运行使用 scanf 进入无限循环

Question

所以我有一些这样的代码（请注意这是在 C89 中）：

void inputChoice(int* choicePtr)
{
    int choice;
    printf(BLUE "\nINPUT: " RESET);                     /* Print input using the ansi colour code for Blue. */
    scanf("%d", &choice);                               /* Parse the choice. */

    while ((choice < 1) && (choice > 5))                /* Loop until choice is one of the menu options. */
    {
        /* Request for a valid menu option, printing this using the ansi colour code for Red, then resetting back to the default colour. */
        printf(RED "\nMenu Selection must be between (1 and 5) inclusive. \nPlease Re-enter. \n" RESET);
        scanf("%d", &choice);                           /* Parse the choice. */
    }
    *choicePtr = choice;                                /* Set the choice pointer to the valid choice after validation. */
}

选择哪个。它适用于整数。但是如果有人输入其他任何东西，例如一个字符。它无限循环。我想以某种方式检查是否输入了字符。

我尝试过的一种方法是添加这个来检查是否输入了字符，因为如果没有正确扫描，整数将为 0。

如下所示，但这也不起作用：

scanf("%d", &choice);

while (choice == 0)
{
    printf("Invalid Choice Re-enter.");
    scanf("%d", &choice);
}

Answer 1

表达式

while ((choice < 1) && (choice > 5))

永远不会为真，因为 choice 不能同时大于 5 和小于 1。

您需要：

while (choice < 1 || choice > 5)

scanf 将尝试解析缓冲区中是否有任何内容但无法解析，它将继续尝试导致无限循环，因为缓冲区中的任何内容都将保留在那里直到成功解析.

由于 scanf 将 return 0 如果没有参数被解析，您可以使用该信息来清除缓冲区以删除导致无限循环的原因：

int choice = 0;
int c; 
int scanned;
//...
if ((scanned = scanf("%d", &choice)) == EOF){ //check for EOF return
    puts("Unexpected error.");       
    //error treatment is upt to you, I would avoid the loop
    *choicePtr = scanned;
    return;
}
if (scanned == 0) {
    while ((c = fgetc(stdin)) != '\n' && c != EOF){}
}

在两个 scanf 中。

Live demo

Answer 2

考虑到评论中的评论，这里是一个修改版本：

/* reading an input choice between 1 and 5 or -1 on error */
void inputChoice(int *choicePtr) {
    int choice;

    printf(BLUE "\nINPUT: " RESET);       /* Print input using the ansi colour code for Blue. */

    for (;;) {
        int res = scanf("%d", &choice);   /* Parse the choice. */
        if (res == EOF) {
            printf(RED "unexpected end of file\n" RESET);
            choice = -1;
            break;
        }
        if (res == 0) {
            int c;
            while ((c = getchar()) != EOF && c != '\n')
                continue;
            printf(RED "invalid entry\n" "Please Re-enter.\n" RESET);
            continue;
        }
        if (choice >= 1 && choice <= 5) {
            break;
        }
        /* Loop until choice is one of the menu options. */
        /* Request for a valid menu option, printing this using the ansi 
           colour code for Red, then resetting back to the default colour. */
        printf(RED "\nMenu Selection must be between (1 and 5) inclusive.\n"
               "Please Re-enter.\n" RESET);
    }
    /* Set the choice pointer to the valid choice after validation. */
    *choicePtr = choice;
}

运行使用 scanf 进入无限循环

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运行 使用 scanf 进入无限循环

Running into infinite loop with scanf

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运行使用 scanf 进入无限循环