How to keep n characters of each row of a pd df, where n differs by row?

I have created a df one column of which contains string values that I want to trim based on a different int value each time. Ex.: From:

length	String
-3	adcdef
-5	ghijkl

I wanna get:

length	String
-3	def
-5	hijkl

What I tried is the following:

for i in range(len(df.index)):
    val = df['string'].iloc[i]
    n = df['length'].iloc[i]
    df['string'].iloc[i] = val[n:]

However, I keep getting this warning:

SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame

Any ideas on how I can avoid getting it?

Thanks!

Answer

Try with apply:

df["String"] = df.apply(lambda x: x["String"][x["lenght"]:], axis=1)

>>> df
   lenght String
0      -3    def
1      -5  hijkl