Optimizing apply and lambda function with pandas

Question

I am trying to optimize a function returning the value (wage)of a variable given a condition (largest enrollment within MSA) for every year. I thought combining apply and lambda would be efficient, but my actual dataset is large (shape of 321681×272) making the computation extremely slow. Is there a faster way of going about this ? I think vectorizing the operations instead of iterating through df could be a solution, but I am unsure of the structure it would follow as an alternative to df.apply and lambda

df = pd.DataFrame({'year': [2000, 2000, 2001, 2001],
                    'msa': ['NYC-Newark', 'NYC-Newark', 'NYC-Newark', 'NYC-Newark'],
                  'leaname':['NYC School District', 'Newark School District', 'NYC School District', 'Newark School District'], 
                  'enroll': [100000,50000,110000,60000],
                   'wage': [5,2,7,3] })


def function1(x,y, var):
    '''
    Returns the selected variable's value for school district with largest enrollment in a given year
    '''

    t = df[(df['msa'] == x) & (df['year'] == y)]
    e = pd.DataFrame(t.groupby(['msa',var]).mean()['enroll'])
    return e.loc[e.groupby(level=[0])['enroll'].idxmax()].reset_index()[var]

df['main_city_wage'] = df.apply(lambda x: function1(x['msa'], x['year'], 'wage'), axis = 1)

Sample Output

   year         msa                 leaname  enroll  wage  main_wage

0  2000  NYC-Newark     NYC School District  100000     5          5
1  2000  NYC-Newark  Newark School District   50000     2          5
2  2001  NYC-Newark     NYC School District  110000     7          7
3  2001  NYC-Newark  Newark School District   60000     3          7

Asked By: Arthur Langlois

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Source

Answer 1

Something like

df['main_wage'] = df.set_index('wage').groupby(['year', 'msa'])['enroll'].transform('idxmax').values

Answered By: crx91

Optimizing apply and lambda function with pandas

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