Question

我一直在尝试理解从著名网站www.geeksforgeeks.org将整数从转换为单词的代码，但是我没有得到关于负号single_digits[*num - '0']的指针算法，因此可以有人请让我明白吗？

/* C program to print a given number in words. The program handles 
numbers from 0 to 9999 */
#include <stdio.h> 
#include <string.h> 
#include <stdlib.h> 

/* A function that prints given number in words */
void convert_to_words(char *num) 
{ 
    int len = strlen(num); // Get number of digits in given number 

    /* Base cases */
    if (len == 0) { 
        fprintf(stderr, "empty string\n"); 
        return; 
    } 
    if (len > 4) { 
        fprintf(stderr, "Length more than 4 is not supported\n"); 
        return; 
    } 

    /* The first string is not used, it is to make 
        array indexing simple */
    char *single_digits[] = { "zero", "one", "two", 
                            "three", "four","five", 
                            "six", "seven", "eight", "nine"}; 

    /* The first string is not used, it is to make 
        array indexing simple */
    char *two_digits[] = {"", "ten", "eleven", "twelve", 
                            "thirteen", "fourteen", 
                            "fifteen", "sixteen", 
                            "seventeen", "eighteen", "nineteen"}; 

    /* The first two string are not used, they are to make 
        array indexing simple*/
    char *tens_multiple[] = {"", "", "twenty", "thirty", "forty", "fifty", 
                            "sixty", "seventy", "eighty", "ninety"}; 

    char *tens_power[] = {"hundred", "thousand"}; 

    /* Used for debugging purpose only */
    printf("\n%s: ", num); 

    /* For single digit number */
    if (len == 1) { 
        printf("%s\n", single_digits[*num - '0']); 
        return; 
    } 

    /* Iterate while num is not '\0' */
    while (*num != '\0') { 

        /* Code path for first 2 digits */
        if (len >= 3) { 
            if (*num -'0' != 0) { 
                printf("%s ", single_digits[*num - '0']); 
                printf("%s ", tens_power[len-3]); // here len can be 3 or 4 
            } 
            --len; 
        } 

        /* Code path for last 2 digits */
        else { 
            /* Need to explicitly handle 10-19. Sum of the two digits is 
            used as index of "two_digits" array of strings */
            if (*num == '1') { 
                int sum = *num - '0' + *(num + 1)- '0'; 
                printf("%s\n", two_digits[sum]); 
                return; 
            } 

            /* Need to explicitely handle 20 */
            else if (*num == '2' && *(num + 1) == '0') { 
                printf("twenty\n"); 
                return; 
            } 

            /* Rest of the two digit numbers i.e., 21 to 99 */
            else { 
                int i = *num - '0'; 
                printf("%s ", i? tens_multiple[i]: ""); 
                ++num; 
                if (*num != '0') 
                    printf("%s ", single_digits[*num - '0']); 
            } 
        } 
        ++num; 
    } 
} 

/* Driver program to test above function */
int main(void) 
{ 
    convert_to_words("9923"); 
    convert_to_words("523"); 
    convert_to_words("89"); 
    convert_to_words("8989"); 

    return 0; 
}

Answer 1

single_digits[*num - '0']

*num-您将获得此指针引用的字符。

*num - '0'得到的数字等于字符（假设字符是数字）

single_digits[*num - '0']获得带有字符十进制值索引的数组元素。

[* num-'0']是什么意思？

1 个答案: