c - 循环在完成之前会重复自身

Question

这个问题已经有答案了:

scanf() leaves the newline character in the buffer (7 个回答)

已关闭 4 年前。

我正在尝试用 c 编写数据文本，但进程顺序有问题。循环在完成之前会重复自身。它必须是“是或否”，但它在询问和稍后的 scanf 过程之后打印“go”。该代码有什么问题？

#include <stdio.h>

int main(){

    FILE *potr;

    char go = 'e',name[20],sname[20];
    int age;

    potr = fopen("example.txt","w");

    if(potr == NULL) printf("File cannot open");

    while(go != 'n'){
        printf("go\n");
        scanf("%s %s %d",name,sname,&age);
        fprintf(potr,"%s %s %d",name,sname,age);
        printf("\ny or n :");
        scanf("%c",&go);
    }

    fclose(potr);

    return 0;
}

Answer 1

首先fopen()错误处理不正确，这个

potr = fopen("example.txt","w");
if(potr == NULL) 
printf("File cannot open"); /* doesn't make any sense */

应该是

potr = fopen("example.txt","w");
if(potr == NULL) {
    printf("File cannot open");
    return 0; /* return if open failed */
}  

其次，使用

scanf(" %c",&go); /* To avoid buffering issue i.e after char you pressed ENTER i.e 2 input. space before %c will work */

而不是

scanf("%c",&go);

示例代码:

int main(void){
        FILE *potr = fopen("example.txt","w");
        if(potr == NULL){
                 printf("File cannot open");
                return 0;
        }
        char go = 'e',name[20],sname[20];
        int age;

        while(go != 'n'){
                printf("go\n");
                scanf("%19s%19s%d",name,sname,&age);
                fprintf(potr,"%s %s %d",name,sname,age);
                printf("\ny or n :");
                scanf(" %c",&go); /* whitespace before %c */
        }
        fclose(potr);
        return 0;
}

此外，在这种特殊情况下，您可以使用 do..while 而不是 while 作为第一次进入循环，无论条件是真还是假，通过分配go 作为 e。例如

do{
        printf("go\n");
        scanf("%19s%19s%d",name,sname,&age);
        fprintf(potr,"%s %s %d",name,sname,age);
        printf("\ny or n :");
        scanf(" %c",&go); /* whitespace before %c */
}while(go == 'y' || go == 'Y');