Use string from one column of dataframe to reference value in another column

trying to use values from a column in the dataframe (placeholder) to reference a specific column in the same dataframe... was wondering if this is even possible. Example input and output below:

Input:

ID	1	2	3	placeholder
9234	923	12	942	2
203841	1230	438	1029	1
94532	4380	312	349	3

Output:

ID	1	2	3	placeholder	final
9234	923	12	942	2	12_ID
203841	1230	438	1029	1	1230_ID
94532	4380	312	349	3	349_ID

Any help would be much appreciated!

Solution

Try:

df["final"] = df.apply(lambda x: str(x[str(x["placeholder"])]) + "_ID", axis=1)
print(df)

Prints:

       ID     1    2     3  placeholder    final
0    9234   923   12   942            2    12_ID
1  203841  1230  438  1029            1  1230_ID
2   94532  4380  312   349            3   349_ID

If the columns are of type integer, remove the inner str():

df["final"] = df.apply(lambda x: str(x[x["placeholder"]]) + "_ID", axis=1)