How to exit from a double loop in a pythonic way?

Question

yellow = ['255', '255', '0']

for row in range(50):
for col in range(50):
pixel = data[row][col]
if pixel == yellow:
upper_left = (row, col)
break

When pixel is yellow, how should be this loop terminated? Now it continues with the outer loop again which is not desired.

Answer 1

for row in range(50):
for col in range(50):
pixel = data[row][col]
if pixel == yellow:
upper_left = (row, col)
break
else:
continue
break

if second loop breaks then first loop breaks too

Answer 2

The use of itertools.product or a function with return to terminate both loops that you already mentioned seem nice and concise, if that is what Pythonic means.

Answer 3

On another note, would it be better (or more Pythonic) to use a tuple rather than a list to represent each colour, as the number of colour components is always constant(3) and doesn't grow?

Answer 4

I would have also used a control flag(boolean variable) to terminate the outer loop but I guess it's not that Pythonic.

Answer 5

guillem ardanuy

flag = False
for i in range(8):
for j in range(8):
if whatever:
flag = True
break
if flag:
break

This is what I called C-ish.

If I knew a shorter, *easily readable* technique, I would choose it.

Maybe it's also a bit a matter of getting used to the pattern

else
break

Answer 6

Gabriel Ilie Another solution, seems to me more pythonic:

def find_yellow():
for raw in range(50):
for col in range(50):
if data[raw][col] == yellow:
return (raw, col)

upper_left = find_yellow()

Answer 7

guillem ardanuy Many people use a boolean flag as control variable but the code is longer, eg:

found = False

while not found:
for row in range(50):
for col in range(50):
pixel = data[row][col]
if pixel == yellow:
upper_left = (row, col)
found = True
break
if found:
break

Answer 8

guillem ardanuy Mirielle👽 For me, this 4-liner seems to be the most pythonic:

from itertools import product

for row, col in product(range(50), range(50)):
if data[row][col] == ['255', '255', '0']:
break

Answer 9

Mirielle👽 Thanks for your comment and sorry for my unclear or uncomplete question.

The code actually works because the list is a matrix of strings, eg. [[[‘0’, ‘0’, ‘0’], [‘0’, ‘0’, ‘0’]...]]. In this way, list items can be matched the variable yellow, eg:

data[25][25] # it is ['255', '255', '0']

yellow = ['255', '255', '0']

so data[25][25] == yellow is True

Answer 10

Sonic “Pythonic” is somewhat subjective but there are guidelines, ie. The Zen of Python or PEP8.

“Exploiting the features of the Python language to produce code that is clear, concise and maintainable.

Pythonic means code that doesn't just get the syntax right but that follows the conventions of the Python community and uses the language in the way it is intended to be used.”

https://stackoverflow.com/questions/25011078/what-does-pythonic-mean

Answer 11

HonFu
You are very advanced. Which solution would you prefer for exiting double loops?

Answer 12

guillem ardanuy Just a quick tipp: you can try to use the @ sign before usernames.

Answer 13

Mirielle👽 Thanks, Mirielle. How to break the first loop (row) when yellow is found?

Answer 14

Mirielle👽 Actually, this list is a matrix of strings, eg. [[[‘0’, ‘0’, ‘0’], [‘0’, ‘0’, ‘0’]...]] which represents an 50x50 image’s RGB pixels

Answer 15

Gabriel Ilie Yes, this works!

Answer 16

🤣 double loop is double loop🧐🧐
You might hide it through some syntactic sugar...
Double loop stays double loop

Answer 17

from itertools import product

for row, col in product(range(50), range(50)):
if data[row][col] == ['255', '255', '0']:
upper_left = (row, col)
break

Answer 18

Its a good idea to put all the code inside a while statement, and when yellow,
The control variable equal false?

...
Control=true

While(control)
...
If yellow
Control=false

....

Good or bad idea?

Answer 19

Sonic Yes, that’s right! Tuples are even faster for iteration

Answer 20

Gabriel Ilie, funny hack, never saw that before. :)