6.5. Definite loops using for¶
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.
- it loops through a known set of items so it runs through as many iterations as there are items in the set.
- Incorrect! A while loop does not run through a known set of items. Try again.
- it loops until some condition becomes True.
- Incorrect! A while loop will keep iterating while the condition is True. Try again.
- it loops until some condition becomes False.
- Correct! A while loop continues until the condition is False. There is no definite answer to when that happens; it all depends on the incrementation.
- it loops until it reaches a "break" statement.
- Incorrect! A while loop can stop at a break statement, but so can any other loop. Try again.
Q-1: The while loop is an “indefinite” loop because…
- it loops through a known set of items so it runs through as many iterations as there are items in the set.
- Correct! A for loop only runs through a distinct set of items.
- it loops until some condition becomes True.
- Incorrect! A for loop does not check any condition to decide whether it should continue to iterate. Try again.
- it loops until some condition becomes False.
- Incorrect! This is the condition for a while loop to stop. Try again.
- it loops until it reaches a "break" statement.
- Incorrect! A for loop can stop at a break statement, but so can any other loop. Try again.
Q-2: The for loop is not an “indefinite” loop because…
The syntax of a for loop is similar to the
while loop in that there is a for statement
and a loop body:
In Python terms, the variable friends is a list (We will examine lists
in more detail in a later chapter) of
three strings. 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.
Construct a block of code that prints “Hello, Prisha”, “Hello, Kahlil”, “Hello, Nirav”, “Hello, Aliyah”, and “Hello, Antonella”. After saying hello to each name in the list, print “All done!”. Watch out for three extra pieces of code and make sure your indentation is correct.