CODEWITHSUNDEEP
cscanfenter
Mif Ano
Mif Ano

Reputation: 15

C How to use sscanf properly support

I am currently learning reading from files in C. Anyway, cutting to the chase:

Text file content:

123456 James Doakes; 0
987987 Dexter Morgan; 0
010203 Masuka Perv; 0

int main()
{
char accountNr[ACCOUNTNRSIZE], ownerName[NAMESIZE], enter[3];
int accountBalance = 0;
char filename[] = "breg.txt";
FILE *file = fopen(filename, "r");
if (file != NULL) {
    char line[128];
    while (fgets(line, sizeof(line), file) != NULL) {
        sscanf(line, "%s %[^;] %d ", accountNr, ownerName, &accountBalance);
        printf("%s", ownerName);
        //fflushstdin();
    }
    fclose(file);
} else {
    perror(filename);
}
return 0;
}

I wrote this to check if the name for instance James Doakes was registered correctly :

printf("%s", ownerName);

But when it prints that out it's like the stdout is still active and I can push Enter and it will type the name again. My goal is to of course be able to sscanff the number, the full name, and the last number as seperate variables. But it obviously doesn't work. I am guessing a \n gets registered as well. Dunno, I am just speculating.

What am I doing wrong? Why? And how do I solve this?

Much appreciated, Mif

Upvotes: 1

Views: 92

Answers (1)

paxdiablo
paxdiablo

Reputation: 882586

%s %[^;] %d

means a string terminated by white space, optional white space, a sequence of characters that are not ;, optional white space, then a number.

You appear to be not scanning for the actual ; character itself so that, when you try to get the number, the ; in the input stream will cause it to fail. You can see this with:

#include <stdio.h>

#define ACCOUNTNRSIZE 100
#define NAMESIZE 100

int main (void) {
    char accountNr[ACCOUNTNRSIZE], ownerName[NAMESIZE], enter[3];
    int accountBalance = 0;
    char filename[] = "breg.txt";
    FILE *file = fopen(filename, "r");
    if (file != NULL) {
        char line[128];
        while (fgets(line, sizeof(line), file) != NULL) {
            int count = sscanf(line, "%s %[^;] %d ", accountNr, ownerName, &accountBalance);
            printf ("%d [%s] [%s] [%d]\n", count, accountNr, ownerName, accountBalance);
        }
        fclose(file);
    } else {
        perror(filename);
    }
    return 0;
}

which outputs:

2 [123456] [James Doakes] [0]
2 [987987] [Dexter Morgan] [0]
2 [010203] [Masuka Perv] [0]

In fact, even if you change the breg.txt file to be:

123456 James Doakes; 314159
987987 Dexter Morgan; 271828
010203 Masuka Perv; 42

you still get 0 for the account balance because the scanning only successfully reads two items.

Whenever you use one of the scanf-family functions, you should check the return code to ensure it's scanning the correct number of items, as in:

int count = sscanf (line, "%s %[^;] %d ", accountNr, ownerName, &accountBalance);
if (count != 3) {
    fprintf (stderr, "Catostrophic failure, count is %d\n", count);
    return 1;
}

The fix here is relatively simple, just use %s %[^;]; %d as the format string.

With that change, the output you see is:

3 [123456] [James Doakes] [314159]
3 [987987] [Dexter Morgan] [271828]
3 [010203] [Masuka Perv] [42]

Keep in mind you don't actually need a space before the %d (though it causes no harm). That particular format specifier skips white space before attempting to scan the number.

Upvotes: 3

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