程序运行时学习C scanf函数的问题（编译期间没有错误）C编程绝对初学者指南第8章

Question

我从书中逐字输入了以下代码：

// Ex program #2 from Ch 8 of ABGTC
// File Ch8Ex2.c

// This is a sample program that asks users for some basic data and prints it on screen in order to show what was entered

#include <stdio.h>

main()
{
    float cost;
    char topping[24];
    int slices;
    int month, day, year;

    // The first scanf will look for a floating-point variable, the cost of a pizza
    // If the user doesn't enter a $ before the cost, it could cause problems

    printf("How much does a pizza cost in your area? Enter as $XX.XX\n");
    scanf(" $%f", &cost);

    // The pizza topping is a string, so your scanf doesn't need an &

    printf("What is your favorite one-word pizza topping?\n");
    scanf(" %s", topping);

    printf("How many slices of %s pizza can you eat in one sitting?", topping);
    scanf(" %d", &slices);

    printf("What is today\'s date (enter it in XX/XX/XX format).\n");
    scanf(" %d/%d/%d", &month, &day, &year);

    printf("\n\nWhy not treat yourself to a dinner on %d/%d/%d", month, day, year);
    printf("\nand have %d slices of %s pizza!\n", slices, topping);
    printf("It will only cost you $%.2f!\n\n\n", cost);

    return 0;
}

编译时出现零错误（使用代码：块，如书中推荐和使用的那样）；然而，一旦我运行程序，在输入请求的用户数据的第一位（一片披萨的价格，假设我说它是 03.45）后，程序就会打印出来：

"What is your favorite one-word pizza topping?"
"How many slices of 03.45 pizza can you eat in one sitting?"

感觉就像是跳过了 printf "What is your favorite...." 后面的 scanf 行，不仅提示了 "How many slices...." 后面的 printf 行，而是插入了 03.45 作为应该由用户在上一行输入的字符数组/字符串值。

我尝试了一些不同的调整，但我终其一生都无法弄清楚我做错了什么。有任何想法吗？

Answer 1

问题是我在输入回复时没有使用 $。在回答第一个问题“您所在地区的比萨饼要多少钱？输入 $XX.XX”时，我的回答是 03.45，而我应该回答 $03.45，因为 scanf 是 $%f。从 "$%f" 中删除 $ 并使其余部分保持不变仍然会产生最终打印函数中所需的结果。

希望这可以帮助其他人使用这本书！

Answer 2

您没有检查scanf() 的返回值。 scanf() 返回正确解析和存储的输入数。

在失败的情况下，例如不匹配格式字符串" $%f" 的缺失$，有问题的输入将保留在标准输入中，并将在下次调用scanf() 或任何其他输入函数时读取。

您可以通过使用fgets() 读取输入并使用sscanf() 解析它来更正程序，直到执行正确的转换或到达文件末尾。

请注意，您应该保护格式%s以防止缓冲区溢出：将要读取的最大字符数传递为scanf(" %23s", topping);

这是一个例子：

#include <stdio.h>

int main(void) {
    char line[80];
    float cost;
    char topping[24];
    int slices;
    int month, day, year;

    for (;;) {
        printf("How much does a pizza cost in your area? Enter as $XX.XX\n");
        if (!fgets(line, sizeof line, stdin))
            return 1;
        if (sscanf(line, "$%f", &cost) == 1)
            break;
        printf("invalid input\n");
    }

    for (;;) {
        printf("What is your favorite one-word pizza topping?\n");
        if (!fgets(line, sizeof line, stdin))
            return 1;
        if (sscanf(line, "%23s", topping) == 1)
            break;
        printf("invalid input\n");
    }

    for (;;) {
        printf("How many slices of %s pizza can you eat in one sitting?\n", topping);
        if (!fgets(line, sizeof line, stdin))
            return 1;
        if (sscanf(line, "%d", &slices) == 1)
            break;
        printf("invalid input\n");
    }

    for (;;) {
        printf("What is today\'s date (enter it in XX/XX/XX format).\n");
        if (!fgets(line, sizeof line, stdin))
            return 1;
        if (sscanf(line, "%d/%d/%d", &month, &day, &year) == 3)
            break;
        printf("invalid input\n");
    }

    printf("\n\nWhy not treat yourself to a dinner on %d/%d/%d\n",
           month, day, year);
    printf("and have %d slices of %s pizza!\n", slices, topping);
    printf("It will only cost you $%.2f!\n\n\n", cost);

    return 0;
}

Answer 3

将浮点变量作为输入后，您缺少flush 标准输入。

只需使用下面给出的以下代码即可解决您的问题。

#include <stdio.h>

main()
{
    float cost;
    char topping[24];
    int slices;
    int month, day, year;

    printf("How much does a pizza cost in your area? Enter as $XX.XX\n");
    scanf(" $%f", &cost);

    fflush(stdin);    //Just add this line here and your problem is solved.

    printf("What is your favorite one-word pizza topping?\n");
    scanf(" %s", topping);

    printf("How many slices of %s pizza can you eat in one sitting?\n", topping);
    scanf(" %d", &slices);

    printf("What is today\'s date (enter it in XX/XX/XX format).\n");
    scanf("%d/%d/%d", &month, &day, &year);

    printf("\n\nWhy not treat yourself to a dinner on %d/%d/%d", month, day, year);
    printf("\nand have %d slices of %s pizza!\n", slices, topping);
    printf("It will only cost you $%.2f!\n\n\n", cost);

    return 0;
}