[R] Question about assigning values in a matrix, conditional on column first row; how to do the loop.

Hello, I’m new at programming and I will greatly appreciate if you can help me with this. I have a very large matrix (hundreds of rows and columns), with the first raw filled with different numbers (between 0 and 1). The rest of the matrix is filled with values 0, 1, 2. What I need is to

Re: [R] Question about assigning values in a matrix, conditional on column first row; how to do the loop.

try this: matr [,1] [,2] [1,] 0.1 0.5 [2,] 1.0 1.0 [3,] 0.0 2.0 [4,] 2.0 2.0 result - apply(matr[-1,], 1, function(.row){ + ifelse(.row == 2, matr[1,], -999) + }) # data back together rbind(matr[1,], t(result)) [,1] [,2] [1,]0.10.5 [2,] -999.0 -999.0 [3,]

Re: [R] Question about assigning values in a matrix, conditional on column first row; how to do the loop.

Paula- Look at this example. R in this case is your data matrix. -tgs R-matrix(sample(0:2,100,replace=T),nrow=10) R[1,]-round(runif(10),3) f-function(x){ show(x) y-x[-1] y[y!=2]--999 y[y==2]-x[1] c(x[1],y) } apply(R,2,f) On Tue, Oct 5, 2010 at 11:54 AM, Paula Fergnani Salvia

Re: [R] Question about assigning values in a matrix, conditional on column first row; how to do the loop.

Paula, Just for fun, another way: ## Create 10 by 10 example matrix set.seed(100) a - matrix(sample(c(0,1,2),100, replace=T), nrow=10, ncol=10) a[1,] - sample(c(0.25,0.5,0.75),ncol(a), replace=T) ## Use apply to substitiute matrix values by row b - apply(a, 1, function(x) { row - x