Loops
While Loops
In Python, a while
loop will repeatedly execute a code block as long as a condition evaluates to True
.
The condition of a while
loop is always checked first before the block of code runs. If the condition is not met initially, then the code block will never run.
while condition:
# Code inside
This loop will only run 1 time:
hungry = Truewhile hungry:print("Time to eat!")hungry = False
This loop will run 5 times:
i = 1while i < 6:print(i)i = i + 1
For Loop
Python for
loops can be used to iterate over and perform an action one time for each element in a list.
Proper for loop syntax assigns a temporary value, the current item of the list, to a variable on each successive iteration:
for <temporary value> in <a list>:
for
loop bodies must be indented to avoid an IndentationError
.
dog_breeds = ["boxer", "bulldog", "shiba inu"]# Print each breed:for breed in dog_breeds:print(breed)
Nested Loops
In Python, loops can be nested inside other loops. Nested loops can be used to access items of lists which are inside other lists. The item selected from the outer loop can be used as the list for the inner loop to iterate over.
groups = [["Jobs", "Gates"], ["Newton", "Euclid"], ["Einstein", "Feynman"]]# This outer loop will iterate over each list in the groups listfor group in groups:# This inner loop will go through each name in each listfor name in group:print(name)
Break Keyword
In a loop, the break
keyword escapes the loop, regardless of the iteration number. Once break executes, the program will continue to execute after the loop.
numbers = [0, 254, 2, -1, 3]for num in numbers:if (num < 0):print("Negative number detected!")breakprint(num)
In this example, the output would be:
02542Negative number detected!
Continue Keyword
In Python, the continue
keyword is used inside a loop to skip the remaining code inside the loop code block and begin the next loop iteration.
big_number_list = [1, 2, -1, 4, -5, 5, 2, -9]# Print only positive numbers:for i in big_number_list:if i < 0:continueprint(i)
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