Error in double for loop with if statement in R

Question

I am looping through a matrix cell by cell and I want to change all of the values less than 1 to their negative reciprocal. Here is the loop I currently have

x <- 1:rowlength
y <- 1:colLength
for (i in x) {
    for (j in y) {
        Cell <- Nmatrix[i,j]
        if (Cell<=1) {Cell = -1/Cell}
    }
}

I keep getting the following error when i try to run the code:

Error in if (Cell <= 1) { : missing value where TRUE/FALSE needed

I am not sure what I am doing wrong. Any help is much appreciated.

Thanks

Missed an answer by a few seconds for the fourth time today. That's it, I'm going to bed. — Roman Luštrik
– Roman Luštrik, Commented Feb 25, 2013 at 22:20

Andrie · Accepted Answer · 2013-02-25 22:25:52Z

5

Since this is R, you don't need a loop for this:

set.seed(42)
Nmatrix <- matrix(runif(20, 0, 2), nrow=4)
Nmatrix
          [,1]      [,2]      [,3]      [,4]      [,5]
[1,] 1.8296121 1.2834910 1.3139846 1.8693445 1.9564529
[2,] 1.8741508 1.0381919 1.4101296 0.5108576 0.2349747
[3,] 0.5722791 1.4731766 0.9154836 0.9245856 0.9499942
[4,] 1.6608953 0.2693332 1.4382245 1.8800290 1.1206655

Now for some magic:

Nmatrix[Nmatrix < 1] <- (-1/Nmatrix)[Nmatrix < 1]
Nmatrix
          [,1]      [,2]      [,3]      [,4]      [,5]
[1,]  1.829612  1.283491  1.313985  1.869344  1.956453
[2,]  1.874151  1.038192  1.410130 -1.957492 -4.255777
[3,] -1.747399  1.473177 -1.092319 -1.081566 -1.052638
[4,]  1.660895 -3.712873  1.438225  1.880029  1.120665

answered Feb 25, 2013 at 22:25

Andrie

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1 Comment

David Robinson Over a year ago

Good. It's worth noting that the reason the OP got the original error is that there is an NA somewhere in the matrix.

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