a square number since 3 × 3 = 9. Write a program that reads an integer n from input and outputs all the positive square numbers less than n, one per line in increasing order. For example, if the input is 16, then the correct output would be

You may enter input for the program in the box below.

Coding Exercise: Skipping
Extend the "Looping through all lines of input" example above (we've copied it for you) by adding a new feature: any line equal to SKIP should not be printed, and should not cause the counter to be increased. Run the program to see an example. Hint
You may enter input for the program in the box below.

Coding Exercise: Finding Factors
If a × b = n, we call a × b a factorization of n. In this exercise, write a program that takes a positive integer n from input, and then outputs all factorizations of n; you should follow the formatting given by the following example for n=10.

1 times 10 equals 10
2 times 5 equals 10
5 times 2 equals 10
10 times 1 equals 10
You may enter input for the program in the box below.