Looping

Code sample link: https://replit.com/@jjoco/python-looping

Looping in Python is used to execute a given set of code repeatedly. There are two ways to loop in Python: for loops and while loops.

For loop

The for keyword is used to iterate over an sequence or some iterable type.

The general syntax for simple for loops is the following:

for elem in <iterable>:
    #Do stuff to elem

When the for loop iterates through the iterable, the value at a given point in the iterable is written into elem (which you can name to whatever you want, by the way). On each iteration, you'll be able to process each element in the iterable.

Let's go through several examples.

Iterate through string

We can process each character in a given string:

string_var = "asdfjk;"
for char in string_var:
    print(char)
'''
Prints out:
a
s
d
f
j
k
;
'''

Iterate through list

Think of lists as an ordered sequence of values, like 1, 5, 6, 7 in that order. We'll go over lists in-depth in the Lists section under the Collections header.

We can iterate through each value in the list like the following:

list_var = [4, 5, 6, 7]
for element in list_var:
    #Process element

Iterate through a range of ints

In other languages, to iterate through lists or arrays, the most common way is to use array indices to access array values. The key is to use Python's in-built range() function, which returns a sequence of integers for 0 to n-1 if you use the range(n) signature:

for i in range(n):
    #Process i

But in case you want to set the sequence's start value, you can use the signature range(start, stop), in which a sequence of numbers from start to stop-1 is returned. If you want to include the step, as well, use range(start, stop, step), which returns a sequence of numbers from start to stop-1 in steps of step:

for i in range(-1, 5):
    print(i)
'''
Prints out:
-1
0
1
2
3
4
'''

for i in range(0, 10, 2):
    print(i)
'''
Prints out:
0
2
4
6
8
'''

So, to iterate through all elements in a list via indices, you'll have to input the length of the list as the argument in the range(n) function:

for i in range(len(list_var)):
  #  Do stuff to listVar[i]
  print(list_var[i])

While loop

One can use while loops to execute code repeatedly. The syntax is follows:

while condition:
   # Do stuff

So, let's say we have a counter that continues incrementing from 0 until 5. We can write:

counter = 0

while counter < 5:
    print(counter)
    counter+=1
'''
Prints out:
0
1
2
3
4
'''

The while continues to execute the increment statement until the counter becomes 5, at which the program exits the while loop.

For those wondering, you can write the same for loop as a while or vice-versa. Generally, I use for-loops when iterating through something or the number of iterations can be known ahead of time; on the other hand, I use while-loops when my looping is dependent on some condition or the number of iterations is variable. For example, you shouldn't use a for-loop to write an infinite loop.

Infinite loop

The easiest way to write an infinite loop is via a while loop:

while True:
   # Do stuff forever or until break

You would want to use an infinite loop if your program is intended to run forever, like a server. However, you can write your infinite loop to end prematurely if some condition is met during the loop execution. In this case, you would use the break statement to exit the loop. You can use break in either a for-loop or a while-loop to exit the current loop block.

while True:
   # Do stuff forever or until break
   x = int(input("Enter a number: "))

   if x == -1:
       break

In the above example, the while-loop executes forever, asking the user to input a number. Assuming the user always inputs a number, the while-loop ends if the user inputs a -1.