If I use variables as indexes to access a char * [] [], the content of the elements is (null)

Question

I'm writing a program to play battleship. I have a string matrix representing the battlefield.

#define NUM_CASELLE 6

char* campo[NUM_CASELLE][NUM_CASELLE];

At the beginning of the program each element of the matrix is initialized with "-".

I noticed that I have problems accessing the matrix elements so I did some debugging to better understand what the problem was and I noticed this: if I write

printf("The content is %s\n", campo[3][1]);

the result is

The content is -

and that's right.

But if I enter coordinates from stdin and memorize them in variables,

printf("row is %c\n", row);
printf("col is %d\n", col);
printf("The content is %s\n", campo[row][col]);

the result is as follows:

The content is (null)

Where am I wrong?

Anyway, I post the whole code because maybe the error is elsewhere. In fact, the coordinates are entered as they are in battleship, for example, a3 or b5 or f1 etc .. and then I convert the letter to the respective row index.

#define NUM_CASELLE 6
#define NUM_NAVI    7

int pos_size = 256;

char* campo[NUM_CASELLE][NUM_CASELLE];

void posizionaNavi(){
    char pos[pos_size];
    int i = 0;
    int col;
    int row;

    printf("Scegli dove posizionare le navi...\n");

    while(i < NUM_NAVI){
        printf("Posizionare nave numero %d...\n", i + 1);
        fgets(pos, pos_size, stdin);

        col = isCommandValid(pos); 
        row = pos[1];

        if(col == -1){
            printf("\n");
            printf(">> ATTENZIONE: formato errato.\n");
            printf(">> Le colonne vanno dalla lettera A alla lettera F\n");
            printf(">> Le righe vanno dal numero 1 al numero 6\n");
            printf(">> Esempi di comando valido: a3 - b6 - f1\n");
            printf("\n");
        }
        else{
            printf("row is %c\n", row);
            printf("col is %d\n", col);
            printf("The content is %s\n", campo[row][col]);
            printf("The content is %s\n", campo[3][1]);

            if(campo[row][col] = " - "){
                campo[row][col] = " x ";
                printf("Nave %d posizionata in %s\n", i + 1, pos);
                i++;
            }
            else{
                printf(">> ATTENZIONE: casella già occupata da una nave.");
                printf(">> Riprovare...\n");
                printf("\n");
            }
        }
    }
}

int isCommandValid(char* pos){
    int ret;

    if(strlen(pos) != 3 || pos[1] > '6' || pos[1] < '1')
        return -1;

    switch(pos[0]){
        case 'a':
            ret = 1;
            break;
        case 'A':
            ret = 1;
            break;
        case 'b':
            ret = 2;
            break;
        case 'B':
            ret = 2;
            break;
        case 'c':
            ret = 3;
            break;
        case 'C':
            ret = 3;
            break;
        case 'd':
            ret = 4;
            break;
        case 'D':
            ret = 4;
            break;
        case 'e':
            ret = 5;
            break;
        case 'E':
            ret = 5;
            break;
        case 'f':
            ret = 6;
            break;
        case 'F':
            ret = 6;
            break;
        default:
            ret = -1;
            break;
    }

    return ret;
}

This is wrong for two reasons: `if(campo[row][col] = " - "){`. First, the comparison operator is `==`, not `=`. Second, you can't compare strings with `==`, you have to use `strcmp()`. — Barmar, Aug 09 '17 at 14:29
Content should be "-" as it is right when I type campo [3] [1] — Filippo Auletta, Aug 09 '17 at 14:31
Don't store an array of strings. Store something simple like numbers in the array, and convert them to/from strings in your input/output functions. — Barmar, Aug 09 '17 at 14:32
`char* campo[NUM_CASELLE][NUM_CASELLE];` and "At the beginning of the program each element of the matrix is initialized with "-"." is inconsistent. The code performs no _initialization_, yet the post says initialization is occurring. — chux - Reinstate Monica, Aug 09 '17 at 14:35
Initialization takes place in another function that I did not post because I'm sure the function does what it has to do. — Filippo Auletta, Aug 09 '17 at 14:37
The actual error is that you assign the `row` from **char**. It is the ASCII code of the row number, not a small integer but 48-57... — Antti Haapala -- Слава Україні, Aug 09 '17 at 14:38
Then you hide your mistake by using `%c` in `printf` when you should use `%d`. — Antti Haapala -- Слава Україні, Aug 09 '17 at 14:38
Antti Haapala you are right. The error is there. How do I convert char to integer? I have not been able to find a way to do it in c. — Filippo Auletta, Aug 09 '17 at 14:42
@FilippoAuletta Detail: In C, _initialization_ has a specific meaning where an initial value is defined in the objects _definition_ like `int x = 42;`. `int x; ... x = 42;` is not initialization, but an _assignment_, even if it is the first assignment. — chux - Reinstate Monica, Aug 09 '17 at 14:54
Basically, this is not how strings work in C. I would recommend reading the chapter about strings in your beginner-level C book. The linked duplicates may also contain good reading. — Lundin, Aug 09 '17 at 14:56

kocica · Accepted Answer · 2017-08-09T14:47:24.713

1.

case 'f':
    ret = 6;
    break;
case 'F':
    ret = 6;
    break;

easier

case 'F':
case 'f':
    ret = 6;
break;

2. Cannot compare string's with ==. You have to use strcmp().

strcmp(campo[row][col], "-")

3. You should write program's in american english, especially if you are pasting them somewhere.

4. Dont forget to check return value's.

5.

char* campo[NUM_CASELLE][NUM_CASELLE];

is pointer to double array, change it to

char campo[NUM_CASELLE][NUM_CASELLE];

so you can do campo[x][x] now.

6.

row = pos[1];

Here you are assigning ascii value of character, do

row = pos[1] - '0';

chux - Reinstate Monica · Answer 2 · 2017-08-09T14:56:32.990

if(campo[row][col] = " - "){ is an assignment and then a test, not a string nor pointer compare @barmar.

Likely code should use strcmp() to compare strings.

if(strcmp(campo[row][col], " - ") == 0) {

OP says "matrix is initialized with "-"." This differs from the above compare target.

// This is not initialization, only a defintion without initialization    
char* campo[NUM_CASELLE][NUM_CASELLE];

...
// potential unposted "initialization" code, needed for each campo[row][col] 
campo[row][col] = "-";  // this will not compare as above
campo[row][col] = " - ";  // this will compare as above

I suspect unposted code has trouble with campo[row][col] and other issues.

0___________ · Answer 3 · 2017-08-09T14:51:46.693

Yes. It is correct. Your table is two dimensional array of pointers. And it can't store anything else but the pointers.

The cell can point to something valid if you:

Allocate the memory and then store something in that allocated memory
```
campo[x][y] = malloc(size);
strcpy(campo[x][y], "Hello.");
```

Assign the cell with the valid pointer

char str[] = "hello";
char *str1 = "HELLO";
campo[x][y] = str;
campo[n][n] = str1;
campo[a][b] = "Hello Again";

If you have already assigned the pointer with the address of the valid string, there is nothing in like string comparition operator

if(campo[row][col] = " - ") - will assign campo[row][col] with the address of the string literal " - " and if will consider it as the truth as the valid pointer always cast to non zero integer (but you get the warning)

if(campo[row][col] == " - ") will compare the pointer campo[row][col] with the address of string literal " - "

if(!strcmp(campo[row][col] , " - ")) will check if both string are equal

Not my downvote, but it was probably downvoted because the first version of your answer was too short and didn't actually answer the question. — Jabberwocky, Aug 09 '17 at 14:43

If I use variables as indexes to access a char * [] [], the content of the elements is (null)

3 Answers3