replacing randomly values in an existing matrix in R

Question

I have an existing matrix and I want to replace some of the existing values by NA's in a random uniform way. I tried to use the following, but it only replaced 392 values with `NA`, not 452 as I expected. What am I doing wrong? ``` N <- 452 ind1 <- (runif(N,2,length(macro_complet$Sod))) macro_complet$Sod[ind1] <- NA summary(macro_complet$Sod) Min. 1st Qu. Median Mean 3rd Qu. Max. NA's 0.3222 0.9138 1.0790 1.1360 1.3010 2.8610 392.0000 ``` My data looks like this ``` > str(macro_complet) 'data.frame': 1504 obs. of 26 variables: $ Sod : num 8.6 13.1 12 13.8 12.9 10 7 14.8 11.3 4.9 ... $ Azo : num 2 1.7 2.2 1.9 1.89 1.61 1.72 2.1 1.63 2 ... $ Cal : num 26 28.1 24 28.5 24.5 24 17.4 26.6 24.8 10.5 ... $ Bic : num 72 82 81 84 77 68 66 81 70 37.8 ... $ DBO : num 3 2.2 3 2.7 3.3 3 3.2 2.9 2.8 2 ... $ AzoK : num 0.7 0.7 0.9 0.8 0.7 0.7 0.7 0.9 0.7 0.7 ... $ Orho : num 0.3 0.2 0.31 0.19 0.19 0.2 0.16 0.24 0.2 0.01 ... $ Ammo : num 0.12 0.16 0.15 0.13 0.19 0.22 0.19 0.16 0.17 0.08 ... $ Carb : n…

Accepted Answer

you must apply `runif` in the right spot, which is the index to vec. (The way you have it now, you are asking `R` to draw random numbers from a uniform distribution between `NA` and `NA`, which of course does not make sense and so it gives you back `NaN`s)

Try instead:

```
        N  <-  5                                   # the number of random values to replace
      inds <- round ( runif(N, 1, length(vec)) )   # draw random values from [1, length(vec)]
 vec[inds] <- NA                                   # use the random values as indicies to vec, for which to replace
```

Note that it is not necessary to use `round(.)` since `[[` will accept numerics, but they will all be rounded down by default, which is just slightly less than a uniform dist.

replacing randomly values in an existing matrix in R

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