Hi Tom,
That's once again the floating point number issue: see FAQ 7.31.
Look at this:
sum(m[161,])
[1] 1
sum(m[161,])==1
[1] FALSE
sum(m[161,])-1
[1] 2.220446e-16
So 0.6+0.3+0.1 is indeed greater than 1
Try this instead:
round(sum(m[161,]))==1
[1] TRUE
HTH,
Ivan
Le 3/7/2011 08:08, thomas.salve...@syngenta.com a écrit :
I am trying to construct a data set with some sequences for example:
a = seq(0,1,0.1)
m = matrix(nrow = 1331, ncol = 3)
m[,1] = rep(a,121)
m[,2] = rep(a,11,each = 11)
m[,3] = rep(a,1,each = 121)
I realize that there may be better ways of doing this, but this approach
demonstrates the problem I'm having.
I then want to get the sum of the rows and delete any row with a sum of greater
than 1. But have a problem with rows containing any combination of the values
0.6, 0.3 and 0.1 as the sum of these is clearly 1, but a request for which rows
have a sum greater than 1 will return rows with these values. Row 161 is the
first row containing these values:
[161,] 0.6 0.3 0.1
which(rowSum(m)>1)
[53] 119 120 121 132 142 143 152 153 154 161 162
As far as I can tell this only affects combinations of 0.6, 0.3 and 0.1 (though
I haven't checked every value in the matrix)
If I try the following:
q=rowSums(m)
which(q>1)
[53] 119 120 121 132 142 143 152 153 154 161 162
But if I add and subtract 1 from this:
q=q+1
q=q-1
which(q>1)
[53] 119 120 121 132 142 143 152 153 154 162
What exactly is going on here? I don't have the problem with other
combinations (eg 0.7, 0.2, 0.1). I assume that there is something about the
data format that I don't understand, but if I make a data frame of the matrix I
found the same effect.
Any help would be great
Tom
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