Python indentation - simple error [duplicate]

Question

So brand new to python, and I've come across something I can't explain, much less put in to words to find a possible answer for. I've made a little coin flipping program:

import random

print("I will flip a coin 1000 times")
input()

flips = 0
heads = 0

while flips < 1000:
    if random.randint(0, 1) == 1:
        heads = heads + 1
        flips = flips + 1

print()
print("Out of 1000 coin tosses, heads came up " + str(heads) + " times!")

This version of the program does not work, it tells me after 1000 flips, there have been 1000 heads every time.

import random

print("I will flip a coin 1000 times")
input()

flips = 0
heads = 0

while flips < 1000:
    if random.randint(0, 1) == 1:
        heads = heads + 1
    flips = flips + 1

print()
print("Out of 1000 coin tosses, heads came up " + str(heads) + " times!")

This version of the program works perfectly however, notice I have changed the indentation of "flips" in the while loop. Can anyone tell me why this is? Thanks in advance!

Answer 1

Python language is indentation dependent. Unlike most C-based languages, it uses indentation to delimit blocks.

So your two scripts have a different semantic:

if random.randint(0, 1) == 1:
    heads = heads + 1
    flips = flips + 1

...will increment both variables if the condition is True.

if random.randint(0, 1) == 1:
    heads = heads + 1
flips = flips + 1

...will increment heads only if the condition is True, and will always increment flips

Answer 2

That's because if that "flips" line is in the if, then it will only execute if it is heads. Therefore, your coin flip count only increments when it's a head, and so by the time flips reached 1000, it means you've executed the if 1000 times and got 1000 heads.

(When you get a tail, flips won't increment and the loop keeps going and nothing happens)