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Engineering LibreTexts

5.6: Definite loops using for

  • Page ID
    3075
  • Sometimes we want to loop through a set of things such as a list of words, the lines in a file, or a list of numbers. When we have a list of things to loop through, we can construct a definite loop using a for statement. We call the while statement an indefinite loop because it simply loops until some condition becomes False, whereas the for loop is looping through a known set of items so it runs through as many iterations as there are items in the set.

    The syntax of a for loop is similar to the while loop in that there is a for statement and a loop body:

    Code \(\PageIndex{1}\) (Python):

    %%python3
    
    friends = ['Joseph', 'Glenn', 'Sally']
    for friend in friends:
        print('Happy New Year:', friend)
    print('Done!')
    

    In Python terms, the variable friends is a list1 of three strings and the for loop goes through the list and executes the body once for each of the three strings in the list resulting in this output:

    Happy New Year: Joseph
    Happy New Year: Glenn
    Happy New Year: Sally
    Done!

    Translating this for loop to English is not as direct as the while, but if you think of friends as a set, it goes like this: "Run the statements in the body of the for loop once for each friend in the set named friends."

    Looking at the for loop, for and in are reserved Python keywords, and friend and friends are variables.

    for friend in friends:
        print('Happy New Year:', friend)

    In particular, friend is the iteration variable for the for loop. The variable friend changes for each iteration of the loop and controls when the for loop completes. The iteration variable steps successively through the three strings stored in the friends variable.