CODEWITHSUNDEEP
cfileloopsinput
David Tamrazov
David Tamrazov

Reputation: 597

File processing in C; won't take input

I'm starting to learn file processing in C. The point of this specific program is to make a file called "clients.dat" where I store the account number, name, and balance of clients at a bank, lets say. I've worked and refined the code so that its a perfect replica of what the textbook provides as an example, yet for some reason mine loops endlessly after the first "scanf" and reprints question marks unto oblivion, without ever making it to the scanf statement inside the while loop. Would anyone have any suggestions as to why this is happening? My compiler is Netbeans and I'm running it on Linux-Ubuntu.

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

/*
 * 
 */
int main() {

    unsigned int actNumber;
    char actName[30];
    long double actBalance;

    FILE *fPtr;

    if((fPtr = fopen("clients.dat", "w")) == NULL) {
        printf("File could not be found.\n");
    }

    else {
        printf("Enter the Account Number, Name, and Balance.\n Hit the EoF to exit.\n");
        printf("%s","?");
        scanf("%d%29s%lf", &actNumber, actName, &actBalance);

        while (!feof(stdin)) {            
            fprintf(fPtr, "%d, %29s, %.2lf\n", actNumber, actName, actBalance);
            printf("%s", "?");
            scanf("%d%29s%lf", &actNumber, actName, &actBalance);                        
        }

        fclose(fPtr);
    }
    return;
}

Upvotes: 0

Views: 84

Answers (1)

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53026

The end of file marker is set on stdin only when you press a special key combination on the console.

You can make your loop work correctly by using the return value of scanf(), like this

while (scanf("%d%29s%lf", &actNumber, actName, &actBalance) == 3) 
{            
    fprintf(fPtr, "%d, %29s, %.2lf\n", actNumber, actName, actBalance);
    printf("%s", "?");
}

After the first scanf() a '\n' character is left in the input stream, when you call scanf() again inside the loop, the character is consumed then ignored, and scanf() fails returning a value that is less than 3, the process is repeated over and over causing the infinite loop.

The following solution, however, is better. Using fgets() allows a better handiling of the '\n' character left unread in the stdin,

char line[100];
while (fgets(line, sizeof(line), stdin) != NULL)
 {
    if (sscanf(line, "%d%29s%lf", &actNumber, actName, &actBalance) != 3)
        continue;
    fprintf(fPtr, "%d;%29s;%.2lf\n", actNumber, actName, actBalance);
    fprintf(stdout, "?");
 }

note that I remove the white spaces in the printf() format, and replaced the , with ; because in some locales the , is the decimal separator, it's just instintictive not to use it, you can use it if you ensure that . is the decimal separator.

Upvotes: 2

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