list comprehension in pandas

问题

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!

回答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'])]

回答2:

The reason why your new method doesn't work is because the list comprehension produces data that is longer than the number of indices in your dataframe. A quick fix for that would be something like:

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

回答3:

If you do not want to repeat df2 for each column:

[row[0][0]*row[0][1]-row[0][2] for row in zip(df2[['apples', 'oranges', 'bananas']].to_numpy())]

def func(row):
    print(row[0]*row[1]-row[2])

[func(*row) for row in zip(df2[['apples', 'oranges', 'bananas']].to_numpy())]