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calculate column-wise sum of a matrix in C

This is a discussion on calculate column-wise sum of a matrix in C within the C Programming forums, part of the General Programming Boards category; Hi All, I am new to C programming. I was trying to do column-wise sum of a matrix whose elements ...

  1. #1
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    calculate column-wise sum of a matrix in C

    Hi All,

    I am new to C programming. I was trying to do column-wise sum of a matrix whose elements are integers. This is very easy to do in R (by calling the 'colSums' function). However, my c code below does not yield right output. I would appreciate if anyone can help.

    Code:
    #include <stdio.h>
    #define n_sample 4
    int *colSums_IntMat(int IntMat_In[][n_sample],int nrow){
    	int i,j,array[n_sample];
    	int *ptr;
    	for (j=0;j<n_sample; j++) {
    		array[j]=0;
    		for (i=0; i<nrow; i++) {
    			array[j]=array[j]+IntMat_In[i][j];
    		}
    	}
    	ptr=array;
    	return ptr;
    }
    
    
    int main (int argc, const char * argv[]) {
        int mat[4][n_sample]={{1,0,1,0},
    		                     {0,0,1,1},
    		                     {0,1,0,0},
    		                     {1,0,0,0}};
    	int *colsum,i;
    	colsum=colSums_IntMat(mat,4);
    	for (i=0; i<n_sample; i++) {
    		printf("Column sum for %d column is %d.\n",i,*(colsum++));
    	}
        return 0;
    }
    -----------------------------------------------------------------------------------
    The output reads:
    Column sum for 0 column is 2.
    Column sum for 1 column is 0.
    Column sum for 2 column is 0.
    Column sum for 3 column is 0.
    which is not correct. The correct output should be:
    Column sum for 0 column is 2.
    Column sum for 1 column is 1.
    Column sum for 2 column is 2.
    Column sum for 3 column is 1.

    Can anyone help? Thanks a lot.

    On another note, can we calculate column-wise sum of an arbitrary 2-d array with unknown row and column numbers? Thanks again!
    Last edited by Raymond2010; 06-19-2011 at 11:57 PM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Your array (in colSums_IntMat) is a local variable, and you're returning a pointer to it.
    The problem is the array goes out of scope when the function returns, and your pointer is then invalid (anything can happen).

    Declare an actual array in main, and pass that array as another parameter, and store the results in that parameter.
    whiteflags likes this.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
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    Thanks. I will try to modify that and see if it works. I will update my thread then.

  4. #4
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    I tried to follow Salem, still got the same output. Totally confused now!

  5. #5
    and the hat of wrongness Salem's Avatar
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    Well so are we - we can't see your latest code.
    anduril462 likes this.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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