I personally don't like atoi function. I would suggest sscanf:

char myarray[5] = {'-', '1', '2', '3', '\0'};
int i;
sscanf(myarray, "%d", &i);

It's very standard, it's in the stdio.h library :)

And in my opinion, it allows you much more freedom than atoi, arbitrary formatting of your number-string, and probably also allows for non-number characters at the end.

EDIT I just found this wonderful question here on the site that explains and compares 3 different ways to do it - atoi, sscanf and strtol. Also, there is a nice more-detailed insight into sscanf (actually, the whole family of *scanf functions).

EDIT2 Looks like it's not just me personally disliking the atoi function. Here's a link to an answer explaining that the atoi function is deprecated and should not be used in newer code.

You might be overthinking things. Here's a simply way to input numbers from a user that doesn't involve formatting or parsing strings.

int numArray[31];
int i = 0;
int num = 0;

while( scanf("%d", &num) > 0 && i < 31 && num >0) {
    numArray[i] = num;
    i++;
}

The loop will stop when the user enters a negative number, or has entered all 31 possible numbers.

You still have to make sure the user enters a number between 0 and 31 though!

int i;
for (i = 0; input[i] != 0; i++){
    output[i] = input[i] - '0';
}

Use sscanf, or strtol in a loop.

If you have a C string containing ASCII numbers with a common delimiter (comma in this case) you can use the strtok() function to split it into individual strings. Then you can use atoi() to convert those individual strings into integers:

char array[] = "10,11,12,1,0,1,0";
int intArray[7]; // or more if you want some extra room? 

int ipos = 0;
// Get the first token from the string
char *tok = strtok(array, ",");
// Keep going until we run out of tokens
while (tok) {
    // Don't overflow your target array
    if (ipos < 7) {
        // Convert to integer and store it
        intArray[ipos++] = atoi(tok);
    }
    // Get the next token from the string - note the use of NULL
    // instead of the string in this case - that tells it to carry
    // on from where it left off.
    tok = strtok(NULL, ",");
}

Note that strtok() is a destructive function. It will decimate your existing string with NULL characters as it slices it up.

Despite using some C++ features, most of your code looks more like C. Might I recommend something more like:

struct to_int { 
    int operator()(char c) { return c - '0'; }
};

const int num = 67;
std::vector<char> patrons(num);
std::vector<int> patron_values(num);

std::ifstream archivo("1_0.txt");  
archivo.read(&patrons[0], num);

std::cout.write(&patrons[0], num);

std::transform(patrons.begin(), patrons.end(), patron_values.begin(), to_int());
std::copy(patron_values.begin(), patron_values.end(), 
          std::ostream_iterator<int>(std::cout, "\n"));

scanf("%s", &string[0]);  

read into input character array. you are reading into some other variable.

scanf("%s",input);

or even best

fgets(input,sizeof input, stdin );

  digit[x++] = input[y++]-'0';   
   // substract `'0'` from your character and assign to digit array.

For example if input[i]=='3' ==> '3'-'0' will result in integer digit 3