invalid operands to binary * (have 'double ' and 'double ')

Question

I have a program that multiplies matrices sequentially in C that I'm trying to finish up. I'm getting the error listed in the title.

Here is the line that is giving me the trouble:

C[i,j] = C[i,j] + A[i,k] * B[k,j];

A, B, and C are 2-dimensional arrays. They are defined with this code:

A = (double **) malloc(sizeof(double *)*n);
for (r = 0; r < n; r++) {
  A[r] = (double *) malloc(sizeof(double)*n);
}

The definition of B and C are the same as this. n is an integer value which defines the size of the columns and rows.

I don't know why I'm getting this error. From some of the other questions I've looked at, this error comes up when the types for an operation are incorrect, but I don't understand why that's the case here. Does anyone have any ideas?

[Don't cast the result of malloc (and friends)](http://stackoverflow.com/q/605845). — Deduplicator, Nov 05 '14 at 18:47
@Deduplicator This didn't come when I was doing my search. Thank you for pointing it out to me. — redeagle47, Nov 05 '14 at 18:49
Yes. The comma would be interpreted as the comma-operator, so the array would only by singly indexed, yielding a pointer expression. — wildplasser, Nov 05 '14 at 18:51

score 3 · Accepted Answer · edited May 23 '17 at 12:13

3

There is no multi-index indexing-operator in C.

What you have is actually single-indexing with an expression which contains the comma-operator.

The comma-operator always returns its second argument.

So, use normal indexing twice instead of trying to cram a second index in there somehow.

C[i,j] = C[i,j] + A[i,k] * B[k,j];

Is equivalent to:

C[j] = C[j] + A[k] * B[j];

Not to what you seem to want:

C[i][j] = C[i][j] + A[i][k] * B[k][j];

As an aside, Don't cast the result of malloc (and friends):

A = (double **) malloc(sizeof(double *)*n);

should be the less error-prone:

A = malloc(n * sizeof *A);

edited May 23 '17 at 12:13

Community

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answered Nov 05 '14 at 18:49

Deduplicator

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I think I must have copied that over from the algorithm I was looking at. Thank you for your help. – redeagle47 Nov 05 '14 at 18:53
Yes, those pseudo-code algorithm normally use more mathematical notation. Also, you are a step up from most if you really read such things. – Deduplicator Nov 05 '14 at 18:56

score 2 · Answer 2 · answered Nov 05 '14 at 18:49

2

C[i,j] is equivalent to C[j]. i,j in this context is treated as a comma operator whose value is the last expression.

Instead of

C[i,j] = C[i,j] + A[i,k] * B[k,j];

use

C[i][j] += A[i][k] * B[k][j];

answered Nov 05 '14 at 18:49

R Sahu

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invalid operands to binary * (have 'double *' and 'double *')

2 Answers2

invalid operands to binary * (have 'double ' and 'double ')