Calculating mean of column based on the occurence of a number in another column Pandas dataframe Python

I’ve got a Pandas dataframe like the one below. What I’m trying to do is calculating the mean of column s2, for every time that ‘5’ occures in s1.

s1  s2
5   0.5
1   0.43
5   1
5   1

In this case, 5 occures three times, so we take the average over 0.5+1+1=0.83.
Can someone help me to calculate this? Thanks!

>Solution :

Try this

df[df['s1']==5]['s2'].mean()