Using For and While Loops in Python

Here are some tools to use to do loops in the Python language.

The While Loop

The while statement will continually cycle through the block of code until the condition is no longer true.

A forever loop:

while True:
	print('This will never end!')

To use a while loop to cycle through something 5 times, you could use this code.

repeat = 1
while repeat <= 5:
    print('Loop number: ' + str(repeat))
    repeat += 1

Using Continue and Break

The continue statement is used when you want to go back to the beginning of the while loop.

The break statement is used when you want to leave the while loop all together.

while True:
	if(we_want_to_start_the_loop_over == True):
		continue
	if(we_want_to_end_the_loop == True):
		break

The For Loop

Another way to do a loop is using a for statement.

Using Range

Here is an example:

sum = 0 
for num in range(10):
	sum = sum+num

print(sum)

This will loop through 10 times because of the range(10) method and print out the value 45.

The range() method allows 3 arguments to be passed to it. Here are 3 ways to use it:

for i in range(10):       # will go from 0 to 10 counting by ones

for i in range(5, 10):    # will go from 5 to 10 counting by ones

for i in range(5, 1, -1): # will go from 5 to 1 counting backwards 1 at a time

For Each Item in a List

If you want to do something with the values of a list, you can cycle through it like this.

grades = ['A', 'C', 'B', 'F']
for grade in grades:
	print(grade)

Using Enumerate

If you want to iterate through a list and make edits to the list you can do this:

grades = ['A', 'C', 'B', 'F']
for n,i in enumerate(grades):
	if i=='F':
		grades[n]='A'

This changes all your F grades to As!

Looping Through all Keys

If using a dictionary, you can loop through all keys this way:

inventory = {'apples': 7, 'peaches': 4}
for fruit in inventory: 
    print("We have " + str(inventory[fruit]) + ' ' + fruit)