Loops

While loop videos

While Loop | Python | Tutorial 20
- Counter variable
- Loop condition
Beginner Python Tutorial 69 - do while Loop in Python
- Post-test loop (Do while)
- break
Beginner Python Tutorial 70 - Indefinite Loop
- Post-test loop with user input
How to Use “break” and “continue” in Python “while” Loops
- break
- continue

For loop videos

Loop with a counter variable

Three steps to looping with a counter variable:

what is the first value I want to output?
- Set ‘i’ to that value.
what is the final value I want to output?
- Include that in the while condition.
what do I want it to go up by?
- Increase ‘i’ by that amount.

i = 0
while i < 5:
    print("hello")
    i += 1

Practice Question Generator

Try the Loop Practice Question Generator. Press the “play” button to run the program.

Loop with an accumulator variable

total = 0  # accumulator
i = 1  # counter
while i <= 10:
    total += i  # add to the accumulator
    i += 1  # increase the counter

break

i = 0

while i <= 10:
    print(i)
    if i == 5:
        break
    i += 1

continue

i = 0

while i <= 10:
    if i == 5 or i == 7:
        i += 1
        continue
    print(i)
    i += 1

Get user input in a loop

This is broken down in three patterns:

Create a loop that iterates five times.
Take user input in the loop.
Use an accumulator variable to store the input.

i = 0         # counter
total = 0     # accumulator

while i < 5:  # set to loop 5 times
    num = int(input("Enter number: "))
    total += num
    i += 1

print(total)

Quit loop with sentinel value

The sentinel value in the code below is -1. The loop will continue forever until the user enters that vaue. We break when the sentinel value is encountered.

total = 0

while True:
    num = int(input("Enter number, -1 to stop: "))
    if num == -1:
        break
    total += num

print(total)

Loop through a string (while)

name = "Mr. Gallo"

i = 0
while i < len(name):
    print(i, name[i])
    i += 1

M
r
.
3
G
a
l
l
o

Loop through a list (while)

friends = ["Frank", "Sally", "Jimbo"]

i = 0
while i < len(friends):
    print(i, friends[i])
    i += 1

Frank
Sally
Jimbo

Loop through a string (for)

For loops can be a short cut, but, can sometimes be limiting because as they are looping, they don’t have access to the index values of the elements like we see in Loop through a string (while) and Loop through a list (while). For a work-around, see Loop using enumerate.

name = "Mr. Gallo"

for character in name:
    print(character)

M
r
.

G
a
l
l
o

Loop through a list (for)

friend_list = ["Frank", "Sally", "Jimbo"]

# For every friend in my friend list
# print the friend
for friend in friend_list:
    print(friend)

Frank
Sally
Jimbo

Loop using range

# while loop print 50-100
i = 50
while i <= 100:
    print(i)
    i += 1

# range(start, end)... Goes up to, but, doesn't include the end
for num in range(50, 101):
    print(num)


# while loop print 1-25 by 5
i = 0
while i <= 25:
    print(i)
    i += 5

# range(start, end, step)
for num in range(0, 26, 5):
    print(num)

Loop using enumerate

To have access to the index number while looping through a collection, we can use the enumerate function. This allows for the simplicity of a for loop, while still having access to the index value.

name = "Mr. Gallo"

for i, character in enumerate(name):
    print(i, character)

M
r
.
3
G
a
l
l
o

String building and filtering

To build a string with a loop:

Create an empty string (before the loop).
Loop over whatever you need to loop over.
Add to the string inside the loop.

my_string = ""
for n in range(65, 70):
    my_string += str(n) + " "

print(my_string)  # "65 66 67 68 69 "

The same concept applies to building and filtering lists.

nums = [1, 6, -4, 1, -5, 1]

only_ones = []
for n in nums:
    if n == 1:
        only_ones.append(n)

print(only_ones)  # [1, 1, 1]