程序不等待用户输入 scanf("%c",&yn); [英] Program doesn't wait for user input with scanf("%c",&yn);

问题描述

这是我正在编写的程序的基本代码，用于练习使用 C 中的文件.我试图检测输出文件是否已经存在，如果确实存在，我想询问用户是否想覆盖它或不.这就是我首先用 fopen(outfilename,"r"); 打开 outfilename 文件的原因；与 fopen(outfilename,"w"); 相反.

This is the basic code to a program I am writing to practise using files in C. I am trying to detect whether the output file already exists and if it does exist I want to ask the user if they would like to overwrite it or not. This is the reason that I have first opened the outfilename file in with fopen(outfilename,"r"); as opposed to fopen(outfilename,"w");.

它检测文件不存在的情况，但是，如果它确实存在，则执行 printf("Output file already exists, overwrite (y/n):");语句但完全忽略了 scanf("%c",&yn);陈述！

It detects the case of the file not existing, however, if it does exist it executes the printf("Output file already exists, overwrite (y/n):"); statement but completely ignores the scanf("%c",&yn); statement!

如果文件不存在，程序末尾的 printf 读取yn=0"，如果文件存在则读取yn=".有人可以帮我吗?

The printf at the end of the program reads "yn=0" if the file doesn't exist and just "yn=" if it does exist. Can anybody help me?

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

int main(void) {
    FILE *inf;
    FILE *outf;
    char filename[21],outfilename[21];
    char yn='0';

    printf("Please enter an input filename: ");
    scanf("%s",&filename);

    printf("Please enter an output filename: ");    
    scanf("%s",&outfilename);

    /* Open file for reading */
    inf=fopen (filename,"r");
    outf=fopen(outfilename,"r");

    /*check that input file exists*/
    if (inf!=NULL) {

        /*check that the output file doesn't already exist*/
        if (outf==NULL){
            fclose(outf);
            /*if it doesn't already exist create file by opening in "write" mode*/
            outf=fopen(outfilename,"w");
        } else {
            /*If the file does exist, give the option to overwrite or not*/
            printf("Output file already exists, overwrite (y/n):");
            scanf("%c",&yn);
        }
    }
    printf("
 yn=%c 
",yn);
    return 0;
}

推荐答案

printf("Please enter an output filename: ");    
scanf("%s",&outfilename);

当您输入第二个字符串并按 ENTER 键时，输入缓冲区中会放置一个字符串和一个字符，它们分别是:输入的字符串和换行符.该字符串被 scanf 但换行符保留在输入缓冲区中.

When you enter the second string and hit the ENTER key, a string and a character are placed in the input buffer, they are namely: the entered string and the newline character.The string gets consumed by the scanf but the newline remains in the input buffer.

进一步，

scanf("%c",&yn);

您下一个用于读取字符的 scanf 只是读取/消耗换行符，因此从不等待用户输入.

Your next scanf for reading the character just reads/consumes the newline and hence never waits for user input.

解决方案是使用额外的换行符:

Solution is to consume the extra newline by using:

scanf(" %c", &yn);
      ^^^   <------------Note the space

或者通过使用 getchar()

您可能想在此处查看我的回答以获取详细的分步说明问题.

You may want to check out my answer here for a detailed step by step explanation of the problem.

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