解析C中来自字符串的两个字符输入

Question

Okay so I am parsing a 18 character string consisting of '?'s and '0' - '9'. What I am trying to do is use the atoi function to convert two character chunks of the string into integers. The characters that I need to parse are in an array. I am lost on how to implement such a solution.

  char *str = "01";
  int n = atoi(str);
  printf("The string %s as an integer is = %d\n",str,n);

Which gives you "The string 01 as an integer is = 1"

Answer 1

Here's something that may work.

1. Advance pointer to next decimal or end of string
2. If reached end of string, you're done
3. Use strtol , keep the result. This will advance the pointer to the next non-decimal or the end of the string.
4. Go back to step 1.

Here's some source.

#include <stdlib.h>
#include <stdio.h>
#define ARR_LEN 18    
int
main(void)
{
    char *str = "12?456?8??12????78";
    char *ptr = str;
    int result[ARR_LEN];
    int i = 0;
    int j = 0;

    i = 0;
    for (; ;)
    {
        while (*ptr == '?' && *ptr != '\0')
            ++ptr;
        if (*ptr == '\0')
            break;
        result[i++] = (int)strtol(ptr, &ptr, 10);
    }    
    for (j = 0; j < i; ++j)
        printf("%d ", result[j]);
    printf("\n");
    return 0;
}

Answer 2

#define UNKNOWN_CONTROL -1           // integer to recognize unknown pairs
#define CONSTANT_STRING_LENGTH 18    // The string length

int i;
char string[CONSTANT_STRING_LENGTH]; // This is your string
int pairs[CONSTANT_STRING_LENGTH/2]; // Array to store results

for (i=0; i<CONSTANT_STRING_LENGTH/2; i++) {       // For each pair in the string
  if (string[i*2] == '?' || string[i*2]+1 == '?')  // Is it a '??' pair?
    pairs[i] = UNKNOWN_CONTROL;                    // Store some constant (> 99)
  else
    pairs[i] = string[i*2] * 10 + string[i*2+1] - '0' * 11; // Compute the number and store
}

And then your array pairs would be filled with your desired results if I got your question right.

EDIT: To understand the computing bit, you've got to understand that ASCII characters (printable, the ones you store on strings) don't correspond to their integer counterparts. ASCII '0' is integer 48, ASCII '1' is integer 49, ASCII '2' is integer 50, and forth...

By multiplying the first character by ten and adding the first characters, you're summing the ASCII values, not the integer ones, so you've got to subtract the bias. Substracting '0' (the base number for ASCII) would work for one character (eg ASCII '2' - ASCII '0' == 2 ), but you've got to multiply it by 11 for two characters.

Keep in mind '0' * 11 is the same as '0' * 10 + '0'. Doing some redistribution of the math you can see exactly what's being done there:

pairs[i] = (string[i*2] - '0') * 10 + string[i*2+1] - '0';