Week4A Preread: Nested Loops

Week4A Preread: Nested Loops#

Nested loops#

A nested loop is one loop inside of another. In other words, the action of a loop contains another loop. Frequently, a nested loop is a nested for loop, in which one for loop is part of the action of another for loop.

In the following example, a list of words is created. A for loop loops through all of the words, and then for each word, another for loop loops to print each character followed by a space.

wordlist = ['hello', "hi", 'ciao'] 
for myword in wordlist: 
    for c in myword: 
        print(c, end = ' ') 
    print() 
print("That's it!")

h e l l o
h i
c i a o
That's it!

The first loop is called the outer loop. The action of the outer loop consists of two statements:

  • loop to print each character and a space

  • a print() to move the cursor down for the next word

The second loop, that loops over the characters, is called the inner loop. The action of the inner loop is a single print statement. After the nested loop, there is another print statement to print “That’s it!”. The outer loop iterates over the words in the list. For each word, the inner loop iterates over the characters in that word.

So, the first time in the outer loop the variable myword will store ‘hello’. The inner loop variable will then iterate through all of the characters in the variable myword, and for each it will print the character and then a space (all on the same line). After the inner loop has completed, the print() statement will print a newline character, and that is it for the action of the outer loop. Once this action has completed, the variable myword will store the second string in the list, “hi”. The inner loop will then iterate through all of its characters and print each one followed by a space. After both characters have been printed, the print() moves the cursor down to the next line. Finally, myword gets the value ‘ciao’, and the inner loop prints all of its characters on one line and then a newline at the end. Once the variable myword has iterated through all 3 words in the list, the outer loop is done so the code prints “That’s it!” after the nested loop has completed.

The following is an example of a nested loop, in which the outer loop repeats 3 times. Each time the action is executed, the next integer (0 then 1 then 2) is printed, followed by a colon and a space. The inner loop then prints, 5 times, a single ‘*’. The second argument, end=’’, keeps all of the *’s on the same line and does not print anything between them. The print() after the inner loop prints the newline character by default, so each of the 3 actions from the outer loop is on a separate line.

for num in range(3): 
    print(f'{num}:', end=' ')  
    for n in range(5): 
        print('*', end='')  
    print()

0: *****
1: *****
2: *****

The outer loop specifies that the action will be repeated 3 times. The action of the outer loop consists of 3 statements:

  • A print statement to print the value of the outer loop variable

  • A for loop to repeat the action of printing a single ‘*’ 5 times on the same line

  • A print statement to move the cursor down for the next value of the outer loop variable

Instead of looping 5 times to print a ‘*’, the following inner loop repeats num times, where num is the value of the outer loop variable (plus 1, so it repeats once, then twice, then three times).

for num in range(3): 
    print(f'{num}:', end=' ')  
    for n in range(num+1):  
        print('*', end='')  
    print()

0: *
1: **
2: ***

If instead of printing 0, 1, and 2 in the beginning of the lines, we wanted 1, 2, and 3, the code would instead look like this:

for num in range(3): 
    print(f'{num+1}:', end=' ')  
    for n in range(num+1):  
        print('*', end='')  
    print()

1: *
2: **
3: ***

These were examples of nested for loops. Nested loops can also contain while loops. For example, let’s say we want the user to enter 3 positive numbers. That means we will use a for loop to repeat the action 3 times. Each time, we will error-check using a while loop to make sure that each time a positive number is entered. The algorithm is:

  • Loop 3 times

    • Prompt the user for a positive number

    • While the number is not positive

      • Print error message

      • Prompt again

    • Print the positive number

Here is an implementation of the algorithm:

for i in range(3): 
    number = input('Enter a positive number: ')  
    number = float(number) 
    while (number <= 0): 
        print('Please follow directions!')  
        number = input('Enter a positive number: ') 
        number = float(number) 
    print('Thanks for entering', number) 
    print()

Enter a positive number: 4
Thanks for entering 4.0

Enter a positive number: -3.3
Please follow directions!
Enter a positive number: 5.1
Thanks for entering 5.1

Enter a positive number: -6
Please follow directions!
Enter a positive number: 11
Thanks for entering 11.0

An extra print() was added to the action of the outer for loop in order to separate each of the actions of the for loop.

Using if Inside a Loop#

So far we have seen loops that repeat the same action every time. In practice, we often want the action to change depending on the current value of the loop variable. This is where an if statement inside a loop becomes useful.

Example: Even and Odd Numbers#

Let’s look at the numbers from 0 through 9. For each number, we want to decide whether it is even or odd. To do this, we use the modulus operator %, which gives the remainder after division. If a number divided by 2 has no remainder (n % 2 == 0), the number is even. Otherwise, it is odd.

for n in range(10):          # loop through the numbers 0,1,2,...,9
    if n % 2 == 0:           # condition: is n evenly divisible by 2?
        print(n, "is even")  # runs when the condition is true
    else:                    
        print(n, "is odd")   # runs when the condition is false

Step-by-step behavior#

  • On the first iteration, n = 0. Since 0 % 2 == 0, the if branch runs and prints 0 is even.

  • On the next iteration, n = 1. Since 1 % 2 == 0 is false, the else branch runs and prints 1 is odd.

  • The loop continues this way for each number up to 9.

Output

0 is even
1 is odd
2 is even
3 is odd
4 is even
5 is odd
6 is even
7 is odd
8 is even
9 is odd

Why this is important#

Combining loops with conditionals lets us write much more powerful programs.

  • The loop controls how many times the block of code runs.

  • The if/else decides which action to take on each iteration.

This combination is used constantly in programming. With the same idea, you could:

  • Print only the negative numbers in a list.

  • Count how many numbers are above a certain threshold.

  • Skip or handle special cases differently while processing data.

In short: loops let us repeat work, and if statements let us choose actions. Together, they form one of the most common and useful patterns in Python.

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