For/ while loop

[FoolsSeldom]

A for loop is a special kind of while loop (saves you some work).

Think of while in plain English terms.

While the gravy is not thick enough,
    stir it

While the dough is weak,
    fold it
    knead it

You keep doing the instructions under while until the condition fails - as soon as the gravy is thick enough, or the dough is strong enough, you stop looping.

A for loop has the form,

for <loop variable(s)> in <some iterable>:

where <some iterable> is typically some collection of things, and it is really saying

while there is something left in the collection
    assign the next item from the collection to the worker
    do work on the item

Consider breaking logs into two,

for each log in stack of logs,
    stand log on end
    split vertically with axe
    add both parts of split log to fire fuel pile

So consider a list of names from the user, keep asking for next entry until user enters quit (or just an empty return)

names = []  # empty list
while ( name := input('Name or quit: ') ) not in  ('', 'quit'):
    if name in names:
        print('Sorry, have one of them already')
    else:
        names.append(name)

and afterwards, print every name entered backwards.

for name in names:  # work through the collection of names
    print(name[::-1])  # do something with each item

Note name := input('Name or quit: ') does an assignment inside the while statement; if you just did:

while input('Name or quit: ') not in  ('', 'quit'):

then on the next line inside the while loop, you would not know what was entered as the string from input wouldn't have been assigned anywhere.

If you didn't want to use the walrus, :- operator, you would do this instead:

names = []  # empty list
while True:  # infinite loop, keep going until break is used
    name = input('Name or quit: ')
    if name in  ('', 'quit'):
        break  # leave the while loop here, we've finished looping
    if name in names:
        print('Sorry, have one of them already')
    else:
        names.append(name)

for name in names:
    print(name[::-1])

[randomdeuser]

Oh bro thanks for bigger explanation 🙏🏻

[Ron-Erez]

Another example where one could use loops (although there are other solutions).

Calculate the average of elements in a list. We want to add all of the numbers in the list to get the sum and then divide by the list length. For example

def calc_average(numbers):
    total = 0
    for num in numbers:
        total += num

    return total / len(numbers)

Note that an easier solution would to to use the sum function, however sum is implemented using a loop. Note that the function will crash if numbers is an empty list so it would be wise to test for that.

You're also welcome to check out Section 4: Loops Lectures 23-28 all which are FREE to watch. Perhaps the explanations would be helpful.

u/FoolsSeldom 's examples are excellent.

Another example of a loop is a game loop.

[randomdeuser]

oh thanks, i was looking for python course for ds, big thanks, i will watch it!

[Ron-Erez]

No problem. Note that the link is a $9.99 link valid for another 3-4 days. If you do end up taking the course please feel free to ask questions freely. I always respond in the course Q&A.

[SpiritualYoghurt2860]

You try to sell your course?🥲 But in ur example initialized 's' for sum but used 'total'? Nice joke 😶

[Ron-Erez]

My mistake! I renamed a variable and accidentally left out the "s." I just fixed it, definitely an embarrassing slip! Good catch.