How do I subtract values in a dataframe from the previous row sequentially?

Supposing I have a data frame that looks like:

col1 col2
0    10
1    23
2    21
3    15

I want to subtract each value in col2 with the previous row sequentially, so that we are subtracting the previously subtracted value to get:

col1 col2
0    10 # left unchanged as index == 0
1    13 # 23 - 10
2    8  # 21 - 13
3    7  # 15 - 8

Other solutions that I have found all subtract the previous values as is, and not the new subtracted value. I would like to avoid using for loops as I have a very large dataset.

>Solution :

Try below to understand the ‘previously subtracted’

b2 = a2 - a1
b3 = a3 - b2 = a3 - a2 + a1
b4 = a4 - b3 = a4 - a3 + a2 - a1
b5 = a5 - b4 = a5 - a4 + a3 - a2 + a1

So we just do

s = np.arange(len(df))%2
s = s + s - 1
df['new'] = np.tril(np.multiply.outer(s,s)).dot(df.col2)
Out[47]: array([10, 13,  8,  7])