Multiply a line of a matrix by a number given

Question

I'm supposed to multiply a certain line (I specify what line exactly in the 4th argument of the function) of a given matrix by a number.

The main function:

int main_tp05(int argc, const char *argv[]){
    int mNx100[][MAXCOLS100] = {{1,2,3},{4,5,6},{7,8,9}};
    multiply_matrixNx100_line_by_scalar(mNx100,3,3,1,2);
    return  0;
}

I've tried to solve it like so:

void multiply_matrixNx100_line_by_scalar(int mNx100[][MAXCOLS100], int lines, int columns, int line, int scalar){
    for (int i = 0; i < lines; i++) {
        for (int j = 0; j < columns; j++) {
            if(i == line){
                printf("%d\n", mNx100[i*scalar][j] );
            }
        }
        printf("\n");
    }
}

To note that:

1- I can´t change the parameters.
2- MAXCOLS100 is a macro on the .h file. I put it with the value of 3.
3- The scalar is the number I want to multiply the line by. 

Answer 1

I WROTE THIS CODING AND VERIFIED BY MYSELF IT WORKS SUCCESSFULLY.

    #include <stdio.h>
    #include <stdlib.h>
    int main() 
    {
        int i,j,a[5][5],b,c,d,m,n,r;
        printf("Enter the number of rows and columns\n");
        scanf("%d %d",&m,&n);
        printf("Enter the matrix\n");
        for(i=1;i<=m;i++)
        {
            for(j=1;j<=n;j++)
            {
                scanf("%d",&a[i][j]);
            }
        }
        printf("Enter the number to multiply the matrix\n");
        scanf("%d",&b);
        printf("Enter 0 to multiply row or 1 to multiply column\n");
        scanf("%d",&d);
        if(d==0)
        {
            printf("Enter the row number to be multiplied\n");
            scanf("%d",&r);
            for(i=r,j=1;j<=n;j++)
            {
                a[i][j]*=b;
            }
        }
        else if(d==1)
        {
            printf("Enter the row number to be multiplied\n");
            scanf("%d",&c);
            for(j=c,i=1;i<=m;i++)
            {
                a[i][j]*=b;
            }
        }
        printf("matrix after multiplied\n");
        for(i=1;i<=m;i++)
        {
            for(j=1;j<=n;j++)
            {
                printf("%d  ",a[i][j]);
            }
            printf("\n");
        }
        return 0;
    }

OUTPUT OF THIS CODING

Answer 2

I'm supposed to multiply a certain line (I specify what line exactly in the 4th argument of the function) of a given matrix by a number

Usually, "multiply a row of a matrix by a scalar" means to multiply every element of the row by a certain scalar value. That's not what the posted function does, it multiplies the index of the row by the passed argument:

void multiply_matrixNx100_line_by_scalar(int mNx100[][MAXCOLS100], int lines, int columns,
                                         int line, int scalar) {
    for (int i = 0; i < lines; i++) {        // <-- Useless, the row is known
        for (int j = 0; j < columns; j++) {
            if(i == line){
                printf("%d\n", mNx100[i*scalar][j] );
                //                    ^^^^^^^^
            }
        }
        printf("\n");
    }
}

If the intent is to only print the modified row, the previous function could be rewritten as

void multiply_matrixNx100_line_by_scalar(int mNx100[][MAXCOLS100], int lines, int columns,
                                         int line, int scalar) {
    if (line < 0  ||  line >= lines)
        return;
    for (int j = 0; j < columns; j++) {
        printf("%d\n", mNx100[line][j] * scalar);
    }
    printf("\n");
}

If instead the function is supposed to only modify the matrix, without printing it, we could use

mNx100[line][j] *= scalar;

Inside the loop, in place of the call to printf.