Apply an operation to an entire block of memory in C - c

Question

As I know in C language in order to multiply an array by a scalar, you have to iterate over each element using a for-loop. And as I know also the source code for the R software environment is written primarily in C. And from there when I have a big matrix in R like mat = matrix(5, nrow = 1100, ncol = 1100) and then multiply it by a constant and measure the time of this operation, just like so:
t_start = Sys.time()
mat = mat *5
print(Sys.time()-t_start)
output:
Time difference of 0.005984068 secs
But doing the same thing using for-loops, it takes too much time:
t_start = Sys.time()
for(i in 1:1100)
{
for(j in 1:1100)
{
mat[i,j] = mat[i,j] * 5
}
}
print(Sys.time()-t_start)
output:
Time difference of 0.1437349 secs
The second way is ~24 times slower, now I'm assuming that behind the scene the first way is also has been done using for-loops, if so, why the time difference is too big?!
I'm wondering if there is a better way to apply an operation to an entire block of memory in C, without iterating over each element using loops.
I would like to get some answers from C language perspective, as I'm working currently with C. And those pieces of R-code was just to show two different ways of doing this that R provides and C do not.

Answer 1

Even use the C language for loop, it is faster than the first way In R language.
so you don't have to worry about for-loop slower in c language.
See the results below.
C language for loop: 0.00093478 secs
gcc -otest test.c -g -Wall -O2
./test
Time difference of 0.00093478 secs
the first way In R language: 0.004915237 secs
./Rscript first.R
Time difference of 0.004915237 secs
test.c code:
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
typedef struct matrix {
int nrow;
int ncol;
int *buf;
int *(array[]);
} matrix;
double sys_time()
{
struct timespec ts;
clock_gettime(CLOCK_MONOTONIC, &ts);
return ts.tv_sec + ts.tv_nsec / 1000000000.0;
}
void test(matrix *mat)
{
int i, j;
double t_start, t_end;
t_start = sys_time();
for(i = 0; i < mat->nrow; i++) {
for(j = 0; j < mat->ncol; j++) {
mat->array[i][j] *= 5;
}
}
t_end = sys_time();
printf("Time difference of %g secs\n", t_end - t_start);
}
matrix *create_matrix(int val, int nrow, int ncol)
{
matrix *mat;
int *buf;
int i, j;
mat = (matrix *)malloc(sizeof(*mat) + nrow * sizeof(int *));
buf = (int *)malloc(sizeof(int) * nrow * ncol);
mat->buf = buf;
mat->nrow = nrow;
mat->ncol = ncol;
for(i = 0; i < nrow; i++) {
for(j = 0; j < ncol; j++)
buf[j] = val;
mat->array[i] = buf;
buf += ncol;
}
return mat;
}
void destroy_matrix(matrix *mat)
{
free(mat->buf);
free(mat);
}
int main()
{
matrix *mat;
mat = create_matrix(5, 1100, 1100);
test(mat);
destroy_matrix(mat);
return 0;
}