# Re: [R] Sampling from a Matrix

From: Daniel Gerlanc <dgerlanc_at_gmail.com>
Date: Fri 11 Aug 2006 - 01:07:53 EST

Once again, thanks for your help.

I did not state the problem correctly, though the code is correct for what I want to do.

A better description of the problem would be that there is a matrix of probabilities:

> set.seed(1)
> probs <- array(abs(rnorm(25, sd = 0.33)), dim = c(5,5), dimnames = list(1:5, letters[1:5]))
> probs

a b c d e

```1 0.21 0.27 0.50 0.0148 0.303
2 0.06 0.16 0.13 0.0053 0.258
3 0.28 0.24 0.21 0.3115 0.025
4 0.53 0.19 0.73 0.2710 0.656
5 0.11 0.10 0.37 0.1960 0.205

```

The column names, dimnames(probs)[], are the names of units to be sampled. Each row is a trial. For each row (trial), I want to sample 3 of the units such that for each row I get a vector like the following:

 "a", "b", "a"

The samples are to be done with replacement, and these vectors could be combined to form a matrix of the samples.

The purpose of the "probs" matrix is to give each unit a probability that it will be sampled.

One way to do this follows:

index <- 1:ncol(probs)

res <- matrix(0,

```                      nrow = dim(probs),
ncol = 3
```

)

for(i in 1:nrow(probs)){

## gets the indexes of the values chosen

res[, i] <- sample(index, size = 3, replace = TRUE, prob = probs[i, ])

}

Using "apply" as Andy described would accomplish the intended result.

• Dan

> > Hmm... If I read Daniel's code (which is different from his description)
> > correctly, that doesn't seem to be what he wanted. Perhaps something like
> > this:
> >
> > apply(probs, 1, function(p) sample(1:ncol(probs), 3, replace=TRUE, prob=p))
> >
> > Andy

```>
```

> Andy,
```>
```

> You are of course correct. I had focused on the description of the
> problem, rather than the code provided, presuming that the code was not
> correct, including the use of 'replace' and 'prob' in sample().
```>
```

> I suppose it would be up to Daniel for clarification.
```>
```

> Regards,
```>
```

> Marc
```>
>
>

```
```--
Daniel Gerlanc
Williams College '07

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