[R] seq(0.05,0.95,by=0.002) and logical error

[Ray Brownrigg]

| That's a computational problem:
|   
|   jj <- seq(0.05, 0.95, by=0.002)
|   options("digits" = 22)
|   jj[351]  # result:  [1] 0.75000000000000011
| 
| So sum(jj==0.75) gives the *correct* answer. 
| 
Also note that:
> jj <- seq(50, 950, by=2)/1000
> sum(jj==0.75)
[1] 1
>
will (should!) always give the answer you require.

The problem is that .002 is not *exactly* .002, and (more generally),
decimal floating point numbers cannot in general be represented exactly
in binary.

> > sum(jj==0.562)
> [1] 1
> > sum(jj==0.564) ## 0.564 seems to be the cutoff point
> [1] 0

This is not quite the true situation. The 'correct' values are scatterd
throughout the vector jj:

> kk <- seq(50, 950, by=2)/1000	# generate 'correct' values
> ll <- as.numeric(jj==kk)
> ll
  [1] 1 0 0 1 1 0 1 1 1 1 1 0 0 0 1 1 1 1 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0
 [38] 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1
 [75] 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[112] 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1
[149] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1
[186] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1
[223] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0
[260] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1
[297] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
[334] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[371] 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
[408] 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[445] 0 0 0 0 0 0 0
> sum(ll)
[1] 296
> 
So more than 60% of them are 'correct'.

Hope this helps,
Ray Brownrigg
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