c - 增加整数和初始化放置会产生奇怪的效果吗？

Question

我用 C 语言编写了以下“刽子手游戏”程序(写得又快又脏，所以不要太在意代码):

#include <stdio.h>

int main() {
    char word[] = "cat";
    int amountGuessesAllowed = 5;
    size_t size = sizeof(word)-1; // -1 to exclude the '\0'

    char guessedWord[size];
    for (int i=0; i<size; i++){
        guessedWord[i] = '_';
    }

    int lettersGuessed = 0;
    int totalTimesGuessed = 0;
    char c;

    printf("%s\n", guessedWord);
    printf("Guess %d - Enter a character: ", totalTimesGuessed+1);
    scanf("%s", &c);

    while (totalTimesGuessed < amountGuessesAllowed) {
        for(int i=0; i<size; i++){
            if (word[i] == c && guessedWord[i] == '_'){
                printf("It matches character at index %d!\n", i);
                guessedWord[i] = c;
                lettersGuessed++;
                printf("letters guessed: %d\n", lettersGuessed);
            }
        }
        totalTimesGuessed++;
        if (lettersGuessed == size) {
            printf("You win!");
            return 0;
        } else if (totalTimesGuessed == amountGuessesAllowed) {
            printf("You lose!");
            return 0;
        } else {
            printf("%s\n", guessedWord);
            printf("Guess %d - Enter a character: ", totalTimesGuessed+1);
            scanf("%s", &c);
        }
    }
}

问题似乎出在 lettersGuessed、totalTimesGuessed 和 c 三个初始化行。

如果您执行该程序并输入几个字母，您会注意到由于某种原因，totalTimesGuessed 值始终显示为 2，但它不再出现是递增的。 lettersGuessed 变量似乎没有任何问题。

___
Guess 1 - Enter a character: b
___
Guess 2 - Enter a character: c
It matches character at index 0!
letters guessed: 1
c__
Guess 2 - Enter a character: j
c__
Guess 2 - Enter a character:

现在，如果您要交换 lettersGuessed 和 totalTimesGuessed 的顺序，那么 lettersGuessed 变量就会出现问题(仍然是 1) !

___
Guess 1 - Enter a character: c
It matches character at index 0!
letters guessed: 1
c__
Guess 2 - Enter a character: a
It matches character at index 1!
letters guessed: 1
ca_
Guess 3 - Enter a character: 

最后，如果我将 char c; 行放在两个整数之前:

char c;
int lettersGuessed = 0;
int totalTimesGuessed = 0;

然后一切都按预期进行!

___
Guess 1 - Enter a character: c
It matches character at index 0!
letters guessed: 1
c__
Guess 2 - Enter a character: a
It matches character at index 1!
letters guessed: 2
ca_
Guess 3 - Enter a character: 

我已经在 Xcode 中使用 gcc 对此进行了测试。

对此必须有一个合乎逻辑的解释吗？我渴望启蒙!

Answer 1

您导致了未定义的行为

scanf("%s", &c);

因为 "%s" 说明符向目标添加一个终止 nul 字节，该目标是单个 char。

相反，你可以尝试

if (scanf(" %c", &c) != 1)
    handleErrorPlease();

"%c" 之前的空格是故意的，它会吃掉之前调用 scanf 留下的任何空白字符。

当发生未定义的行为时，实际发生的情况取决于程序在加载内存时的布局，因此更改已定义变量的顺序会导致行为不同。