atoi() for 2 dimensional array

Question

If I have a 2D char array:

char tempoArray[2][3]={{1,2,3}, {4,5,6}};

How can I convert each entire row to one integer value using atoi() so that

printf("val1 %d\nval2 %d", val1, val2);

Will give me:

123

456

Answer 1

When you write

char tempoArray[2][3]={{1,2,3}, {4,5,6}};

your array contains small integer values like 1, 2, and 3.

atoi operates on strings, which are arrays of characters. For example, the string

"123"

consists of the three characters '1', '2', and '3', plus a trailing null character '\0'.

But the digit characters '1', '2', and '3' do not have the values 1, 2, and 3! If you hadn't known this, I encourage you to run this little program:

#include <stdio.h>

int main()
{
    printf("character '1' has value %d, number 1 has value %d\n", '1', 1);
    printf("character '2' has value %d, number 2 has value %d\n", '2', 2);
    printf("character '3' has value %d, number 3 has value %d\n", '3', 3);
}

Since the rows of your array are not strings (they are not arrays of useful character values, and they are not null-terminated), it is not possible to simply call atoi() on them.

Since the arrays contain integer values already, the most straightforward thing to do would be to do the arithmetic yourself, like this:

int i, j;
int val;

for(i = 0; i < 2; i++) {
    val = 0;
    for(j = 0; j < 3; j++) {
        val = 10 * val + tempoArray[i][j];
    }
    printf("val: %d\n", val);
}

If you really, really wanted to call atoi, you would have to construct an actual string from each array row, like this:

char tmpstring[4];

for(i = 0; i < 2; i++) {
    for(j = 0; j < 3; j++) {
        tmpstring[j] = tempoArray[i][j] + 48;  /* +48 to convert dig to char */
    }
    tmpstring[j] = '\0';

    val = atoi(tmpstring);

    printf("val: %d\n", val);
}

As you can see, this is more work, and more confusing.

One more point. To make it clear what kind of conversion was going on, I wrote

        tmpstring[j] = tempoArray[i][j] + 48;  /* +48 to convert dig to char */

But in real code I would never write this, because that "magic number" 48 is too obscure. In real code I would always write

        tmpstring[j] = tempoArray[i][j] + '0';

By definition, the value of the character '0' is exactly the right value to add to convert the number 1 to the character '1', the number 2 to the character '2', etc.