reading by columns using ptr

Question

Hi

if I want to print by rows of

char boo[] = 
     "abcd"
     "efgh"
     "ijkl"
     "mnop";

I'd go with

for(i = 0; i < 4; i++)
{
  char row[] = "";
  for(j = 0; j < 4; j+)
    printf("%c", *(boo++))
  putchar('\n');
  puts(row);

}

my question is how I can print columns so I get
aeim
bfjn
cgko
dhlp

Answer 1

Because it looks like a homework I'll just give you a clue.

Your boo is not an array of strings - preprocessor glues all adjacent string literals into one string so you got one string "abcdefghijklmnop"

You need to declare it as

char *boo[] = {"abcd", "efgh", ...};

Answer 2

#include <stdio.h>

int main (int argc, char* argv[])
{
    char *boo[] = {"abcd", "efgh", "ijkl", "mnop"};
    int i, j;

    for(i = 0; i < 4; i++) {
        for(j = 0; j < 4; j++)
            printf("%c", boo[j][i]);

        printf("\n");
    }
}

-m

Answer 3

The other solutions involved modifying boo, which you said was against the rules, here a simple solution. Just use an inner loop which prints every 4th character relative to the outer loop.

int main() {
    int i, j;
    for(i = 0; i < 4; ++i) {
        for(j = 0; j < 4; ++j) {
            putchar(boo[i + j * 4]);
        }
        putchar('\n');
    }
}

think of it this way, this is effectivly manually treating boo as if it were a 2d array of characters. Since the formula for indexing a 1d array with 2d is (x + y * width), you can imagine that i is the x-coord and j is the y-coord.