inplace Modifying a vector in R

Question

I have a vector 'participant' in R.

> participant [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

I am using a function 'modify' to change the contents of this vector.

modify <- function(x){ for (i in participant){ if (x[i] > 12) (x[i]=x[i]-12) print (x[i]) }}

When I run the function as modify(participant), it runs OK, but the elements of the vector participant remain unchanged.

Any suggestion, for where am I going wrong ?

I would go with participant[participant > 12] <- participant[participant > 12] - 12 — David Arenburg
– David Arenburg, Commented Nov 15, 2016 at 8:31

Roland · Accepted Answer · 2016-11-15 08:26:13Z

Don't use a loop.

participant <- participant - (participant > 12) * 12

If you insist on using your function, loop over the indices, let you function return the modified vector and assign it:

modify <- function(x){ for (i in seq_along(participant)){ if (x[i] > 12) x[i]=x[i]-12 } return(x) } participant <- modify(participant)

Of course, the loop is more difficult to write and also much slower.

J_F · Accepted Answer · 2016-11-15 08:26:26Z

Your problem is the function return. Use this solution, so the function returns the modified vector x:

modify <- function(x){ for (i in participant){ if (x[i] > 12) (x[i] = x[i] - 12) print (x[i])} return(x) } participant <- modify(participant)

Another solution is the ifelse function:

participant <- ifelse(participant > 12, participant - 12, participant)

2 Answers 2