Pandas : reverse cumulative sum's process

Question

I've got a simple pandas' Serie, like this one :

    st
0   74
1   91
2   105
3   121
4   136
5   157

Datas for this Serie are the result of a cumulative sum, so I was wondering if a pandas function could "undo" the process, and return a new Serie like :

    st     result
0   74     74
1   91     17
2   105    14
3   121    16
4   136    15
5   157    21

---

result[0] = st[0], but after result[i] = st[i]-st[i-1]. 

It's seemed to be very simple (and maybe I missed a post), but I didn't find anything...

Answer 1

Use Series.diff with replace first missing value by original by Series.fillna and then if necessary cast to integers:

df['res'] = df['st'].diff().fillna(df['st']).astype(int)
print (df)
    st  res
0   74   74
1   91   17
2  105   14
3  121   16
4  136   15
5  157   21