Loops

For loop

There are instances in which we do not want to have long messy unreadable code that can result from wanting to repeat an action.

# this is bad!
print("hi")
print("hi")
print("hi")
print("hi")
print("hi")
print("hi")

so instead, we have ways of repeating code over and over again, we can use a loop.

for i in range(6):
	print("hi")

typically we want something to change a little bit in each part of the loop. the i in this for loop is specifying the name of a variable, so we can use it in the loop

for i in range(5):
	print(i) # should print 0, 1, 2, 3, 4 all on seperate lines

just like in conditional statements, loops are indent sensitive, the following should produce an error

for i in range(6):
print("hi") # indentation error!!!

Lets break down each part of the for loop

• for the keyword telling python we want to create a loop that iterates a fixed number of times
• i we are defining a variable with name i to run through this loop
• in just another part of the syntax in a for loop
• range(6) creates an iterator that goes from 0 through 5 (6 is exclusive). We will talk about the difference between an iterator and a list in a future chapter.
  • For the purposes of a loop, all lists are iterables, but not all iterables are lists. You can't call range(6)[0] like you can [1,2,3,4][0] If you wish to treat any iterable as a list, just wrap it in a list function like so:

a = range(6)
print(a)      # => range(0,6)
print(list(a)) # => [0,1,2,3,4,5]

since lists are iterables, we can also do the following:

a = ["sheep", "dog", "cat", "mouse"]
for i in a:
	print(f"{i} is an animal")

note that while convention says that loop variables should be i, j, k, l, there is no reason that they can't be anything else

for cool_var in range(7):
	print(f"This works! {cool_var}")

sometimes, we may need to keep track of the index of some iterable. We can take advantage of len() from before to help us solve these problems:

a = ["sheep", "dog", "cat", "mouse"]
# what if we wanted only the even indices?
for i in range(len(a)):
    if i % 2 == 0: # check if even
        print(a[i])

While Loops

In control flow, we may want to have something run continuously until a certain condition is met. The basic syntax of a while loop is the following:

while condition:
	action

Just like with for loops and if statements, indentation matters for python to correctly parse it

Here is an example while loop

user = ""
while user != "q":
	print("I will keep printing until you enter 'q'")
	user = input()

When possible, you should use for loops as opposed to while loops as Python is more optimized for the former. In cases however in which you need to check a certain condition (above) or are modifying the list/string/iterable that you are attempting to iterate through, you must use a while loop.

Breaking loops

Some times we want to break out of a loop conditionally, we can use the break keyword to exit a loop halfway

for i in range(5):
	print(i)
	if i == 2:
		break # prevents us from printing 3 or 4
# this code should print 0, 1, 2 on seperate lines

rewriting the example from the while loop section to instead use a break

while True: # while true will loop forever!! 
	print("I will keep printing until you enter 'q'")
	if input() == "q":
		break

heres perhaps a more practical example

password = "super secure"
for i in reversed(range(3)):
	if password == input():
		print("Correct!")
		break
	else:
		print("incorrect!")
		print(f"You have {i} tries left for your password")

in this instance, since break is called, the else statement is actually optional, because when python hits break it will immediately cease the loop and not let it clean up

using this knowledge we can rewrite it as the following

password = "super secure"
for i in reversed(range(3)):
	if password == input():
		print("Correct!")
		break
	print("incorrect!") # note no else statement needed
	print(f"You have {i} tries left for your password")

continue

we can also use the continue statement in order to skip execution of anything after the statement if reached

for i in range(5):
	print(f"I will be reached: {i}")
	continue # will stop execution of everything after, but continue the loop
	print("I will never be reached")

for i in range(20):
	print(i)
	if i % 2 == 0: # simple check if the number is even
		print("I'm even")
		continue
	print("I'm an odd number!")

After this, try out Challenge 4

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