6.Python Intro - Loops

By: Carlo Sarli

Posted: February 7, 2021 in (Python) (Intro)

Tech Used:

Python3.x

Reading Time: 2 minutes

What are Loops?

Loops are blocks of code that execute repeatedly (in a loop) until a condition is met. Usually this condition can be a logical statement or a range. They are a great tool to write shorter code although they should be used carefully.

Loops are often the cause of infinitely running code or particularly slow code, so a bit of care should be taken when writing them.

You should always make sure, unless you intend to run it ad infinitum, that the loop has an exit condition.

Break and Continue

A break statement will immediately exit the current loop.

A continue statement will skip the rest of the loop and go back to the start.

Else

An else statement at the end of a loop indicates a block of code that will always run after the loop has finished executing.

While Loop

While loops will execute as long as a Boolean condition is not met, or a break statement is reached.

# ----- LOOPS -----
# While : Execute while condition is True
END_LOOP: int = 5
w1: int = 1
while w1 < END_LOOP:
    print(w1)
    w1 += 1

END_LOOP_TWO = 20

w2: int = 0
while w2 <= END_LOOP_TWO:
    if w2 % 2 == 0:
        print(w2)
    elif w2 == 9:
        # Forces the loop to end all together
        break
    else:
        # Shorthand for i = i + 1
        w2 += 1
        # Skips to the next iteration of the loop
        continue
    w2 += 1
 
# Cycle through list
l4 = [1, 9.2, "Ule", True]
while len(l4):
    print(l4.pop(0))

END_LOOP_THREE: int = 6
i: int = 1
while i < END_LOOP_THREE:
  print(i)
  i += 1
else:
  print("i is no longer less than 6")

For Loop

For loops are used to iterate over sequences. They are really useful when utilised in conjunction with collections.

# For Loop
# Allows you to perform an action a set number of times
# Range performs the action 10 times 0 - 9
# end="" eliminates newline

for x in range(0, 10):
    print(x, ' ', end="")
print('\n')
 
# Cycle through list
l4 = [1, 9.2, "Ule", True]
for x in l4:
    print(x)
 
#You can also define a list of numbers to
# cycle through
for x in [2, 4, 6]:
    print(x)

# You can double up for loops to cycle through lists
numList: list[list[int]] = [[1, 2, 3], [10, 20, 30], [100, 200, 300]]

for l in numList:
    for el in l:
        print(el)

Iterators

Iterators are objects that contain countable values and will return a value at the time.

# ----- ITERATORS -----
# You can pass an object to iter() which returns
# an iterator which allows you to cycle thru a list
l5: list[int] = [6, 9, 12]
itr = iter(l5)
print(next(itr))  # Grab next value
 
# ----- RANGES -----
# The range() function creates integer iterables
print(list(range(0, 5)))
 
# You can define step
print(list(range(0, 10, 2)))
 
for x in range(0, 3):
    for y in range(0, 3):
        print(numList[x][y])

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