c - 返回字符串中的第 i 个单词

Question

实现 char * ithword(char * str, int i) 将返回 str 中的第 i 个单词。这些词是字母数字的，由非字母数字字符分隔。该函数将为返回的单词分配内存，并返回指向它的指针。

我的代码:

char *ithword(char * str, int i)
{   
    /* declarations */
    char *der = malloc(100 * sizeof(int));
    int nw, state, j, counter;
    state = 0;
    nw = 0;
    counter =0 ;

    for(j = 0; (nw <= i) || (j < sizeof(str)); j++) { 
        char c = str[j];
        if (c == '\0')
            return der;
        if (isalnum(c) == 0) {
            state = 0;
            continue;
        }
        if (state == 0) {            
            state = 1;
            nw++;
        }
        if (nw == i) {
            der[counter]=c;
            counter++;
        }
    }
    return der;
}
int main()
{
    char *p = malloc(101);
    p = ithword("bcd&acd*abc",3);
    printf("%s",p); 
}

Answer 1

我看到的一些问题，我添加了评论。

char *ithword(char * str, int i)
{
    char *der = (char *)malloc(101); # Why sizeof(int) when it has to store characters.
    memset(der ,0, 101); # Always memset a malloc allocated memory else garbage value would be printed.
    int nw, state, j, counter;
    state = 0;
    nw = 0;
    counter =0 ;

    for(j = 0; (nw <= i) || (j < sizeof(str)); j++) { 
        char c = str[j];
        if (c == '\0')
            return der;
        if (isalnum(c) == 0) {
            state = 0;
            continue;
        }
        if (state == 0) {            
            state = 1;
            nw++;
        }
        if (nw == i) {
            der[counter]=c;
            counter++;
        }
    }
    der[counter] = '\0'; # The output string has to be null terminated else all the characters would be printed from the memory location until a null is encountered.
    return der;
}

int main()
{
    char *p = NULL; # Do not allocate here. What you did would create dangling pointer because you have allocated it and then pointed it to some other location.
    p = ithword("bcd&acd*abc",3);
    printf("%s",p);
    return 0; # Return an exit code (0) since it has to return an int.
}

再次指出问题:

• main() 中的悬挂指针。
• 最好根据定义从 main() 返回退出代码。
• 如果内存是通过 malloc 分配的，则始终 memset 内存，否则它将具有垃圾值，这可能导致不确定的输出。
• 始终以 null 终止自创建的字符串，因为在打印时编译器会打印所有字符，直到遇到“\0”。
• 建议:不要盲目分配 101，找到输入字符串 str 的长度并分配该大小，以便它可以处理小字符串和大字符串。