5.6: Definite loops using for
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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 5.6.1 (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!
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
friends are variables.
for friend in friends: print('Happy New Year:', friend)
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