WhitneyChia WhitneyChia - 1 year ago 123
Python Question

list comprehension in pandas

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' that is apples * orange - bananas.

I can do something like this to get it.

df2['optimal_fruit'] = df2['apples'] * df2['oranges'] - df2['bananas']

apples oranges bananas optimal_fruit
1 6 11 -5
2 7 12 2
3 8 13 11
4 9 14 22
5 10 15 35

What is happening if I try to do something like this? And how could I do this in a list comprehension?

df2['optimal_fruit'] = [x * y - z for x in df2['apples'] for y in df2['oranges'] for z in df2['bananas']]

I get an error of:

ValueError: Length of values does not match length of index

As always, thank you all so much for your help!

Answer Source

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'])]
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