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!
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.append(x * y - z)
The length of your resultant list will be power-of-3 times the length of your DataFrame (5x5x5 = 125). 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'])]