R: Apply function to matrix with elements of vector as argument

Question

Suppose I want to apply a function to each row of a matrix. One of the function\'s arguments takes a vector. I would like to apply the first element of the vector to the fir

Answer 1

As there are two arguments that should be the corresponding rows and elements in matrix/vector respectively, we can loop through the sequence of rows, subset the data and apply the function

sapply(seq_len(nrow(df)), function(i) MYFUNC(df[i,], Var = var2[i]))
#[1] 5.0795111 2.8693537 1.8285747 1.3640238 0.8300597 0.6280441
#[7] 0.7706310 0.6720132 0.5719003 0.4259674

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For the specific example, it can be vectorized with rowSums

rowSums(df)/var2
#[1] 5.0795111 2.8693537 1.8285747 1.3640238 0.8300597 0.6280441 
#[7] 0.7706310 0.6720132 0.5719003 0.4259674

Answer 2

Mapply is definitely a possibility. This should work:

mapply(MYFUNC, x = as.data.frame(t(df)), Var = var2)

#V1        V2        V3        V4        V5        V6        V7        V8        V9       V10 
#5.0795111 2.8693537 1.8285747 1.3640238 0.8300597 0.6280441 0.7706310 0.6720132 0.5719003 0.4259674 

The issue I think you were running into is that mapply takes either vectors or lists. In R matrices aren't lists, but data.frames are. All you need to do is transpose your matrix and convert to a data.frame and then mapply should work. Each column in a data.frame is an element in the list which is why we have to transpose it (so that each row will be mapped to each element in the vector).