Populating 2d array of ints from string in C

Question

I want to populate a 2d array of ints with some numbers.

The numbers is in a grid format like the following :

08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91
22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50
32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48

I want my program to take the string above and store the numbers in a 20x20 2d array of ints.

int main(void)
{

    int grid[20][20];
    for(int i = 0;i<20;i++)
    {
        for(int j = 0;j<20;j++)
        {
            grid[i][j] = //some code to populate int here
        }
    }

}

I am unsure how to get the 'code to populate'

So I tried using strtok as below:

int main(void)
{

    char *input = "08 02 22...48";
    char *output = strtok(input," ");
    printf("%s",output);
}

However, the above is giving me a segmentation fault and I am unsure why.

I am very new to programming, especially in C, and I am a bit unsure how to get an input like above stored as a 2d array of ints. Any alternative routes to the above or any guidance as to why I am getting segmentation faults is appreciated, thanks :)

Answer 1

You may not apply function strtok to string literals. There is a more simple and robust approach by means of function strtol .

Here is a demonstrative program

#include <stdio.h>
#include <stdlib.h>

#define N   20

int main(void) 
{
    char *s = 
        "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08 "
        "49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00 "
        "81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65 "
        "52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91 "
        "22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80 "
        "24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50 "
        "32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70 "
        "67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21 "
        "24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72 "
        "21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95 "
        "78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92 "
        "16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57 "
        "86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58 "
        "19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40 "
        "04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66 "
        "88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69 "
        "04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36 "
        "20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16 "
        "20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54 "
        "01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48 ";
        int a[N][N] = { { 0 } };

        int *p = ( int * )a;

        const char *nptr = NULL;
        char *endptr = s;

        for ( size_t i = 0; i < N * N && nptr != endptr; i++ )
        {
            int x;
            nptr = endptr;
            x = ( int ) strtol( nptr, &endptr, 10 );
            if ( nptr != endptr ) p[i] = x;
        }

        for ( size_t i = 0; i < N; i++ )
        {
            for ( size_t j = 0; j < N; j++ ) printf( "%3d", a[i][j] );
            puts( "" );
        }

    return 0;
}

And this is its output

 8  2 22 97 38 15  0 40  0 75  4  5  7 78 52 12 50 77 91  8
 49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48  4 56 62  0
 81 49 31 73 55 79 14 29 93 71 40 67 53 88 30  3 49 13 36 65
 52 70 95 23  4 60 11 42 69 24 68 56  1 32 56 71 37  2 36 91
 22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
 24 47 32 60 99  3 45  2 44 75 33 53 78 36 84 20 35 17 12 50
 32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
 67 26 20 68  2 62 12 20 95 63 94 39 63  8 40 91 66 49 94 21
 24 55 58  5 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
 21 36 23  9 75  0 76 44 20 45 35 14  0 61 33 97 34 31 33 95
 78 17 53 28 22 75 31 67 15 94  3 80  4 62 16 14  9 53 56 92
 16 39  5 42 96 35 31 47 55 58 88 24  0 17 54 24 36 29 85 57
 86 56  0 48 35 71 89  7  5 44 44 37 44 60 21 58 51 54 17 58
 19 80 81 68  5 94 47 69 28 73 92 13 86 52 17 77  4 89 55 40
  4 52  8 83 97 35 99 16  7 97 57 32 16 26 26 79 33 27 98 66
 88 36 68 87 57 62 20 72  3 46 33 67 46 55 12 32 63 93 53 69
  4 42 16 73 38 25 39 11 24 94 72 18  8 46 29 32 40 62 76 36
 20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74  4 36 16
 20 73 35 29 78 31 90  1 74 31 49 71 48 86 81 16 23 57  5 54
  1 70 54 71 83 51 54 69 16 92 33 48 61 43 52  1 89 19 67 48

Answer 2

You're almost there. Inside your inner loop,

if (scanf("%d", &grid[i][j]) < 1)
    break; // oops, looks like we ran out of numbers to read!

Answer 3

The reason that strtok is giving you a segmentation fault is that you are using a const char* as the input, but strtok expects to modify the string (namely, it inserts a '\0' at the first instance of the terminating character). If you just changed your code to

char input[] = "08 02 22...48";
char *output = strtok(input," ");
printf("%s",output);

You would find that it works (because that initializer creates a string that is not a const).

Obviously you can then use sscanf to convert the string to a numerical value - make sure that you pass it the address of the location where you want to store:

sscanf(output, "%d", &grid[i][j]);

You look like you know how to combine these elements into a loop structure. Do note that strtok should be called with a char* argument the first time, and with NULL after that...

EDIT re-reading your question, it seems you would benefit from a complete code example:

#include <stdio.h>
#include <string.h>

int main(void) {

char input[]=\
"08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08 "\
"49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00 "\
"81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65 "\
"52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91 "\
"22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80 "\
"24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50 "\
"32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70 "\
"67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21 "\
"24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72 "\
"21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95 "\
"78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92 "\
"16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57 "\
"86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58 "\
"19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40 "\
"04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66 "\
"88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69 "\
"04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36 "\
"20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16 "\
"20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54 "\
"01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48";

int ii, jj;
int grid[20][20];
char *s = strtok(input, " ");

for(ii=0; ii<20; ii++){
  for(jj=0; jj<20; jj++){
    sscanf(s, "%d", &grid[ii][jj]);
    s = strtok(NULL, " ");
  }
}
for(ii=0; ii<20; ii++){
  for(jj=0; jj<20; jj++){
    printf("%02d ", grid[ii][jj]);
  }
  printf("\n");
}
return 0;
}

Let me know if any of this needs clarification.

Answer 4

#include <stdio.h>
#include <ctype.h>

int populate(const char **p){
    int n = 0;
    while(isspace(**p))
        ++*p;
    for(n=0; isdigit(**p); ++*p)
        n = n * 10 + **p - '0';
    return n;
}

int main() {
    const char *numbers = 
    "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08 "
    "49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00 "
    "81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65 "
    "52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91 "
    "22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80 "
    "24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50 "
    "32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70 "
    "67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21 "
    "24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72 "
    "21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95 "
    "78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92 "
    "16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57 "
    "86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58 "
    "19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40 "
    "04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66 "
    "88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69 "
    "04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36 "
    "20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16 "
    "20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54 "
    "01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48";

    const char *p = numbers;
    int grid[20][20];
    for(int i = 0;i<20;i++){
        for(int j = 0;j<20;j++){
            grid[i][j] = populate(&p);
            printf("%02d ", grid[i][j]);
        }
        printf("\n");
    }
    return 0;
}