Why do I get subscript indices error?

Hi, I have a code like this,

      r2 = randi([2 64]);
    [m, n] = ind2sub([4 16],r2);
    n = (n-1)*60;
    CH1(m:end,n:end) = P2(m:end,n:end);

CH1 and P2 are two matrices. I generated 'r2' randomly and used 'ind2sub' to find the indices of the element r2. So I expected m and n to be numbers, but I get this error:

??? Subscript indices must either be real positive integers or
logicals.

Please help.

Answer 1

[Walter Roberson]

   r2 = 2:4;
   [m, n] = ind2sub([4 16],r2)
   (n-1)*60

[Sherwin]

Sorry, I don't get it. Would you please explain it a little?

[Walter Roberson]

You have

    CH1(m:end,n:end) = P2(m:end,n:end);

so you are using n as a subscript.

You used

    n = (n-1)*60;

That will come at as 0 if n was 1 before executing that statement.

When can n come out as one in reaching that statement? If

   [m, n] = ind2sub([4 16],r2)

is to have the second output come out as 1 then the value being converted, r2, must have been in the first column of the 4 x 16 matrix. That would be the case of r2 was one of 1, 2, 3, or 4. Is that possible? Let us look to see where r2 came from:

      r2 = randi([2 64]);

Well that cannot be 1, but it can be 2, 3, or 4. Let us double check that by setting the values to 2, 3, or 4 specifically and trying:

   r2 = 2:4;
   [m, n] = ind2sub([4 16],r2)
   (n-1)*60

and indeed the 3 outputs of (n-1)*60 are 0, and 0 is not a valid subscript.

The bug is either in your bounds for randi, or in your manipulation n = (n-1)*60

Perhaps you wanted

n = (n-1) * 60 + 1;

[Sherwin]

Thanks a million!!

[Sherwin]

In the rest of the code I used a similar method,

r3 = randi([1 N],[G 1]);
for i = 1:G
  ch = ceil(r3(i,1)/3840);
  [r, c] = ind2sub([4 960],r3-((ch-1)*3840));
  if Ma{ch,1}(r,c) == 0
      Ma{ch,1}(r,c) = 1;
  else
      Ma{ch,1}(r,c) = 0;
  end
end

This time 'r' and 'c' can't be zero, but I get the same error here! Would you please help me with this too?

[Jan]

@Sherwin: The spam flag has been set by accident, most likely. Or perhaps "thanks a million" is too near to "win a million" ;-) I've removed the flag.

[Sherwin]

Thank you so much :)

[Walter Roberson]

You should use the debugger

dbstop if error

and run. When it stops, tell us what the values of r and c and r3(i) are

[Walter Roberson]

You asked this later question at http://www.mathworks.com/matlabcentral/answers/309643-why-do-i-get-an-index-error and it was answered there.