A random number between 1 and 4 that's not another random number

Question

For some reason the following code doesn\'t work.

var a1 = Math.floor(Math.random()*4+1); 

//Answer2
for(a2 = 0; a2 != a1 && a2 != 0; a2 = Math.floo         


        

Answer 1

There are two standard ways of doing this kind of thing. I'm considering a slightly more general framework though

Sample from a filtered list

1. Create the array of base values to select from

   var values = [1,2,3,4]
   

2. Remove the values you're not interested in

   values = Array.filter( values, function(x) { return x!=a1; } )
   

3. Select a value at random from the list

   var a2 = values[Math.floor(Math.random()*values.length)];
   

Use rejection sampling

1. Create the array of values to select

    var values = [1,2,3,4]
   

2. Select a value at random and retry until it succeeds

    var a2 =  values[Math.floor(Math.random()*values.length)];
    while( a2==a1 )
    {
      a2 = values[Math.floor(Math.random()*values.length)];
    }
   

Comparison

If we consider n to be the length of the initial list and m to be the number of elements removed, then the filtered version does n comparisons to build the filtered list of length n-m, but only one random number generation. In comparison the rejection method does on average n/(n-m) comparisons and random number generations.