6.S096 | January IAP 2014 | Undergraduate
Effective Programming in C and C++
Assignments

Sample Solution to Assignment 1, Problem 4

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/*

PROG: matrix2

LANG: C

*/

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

typedef struct Matrix_s {

size_t R, C;

int *index;

} Matrix;

Matrix* allocate_matrix( size_t R, size_t C ) {

Matrix *matrix = malloc( sizeof( Matrix ) );

matrix->R = R;

matrix->C = C;

matrix->index = malloc( R * C * sizeof( int ) );

return matrix;

}

void destroy_matrix( Matrix *matrix ) {

free( matrix->index );

free( matrix );

}

typedef enum {

REGULAR = 0,

TRANSPOSE = 1

} Transpose;

// Allowing reading a matrix in as either regular or transposed

Matrix* read_matrix( FILE *input, Transpose orient ) {

size_t R, C;

fscanf( input, “%zu %zu”, &R, &C );

Matrix *matrix = NULL;

if( orient == REGULAR ) {

matrix = allocate_matrix( R, C );

for( size_t r = 0; r < matrix->R; ++r ) {

for( size_t c = 0; c < matrix->C; ++c ) {

fscanf( input, “%d”, &matrix->index[c + r * C] );

}

}

} else if( orient == TRANSPOSE ) {

matrix = allocate_matrix( C, R );

for( size_t r = 0; r < matrix->C; ++r ) {

for( size_t c = 0; c < matrix->R; ++c ) {

fscanf( input, “%d”, &matrix->index[r + c * R] );

}

}

} else {

fprintf( stderr, “Error: unknown orientation %d.\n”, orient );

exit( EXIT_FAILURE );

}

return matrix;

}

void print_matrix( FILE *output, Matrix *matrix ) {

fprintf( output, “%zu %zu\n”, matrix->R, matrix->C );

for( size_t r = 0; r < matrix->R; ++r ) {

for( size_t c = 0; c < matrix->C - 1; ++c ) {

fprintf( output, “%d “, matrix->index[c + r * matrix->C] );

}

fprintf( output, “%d\n”, matrix->index[matrix->C - 1 + r * matrix->C] );

}

}

Matrix* product_matrix( Matrix *a, Matrix *b ) {

if( a->C != b->C ) {

printf( “Error: tried to multiply (%zux%zu)x(%zux%zu)\n”, a->R, a->C, b->C, b->R );

exit( EXIT_FAILURE );

}

Matrix *prod = allocate_matrix( a->R, b->R );

size_t nRows = prod->R, nCols = prod->C, nInner = a->C;

for( size_t r = 0; r < nRows; ++r ) {

for( size_t c = 0; c < nCols; ++c ) {

prod->index[c + r * nCols] = 0;

for( size_t i = 0; i < nInner; ++i ) {

prod->index[c + r * nCols] += a->index[i + r * nInner] * b->index[i + c * nInner];

}

}

}

return prod;

}

int main(void) {

FILE *fin = fopen( “matrix2.in”, “r” );

if( fin == NULL ) {

printf( “Error: could not open matrix2.in\n” );

exit( EXIT_FAILURE );

}

Matrix *a = read_matrix( fin, REGULAR );

Matrix *b = read_matrix( fin, TRANSPOSE );

fclose( fin );

Matrix *c = product_matrix( a, b );

FILE *output = fopen( “matrix2.out”, “w” );

if( output == NULL ) {

printf( “Error: could not open matrix2.out\n” );

exit( EXIT_FAILURE );

}

print_matrix( output, c );

fclose( output );

destroy_matrix( a );

destroy_matrix( b );

destroy_matrix( c );

return 0;

}

Below is the output using the test data:

matrix2:

1: OK [0.006 seconds]

2: OK [0.007 seconds]

3: OK [0.007 seconds]

4: OK [0.019 seconds]

5: OK [0.017 seconds]

6: OK [0.109 seconds]

7: OK [0.178 seconds]

8: OK [0.480 seconds]

9: OK [0.791 seconds]

10: OK [1.236 seconds]

11: OK [2.088 seconds]

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January IAP 2014
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