list comprehension in pandas

Question

I\'m giving a toy example but it will help me understand what\'s going on for something else I\'m trying to do. Let\'s say I want a new column in a dataframe \'optimal_fruit

Answer 1

Essentially your list comprehension statement is a set of 3 nested loops. In code:

l = []
for x in df2['apples']:
    for y in df2['oranges']:
        for z in df2['bananas']:
            l.extend([x * y - z])

The length of your resultant list will be 3 times the length of your DataFrame. Hence the error. To fix, you need the equivalent of:

for x, y, z in zip(df2['apples'], df2['oranges'], df2['bananas']):
    l.extend([x * y - z])

In terms of list comprehension:

[x * y - z for x, y, z in zip(df2['apples'], df2['oranges'], df2['bananas'])]