Extract a section of a string based on the value of another column in python

Question

I want to extra the first portion of column (B) based on the value of another column (A) minus 1 to get an output. Eg, if column A is 4 than I want the first 3 characters of column B.

Any suggestions? Greatly appreciated!

+---+---------------+------+
| A |       B       | Out  |
+---+---------------+------+
| 4 |  CTAAT        |  CTA |
| 3 |  GKAEI        |  GK  |
+---+---------------+------+

ie first 3 letters of column B in row 1, the fist 2 letters of column B in row 2.

My current approach that isn't working:

df['output']= df.B.str[0:(df[A]-1)]

pc_pyr · Accepted Answer · 2020-04-11 13:05:01Z

2

You can use pandas.apply:

df['output']=df.apply(lambda x: x['B'][:x['A']-1], axis=1)
print(df)

A      B         output
4      CTAAT     CTA
3      GKAEI     GK

edited Apr 11, 2020 at 13:05

answered Apr 10, 2020 at 13:30

pc_pyr

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5 Comments

Raymond Howanski Over a year ago

This is giving me the error:TypeError: slice indices must be integers or None or have an index method

Raymond Howanski Over a year ago

Okay, I see what you are saying. I did not have the 'output' column created yet, but was relying on the column to be created with the line of code. Thanks! That did work for me.

Raymond Howanski Over a year ago

Nevermind, I am still having issues with getting the error I listed above. It did give me the correct output once, but not it is giving me the error again. Any insights to why I wouldn't be able to reference the column? dtypes lists it as an int.

pc_pyr Over a year ago

You are saying that you got the right output and also getting the error ? Could you explain what's the issue

Raymond Howanski Over a year ago

Yes, sorry for the confusion. I am operating in Jupyter and launch the cell, which gave the error "TypeError: slice indices must be integers or None or have an index method". I went back and added an empty 'output' column to the df prior to the pandas.apply function. Then, when I reran the cell with pandas.apply, it worked. But, the same error appears as before whenever I run the entire script from the beginning.

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