indexing in nested loop

Question

Nicolas 2011-4-13

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Hi,

I have nested loops in a code and I create files that increased for every loop which works like that: data1 gives me the number of lines created in the loop for the para meters 'a' and 'b', data2 gives me for each lines created its orientation/angle.

 for a=1:20
 ...
   for b=1:20
    if
    ...
    data1(a,b)=data1(a,b)+1
    data2(data1(a,b),b)= tan(x/y)
    end
   end
 end

But when I try with non-successive values, it doesn't works.

 for a=1:5:101
 ...
   for b=1:20
    if
    ...
    data1(a(i),b)=data1(a(i),b)+1
    data2(data1(a(i),b),b)= tan(x/y)
    end
   end
 end

I thought I needed to use the index value (i) of 'a' but it doesn't work like that I supposed or I'm making a mistake somewhere.

Looking forward your advices.

Cheers,

n.

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Answer 1

Matt Fig 2011-4-14

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You might consider a loop counter or two:

cnt1 = 0;
cnt2 = 0;
for a=1:5:101
   cnt1 = cnt1 + 1;
   for b=1:20
      cnt2 = cnt2 + 1;
      data1(cnt1,cnt2) = ...
   end
 end

Now you can use the counters where you want and the a and b where you want. The alternative is to set your loop variables to integers (say ii and jj), then index into pre-existing vectors a and b (created before the loop). Such as:

a = 1:5:101
b = b=1:20
for ii = 1:length(a)
   for jj = 1:length(b)
      data1(ii,jj) = ... % Now index into a and b.  a(ii),b(jj)
   end
 end

Note that it doesn't really make much sense to index into your loop variables (a and b) as you had it since they are scalar.

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Nicolas 2011-4-14

I like the loop counter ! thanks

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Answer 2

Robert Cumming 2011-4-13

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in your first example in each loop a is:

1 2 3 4 5 6 7 8 9 10 11 ... 20

In your second it is:

1 6 11 16 21 26 31 .... 101

what were you expecting a(i) to do? If your confused on the value of a in each loop - print it to the screen (or check in the debug)

The answer to this question will depend on what you want to do with your data variables.

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Nicolas 2011-4-13

I would like to have data1 and data2 increasing by 1 in every loop, which is ok for the first example, but not for the second example because a=1 6 11 16 ... so the having a(i) give me the position instead of the value.. a(1)= 1; a(2)=6; a(3)=11.

basically, I want data1(a,b) to increase by one column (21*20), to avoid having a matrix (101:20).

i hope it is a bit clearer

cheers

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