How to get specific column key from data frame given a particular element?

Question

Consider the data frame

   c1   c2   c3  
0 a 3    4    2  
1 b 1    2    7  

say I want to know which column in row 1 has the element 7. how will I achieve it? I am trying to achieve it using pandas.

Answer 1

If want get column of first value in row with index 1 :

a = df.loc[1].eq(7).idxmax()
print (a)
c3

Explanation:

First select column with index by loc :

print (df.loc[1])
c1    4
c2    2
c3    7
Name: (1, b), dtype: int64

Compare with 7 :

print (df.loc[1].eq(7))
c1    False
c2    False
c3     True
Name: (1, b), dtype: bool

and for c3 get index of max value, it means first True .

If row contains multiple values (here 7) and need all matched columns use boolean indexing :

a = df.loc[1].eq(7)
a = a.index[a].tolist()

Answer 2

Not sure exactly what output you're looking for, but I believe this will help. I started by creating your dataframe.

df = pd.DataFrame({'c1':['3', '1'],
                   'c2':['4', '2'],
                   'c3':['2', '7']})

The following code reads like so: give me all the records where the column 'c3' equals 7.

df = df[df['c3'] == '7']

Output: