Scanf跳过变量并重用旧变量

编程问答 2022-08-16

问题描述

我正在遍历C编程绝对入门指南第三版，在Chapter8ex2.c学习编程并且遇到问题，我的程序在此花了成本，然后被认为是顶峰，反而绕过了，重用了第一个变量而不是使用正确的第二个变量。

这是我的代码

// This is a sample program that asks users for some basic data and prints it on 
// the screen in order to show what was entered
#include <stdio.h>

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

    // The first scanf will look for a floating-point variable,the cost of the 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?");
    printf("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",topping);
    printf("can you eat in one sitting?\n");
    scanf("%i",&slices);

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

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

    return(0); 
}

这是正在运行的代码

How much does a pizza cost in your area?enter as $XX.XX)
15.00
What is your favorite one-word pizza topping?
How many slices of 15.00 pizzacan you eat in one sitting?

解决方法

scanf存在一些问题。考虑对所有输入使用fgets并根据需要进行解析。
当scanf失败时，有问题的输入将留在输入流中，并可能导致后续扫描失败或行为异常。 fgets将从输入流中读取并删除最大缓冲区的大小。
解析输入可以通过许多功能来完成。这里使用sscanf和strcspn，但还有许多其他可用的。 strspn，strtod，strtol，strchr，strstr，strpbrk，strtok，strsep等。
避免将%i尤其用于日期。每天或每月输入08或09都会造成麻烦。前导零告诉%i处理一个八进制（以8为底）值，但是8和9无效的八进制数字。
do{}while();允许重复输入，直到可接受的结果为止。解析 这里有限制，但可以改进。

#include <stdio.h>
#include <string.h>

int main()
{
    char line[100] = "";
    char topping[100] = "";
    int result = 0;
    int slices = 0;
    int month = 0;
    int day = 0;
    int year = 0;
    float cost;

    do {
        printf ( "How much does a pizza cost in your area?");
        printf ( "enter as $XX.XX)\n");
        if ( ! fgets ( line,sizeof line,stdin)) {
            fprintf ( stderr,"fgets EOF\n");
            return 1;
        }
        result = sscanf ( line," $%f",&cost);
        if ( 0 == result) {
            printf ( "be sure to use a $\n");
        }
    } while ( 1 != result);

    printf ( "What are your favorite pizza toppings?\n");
    if ( ! fgets ( topping,sizeof topping,stdin)) {
        fprintf ( stderr,"fgets EOF\n");
        return 1;
    }
    topping[strcspn ( topping,"\n")] = 0;//remove newline

    do {
        printf ( "How many slices of %s pizza",topping);
        printf ( " can you eat in one sitting?\n");
        if ( ! fgets ( line,"%d",&slices);
        if ( 0 == result) {
            printf ( "enter a number\n");
        }
    } while ( 1 != result);

    do {
        printf ( "What is today's date (enter it in XX/XX/XX format).\n");
        if ( ! fgets ( line,"%d /%d /%d",&month,&day,&year);
        if ( 3 != result) {
            printf ( "be sure to use a / between numbers\n");
        }
    } while ( 3 != result);

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

    return(0);
}
c