[R] for loop query

[Robin Hankin]

Hi

start simple!

Work out *each* row combined with *each* row,
to give (in your case) a 26-by-26 matrix.


Only after you have got this working, start thinking about
 making it run faster [eg by
only evaluating the upper triangular entries]

To do a nested loop, do


M <- matrix(0,n,n)

for(i in seq_len(n)){
  for(j in seq_len(n)){
    M[i,j] <-  f(i,j)
    }
}

which will fill in matrix M for you.

HTH

rksh






Gerard M. Keogh wrote:
> Hi all,
>
> apologies if this is obvious - but I can't see it and would appreciate some
> quick help!
>
> the matrix mhouse is 26x3 and I'm computing odds ratios. The simple code
> below "should" compute the odds vector for every pair (325) i.e. 26C2 in
> cols 1 and 2.
> On the first i=1 outer loop the inner j loop runs from 2 to 26 ok and then
> I get the error (Error: subscript out of bounds)
>
> Why isn't my loop incrementing i - the outer loop to 2 and then resetting
> j=3?
> Am I missing something obvious?
>
> thanks
>
> Gerard
>
>
>          > mhouse
>                [,1] [,2]  [,3]
>           [1,]  275  819    49
>           [2,]  593 1323   192
>           [3,]  813 1181   292
>           [4,] 2177 5189  1320
>           [5,] 1651 2243   270
>           [6,] 1061 5629 11035
>           [7,] 1690 2302   589
>           [8,] 1130 1203   345
>           [9,]  565 1898   655
>          [10,]  580  730   234
>          [11,]  343 1761    73
>          [12,]  372  536    67
>          [13,]  666 1713   397
>          [14,]  382  918   279
>          [15,]  486  921   247
>          [16,] 1141  988   313
>          [17,]  626 1135   666
>          [18,]  438  436   168
>          [19,]  425  691   101
>          [20,]  609  716    99
>          [21,]  467  661   141
>          [22,]  879 1373    79
>          [23,]  444 1101   130
>          [24,]  459  898   351
>          [25,]  995 1801   398
>          [26,]  396 1107   201
>          > # set up the odds vector by declaring it to be null
>          > odds=NULL
>          >
>          > # compute the odds ratios for Individual House vs Scheme House
>          > for (i in 1:25)
>          + {
>          +   for (j in i+1:26)
>          +   {
>          +     todds = (mhouse[i,1]*mhouse[j,2])/(mhouse[j,1]*mhouse[i,2])
>          + #                     compute the todds for row i with row j:
>          j>i
>          +     odds = c(odds,todds)
>          + #                     append todds to the odds vector
>          +   }
>          + }
>          Error: subscript out of bounds
>          > odds
>           [1] 0.7491244 0.4877622 0.8003391 0.4561745 1.7814132 0.4573697
>          0.3574670 1.1279674 0.4226138 1.7239078 0.4838053
>          [12] 0.8636384 0.8069156 0.6363150 0.2907502 0.6087939 0.3342421
>          0.5459312 0.3947703 0.4752623 0.5244818 0.8326321
>          [23] 0.6569199 0.6077702 0.9386447
>          >
>
>
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-- 
Robin K. S. Hankin
Uncertainty Analyst
University of Cambridge
19 Silver Street
Cambridge CB3 9EP
01223-764877