Question

Okay, so I had working code written in all objective c (yes, I know that objc is technically just C. But i mean I had it written with messages and stuff. I only have a java background and do not know much about plain old C) but it ran incredibly slow. So I wrote out (what i thought) was the same code, but now this set of loops produces different values (for only some of the numbers) and I cannot, for the life of me, figure out whats different. What I am doing is looping 10 times and doing 1 multiplication and 1 addition between matricies. I’m hoping someone with more background with the two languages can pick out the part of code that I transcribed incorrectly. I did not change anything beforehand for any of the arrays (those were hardcoded in and uneffected) so A1, A2, etc have the same values in both parts of code.

Current code in C:

    for (int m = 0; m < 10; m++) {

    //Do matrix multiplication between A1 and A2.  Store in temporary B1
    for( int i = 0; i < 13; i++ )
        for( int j = 0; j < 43; j++ ) {
            double tempTotal = 0;
            for( int k = 0; k < 43; k++){
                tempTotal = tempTotal + A1[i][k] * A2[k][j];
            }
            B1[i][j] = tempTotal;
        }

    //Assign B1 data back into A1 after the multiplication is finished
    for(int i = 0; i < 13; i++)
        for(int j = 0; j<43; j++)
            A1[i][j] = B1[i][j];

    //Add C1 and A1.  Store into C1.
    for (int l = 0; l < 13; l++) 
        for (int n = 0; n < 43; n++) 
            C1[l][n] = C1[l][n] + A1[l][n];

}//end m for loop

This was the old Obj c code:

 for (int m = 0; m < 10; m++) {
    //multiply A1 and A2.  Store into A1
    A1 = [LCA_Computation multiply:A1 withArray:A2];    //LCA_Computation is the name of the .m class file in which this all happens.  

    //Add C1 and A1.  Store into C1
    for (int i = 0; i < 13; i++) 
        for (int j = 0; j < 43; j++) 
            [[C1 objectAtIndex:i] replaceObjectAtIndex:j withObject:[NSNumber numberWithDouble: [[[C1 objectAtIndex: i] objectAtIndex: j] doubleValue] + [[[A1 objectAtIndex: i] objectAtIndex: j] doubleValue]]];

}//end m for loop

//multiply method
   + (NSMutableArray*)multiply:(NSMutableArray*)a1 withArray:(NSMutableArray*)a2
{
    int a1_rowNum = [a1 count];
    int a2_rowNum = [a2 count];
    int a2_colNum = [[a2 objectAtIndex:0] count];
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:a1_rowNum];
    for (int i = 0; i < a1_rowNum; i++) {
        NSMutableArray *tempRow = [NSMutableArray arrayWithCapacity:a2_colNum];
        for (int j = 0; j < a2_colNum; j++) {
            double tempTotal = 0;
            for (int k = 0; k < a2_rowNum; k++) {
                double temp1 = [[[a1 objectAtIndex:i] objectAtIndex:k] doubleValue];
                double temp2 = [[[a2 objectAtIndex:k] objectAtIndex:j] doubleValue];
                tempTotal += temp1 * temp2;
            }
            //the String format is intentional.  I convert them all to strings later.  I just put it in the method here where as it is done later in the C code
            [tempRow addObject:[NSString stringWithFormat:@"%f",tempTotal]];
        }
        [result addObject:tempRow];
    }
    return result;
}

Answer 1

This issue had to do with prior memory management issues causing 0’s to be used in some calculations.