Multiplying column value by another value depending on value in certain column R

Question

in the following dataset, I would like to multiply value in column Size by value in column Month1, Month2 or Month3 depending on what number we have in column Month. So if in certain row the Month value is 2, I would like to multiply the value in column Size by the value in column Month2 and save the result in new column NewSize. Many thanks for your help in advance!

Orig = c("A","B","A","A","B","A","A","B","A")
Dest = c("B","A","C","B","A","C","B","A","C")
Month = c(1,1,1,2,2,2,3,3,3)
Size = c(30,20,10,10,20,20,30,50,20)
Month1 = c(1,0.2,0,1,0.2,0,1,0.2,0)
Month2 = c(0.6,1,0,0.6,1,0,0.6,1,0)
Month3 = c(0,1,0.6,0,1,0.6,0,1,0.6)
df <- data.frame(Orig,Dest,Month,Size,Month1,Month2,Month3)
df

  Orig Dest Month Size Month1 Month2 Month3
1    A    B     1   30    1.0    0.6    0.0
2    B    A     1   20    0.2    1.0    1.0
3    A    C     1   10    0.0    0.0    0.6
4    A    B     2   10    1.0    0.6    0.0
5    B    A     2   20    0.2    1.0    1.0
6    A    C     2   20    0.0    0.0    0.6
7    A    B     3   30    1.0    0.6    0.0
8    B    A     3   50    0.2    1.0    1.0
9    A    C     3   20    0.0    0.0    0.6

Answer 1

Here's one alternative using ifelse

> transform(df, NewSize=ifelse(Month==1, Size*Month1, 
                  ifelse(Month==2, Size*Month2, Size*Month3)))
  Orig Dest Month Size Month1 Month2 Month3 NewSize
1    A    B     1   30    1.0    0.6    0.0      30
2    B    A     1   20    0.2    1.0    1.0       4
3    A    C     1   10    0.0    0.0    0.6       0
4    A    B     2   10    1.0    0.6    0.0       6
5    B    A     2   20    0.2    1.0    1.0      20
6    A    C     2   20    0.0    0.0    0.6       0
7    A    B     3   30    1.0    0.6    0.0       0
8    B    A     3   50    0.2    1.0    1.0      50
9    A    C     3   20    0.0    0.0    0.6      12