Question
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");.
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!
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("\n yn=%c \n",yn);
return 0;
}
Answer
printf("Please enter an output filename: ");
scanf("%s",&outfilename);
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.
Further,
scanf("%c",&yn);
Your next scanf for reading the character just reads/consumes the newline
and hence never waits for user input.
Solution is to consume the extra newline by using:
scanf(" %c", &yn);
^^^ <------------Note the space
Or by using getchar()
You may want to check out my answer here for a detailed step by step explanation of the problem.
