Pandas change multiple columns based on multiple conditions

Question

I have a dataset example shown here:

df = pd.DataFrame({'product' : ['a', 'a', 'c', 'c', 'd', 'b', 'a', 'b', 'c'],
               'unit' : ['ng/L', 'k/uL', 'x10(3)/mcL', 'x10(3)/mcL', 'k/uL', 'ng/L', 'ng/L', 'sss', 'sss'],
           'value' : [0.2, 1.0, 67.0, 71.5, 23.2, 71.0, 0.44, 59.3, 12.7],
           'market_penetration_rate' : [0.82, 0.64, 77.5, 12.5, 22.5, 88.0, 0.34, 98.2, 87.4]})

I want to get all rows where the product = 'a' and the 'unit' = 'ng/l' and convert the value and the unit to be value/1000 and unit = 'ng/ml'

I nearly have it working but I don't know how to get the value and divide by 1000 in the code below

df.loc[(df['product'] == 'a') & (df['unit']== 'ng/L'), ['value', 'unit']] = ['value'/1000 ,'ng/mL'] 

In place of 'value'/1000 what do I put? If I just used a constant in the square brackets then it works, but I want to grab the value it already is and divide.

Answer 1

You are close, but I think you need to separate the update commands for different operations:

mask = df['product'].eq('a') & df['unit'].eq('ng/L') 

# update value
df.loc[mask, 'value'] /= 1000

# update unit
df.loc[mask,'unit']='ng/mL'

Output:

  product        unit     value  market_penetration_rate
0       a       ng/mL   0.00020                     0.82
1       a        k/uL   1.00000                     0.64
2       c  x10(3)/mcL  67.00000                    77.50
3       c  x10(3)/mcL  71.50000                    12.50
4       d        k/uL  23.20000                    22.50
5       b        ng/L  71.00000                    88.00
6       a       ng/mL   0.00044                     0.34
7       b         sss  59.30000                    98.20
8       c         sss  12.70000                    87.40

Answer 2

Can use df.assign and np.where to compute required values

df=df.assign(unit=np.where((df['product'].eq('a'))&(df.unit.eq('ng/L')),'ng/ml', df.unit),value=(np.where((df['product'].eq('a'))&(df.unit.eq('ng/L')),df.value/1000, df.value)))


pr

oduct      unit  value  market_penetration_rate
0       a       ng/ml   0.20                     0.82
1       a        k/uL   1.00                     0.64
2       c  x10(3)/mcL  67.00                    77.50
3       c  x10(3)/mcL  71.50                    12.50
4       d        k/uL  23.20                    22.50
5       b        ng/L  71.00                    88.00
6       a       ng/ml   0.44                     0.34
7       b         sss  59.30                    98.20
8       c         sss  12.70                    87.40