Subtracting a constant value from one column when condition on another column holds

I have a Pandas data frame that has the following columns: foo and bar. foo values are integers and bar values are strings. For each row, if the value of bar is some particular value, say, ‘ABC’, then I want to set the value of the foo column (for that row) to its current value minus one.

For example, I want to convert this data frame:

foo bar
98 ‘ABC’
53 ‘DEF’
22 ‘ABC’
34 ‘FGH’

converted to this:

foo bar
97 ‘ABC’
53 ‘DEF’
21 ‘ABC’
34 ‘FGH’

How is this done?

>Solution :

You can do this via the .loc on dataframe:

df.loc[df['bar'] == 'ABC', 'foo'] -= CONSTANT_VAL

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