Apply a multiplication to random elements on my dataframe

Question

So what I want is to randomly chose a given amount of elements on my dataframe, and to those elements, apply an operation (which will be a multiplication by a number which will also be randomly chosen between a range) to an 'eta' column. I'm stuck on achieving this.

atm I've randomly gotten a list of index within my dataframe but what I dont know is how to only apply the multiplication to the elements with those index and not the rest.

Some help would be trully appreciated!

numExtreme = 50 #Amount of registres to apply the random modification.
mod_indices = np.random.choice(df.index, numExtreme, replace=False) #set of "numExtreme" indices randomly chosen.

Answer 1

Use DataFrame.loc for select random indices and only column eta and multiple by random array between 1 and 50:

np.random.seed(123)
df = pd.DataFrame({'eta':np.random.randint(10, size=10),
                   'another':np.random.randint(10, size=10)})
print (df)
   eta  another
0    2        9
1    2        0
2    6        0
3    1        9
4    3        3
5    9        4
6    6        0
7    1        0
8    0        4
9    1        1

numExtreme = 5 #Amount of registres to apply the random modification.
mod_indices = np.random.choice(df.index, numExtreme, replace=False)

print (mod_indices)
[5 1 0 8 6]

arr = np.random.randint(1, 50, len(mod_indices))
print (arr)
[49  8 42 36 29]

---

df.loc[mod_indices, 'eta'] *= arr
print (df)
   eta  another
0   84        9
1   16        0
2    6        0
3    1        9
4    3        3
5  441        4
6  174        0
7    1        0
8    0        4
9    1        1