在 for 循环中使用 do-while 循环是否正确？ 为什么，为什么不呢？

Question

//program to count words
#include<stdio.h>
int main()
{
    int i;
    int word = 0;
    char sent[] = "How are you doing mister";  //character pointer
    for (i = 0; sent[i] != '\0'; i++)
    {
        do
        {
            word++;

        } while (sent[i] == ' ');
    }
    printf("There are %d words in the sentence\n", word + 1);  //words are always 1 more than the no. of spaces.
    return 0;                                                  //or word=1;
}

This is a code for counting the number of words. Please tell me why cannot we use do-while loops inside a for loop. Or if we can, how to do it.

Answer 1

Nesting the various composite statements such as for and do / while is allowed in C at least up to 127 levels, as specified in 5.2.4.1 Translation limits .

The problem is not a syntax issue, but rather a conceptual one:

• your do / while loop iterates on a constant condition as neither i , nor sent is modified in the body or in the condition of the loop, leading to an infinite loop if sent[i] is a space.

• counting spaces is not a correct approach to counting words in a string: "" has 0 words, not 1 per your intended code, " " also but you would get 2 and "AB" only has 2 words, not 3 .

• you should count the number of transitions from space to non space, starting with an implicit space before the beginning of the string.

• note also that char sent[] = "..."; is not a character pointer, but a character array.

Here is a modified version:

//program to count words
#include <stdio.h>

int main() {
    int i, words, last;
    char sent[] = "How are you doing mister?";

    words = 0;
    last = ' ';
    for (i = 0; sent[i] != '\0'; i++) {
        if (sent[i] != ' ' && last == ' ')
            word++;
        last = sent[i];
    }
    printf("There are %d words in the sentence '%s'\n", words, sent);
    return 0;
}

In my experience proof reading code, do / while loops tend to be written incorrectly, especially by beginners, missing test conditions or broken in some other way. I you think a do / while loop solves a given problem, think again, a for loop might be a safer approach. The only place do / while loops are needed is in a macro expansion where you want to combine multiple statements into a single compound statement:

#define swap_ints(a, b)  do { a ^= b; b ^= a; a ^= b; } while (0)

Note however that the swapping method in this macro is inefficient and macros are very error prone and should be avoided even more than do / while loops :)

Answer 2

It is completely valid to nest a do-while loop within a for loop. Syntactically, there is nothing wrong with your program.

However, as others have described, your nested loop will never terminate when sent[i] == ' ' . Your program has bugs, but they have nothing to do with the validity of nesting loops.