Replace subscripts with pointers in C

Question

I've got a C program that I'd like to have pointers do the calculations instead of the subscripts. Here's the code:

/* rain */
#define TWLV 12
#define YRS 5 
main()

{
     static float rain [YRS][TWLV] = {
{10.2, 8.1, 6.8, 4.2, 2.1, 1.8, 0.2, 0.3, 1.1, 2.3, 6.1, 7.4},
{9.2, 9.8, 4.4, 3.3, 2.2, 0.8, 0.4, 0.0, 0.6, 1.7, 4.3, 5.2},
{6.6, 5.5, 3.8, 2.8, 1.6, 0.2, 0.0, 0.0, 0.0, 1.3, 2.6, 4.2},
{4.3, 4.3, 4.3, 3.0, 2.0, 1.0, 0.2, 0.2, 0.4, 2.4, 3.5, 6.6},
{8.5, 8.2, 1.2, 1.6, 2.4, 0.0, 5.2, 0.9, 0.3, 0.9, 1.4, 7.2}
        };
/* initialize rainfall data for 1970-74 */
int year, month;
float subtot, total;

printf( " YEAR  RAINFALL (inches)\n");
for ( year = 0, total = 0; year < YRS; year++ )
{ /* for each year, sum rainfall for each month */
    for ( month = 0, subtot = 0; month < TWLV; month++)
        subtot += rain[year][month];
    printf("%5d, %15.1f\n", 1970 + year, subtot);
    total += subtot; /*total for all years */
}
printf("\nThe yearly average is %.1f inches. \n\n", total/YRS);
printf("MONTHLY AVERAGES:\n\n");
printf("Jan Feb Mar Apr May Jun Jul Aug Sep Oct");
printf("Nov Dec\n ");

for ( month = 0; month < TWLV; month++)
{ /* for each month, sum rainfall over years */
for ( year = 0, subtot =0; year < YRS; year++)
    subtot += rain[year][month];
printf("%4.1f ", subtot/YRS);
}
printf("\n");
}

I'm struggling with arrays / pointers being used interchangably so if someone could provide an explanation that would be great.

Thanks!

And your definition of `main()` is wrong, are you using a very old book? — Iharob Al Asimi, Oct 02 '15 at 01:51
`main()` should be `int main()` and should have a return value. — Mike -- No longer here, Oct 02 '15 at 02:04
For most basic uses of indexing with subscripts, the compiler is going to convert to pointer arithmetic for you (assuming optimization isn't intentionally disabled; even the most basic optimizations tend to convert to pointer arithmetic). — ShadowRanger, Oct 02 '15 at 02:04
main is an int - and it returns an int. Always. Pls: why are you not happy, specifically, with subscripts? — jim mcnamara, Oct 02 '15 at 02:04
Here, look at this answer for an idea for using pointers with arrays: http://stackoverflow.com/a/29485321/2079103 — clearlight, Oct 02 '15 at 02:22
Even though it may not matter to you, and you may not be able to relate to people who ask you to format the code nicely, it is easier for people giving you free time and help when you make the effort to make it easy to read and prepare it better. I always do. — clearlight, Oct 02 '15 at 02:30

score 0 · Accepted Answer · answered Oct 02 '15 at 03:35

If the type of your array is float rain[YRS][TWLV], you can replace every instance of rain[i][j] with *(&rain[0][0] + (i*TWLV) + j). This won’t actually gain you anything, because the compiler does it anyway. However, you can also declare for ( float* p = &rain[0][0]; p < &rain[0][0] + YRS*TWLV; ++p ) and that might be faster because it eliminates pointer arithmetic inside the loop.

Replace subscripts with pointers in C

1 Answers1