gussilago - 3 months ago 13

Python Question

Is there a simple way to calculate the average for each column in a pandas DataFrame and for each row *exclude* the specific value? The

`x`

`a b a b a b`

0 1 2 0 x x 0 1 2

1 2 4 first loop 1 2 4 second loop 1 x x etc.

2 3 6 ---> 2 3 6 ---> 2 3 6 --->

3 4 8 3 4 8 3 4 8

4 5 10 4 5 10 4 5 10

____________ _____________

col_avg: 3.5 7.0 col_avg: 3.25 6.5

Using only 4 values at each iteration, as the "x" is excluded from data set

resulting in a new DataFrame

`a_x b_x`

0 3.5 7.0

1 3.25 6.5

2 3.0 6.0

3 2.75 5.5

4 2.5 5.0

Thanks

/N

Answer

To start off with the first step, let's say we were interested in summing instead of calculating the average values. In that case, we would be adding all elems along each col except the current elem. Other way to look at it/solve it would be to sum all elems along each col and subtract the current elem itself. So, essentially we could get the sum for all columns with `df.sum(0)`

and simply subtract `df`

from it, keeping the axis
aligned. `Broadcasting`

would take care of performing these operations across all cols in one go.

To get the second step of averaging, we simply divide by the number of elems involved for each col's summing, i.e. `df.shape[0]-1`

.

Thus, we would have a vectorized solution, like so -

```
df_out = (df.sum(0) - df)/float(df.shape[0]-1)
```

Sample run -

```
In [128]: df
Out[128]:
a b
0 1 2
1 2 4
2 3 6
3 4 8
4 5 10
In [129]: (df.sum(0) - df)/float(df.shape[0]-1)
Out[129]:
a b
0 3.50 7.0
1 3.25 6.5
2 3.00 6.0
3 2.75 5.5
4 2.50 5.0
```

To set the column names to the desired ones, do : `df_out.columns = ['a_x','b_x']`

.