Reading a specific string using Scanf()

Question

I'm triying to read a string with a specific format in C using scanf() The string has the format:

<LET,LET,LET> op

where LET is a capital letter and op has to be '+' or '%'.

These are valid entries:

<A,B,C> +
<A,W,Z> %
<Q,   X,W>    +

These are not:

<A,b,C> +
<A,W,Zddddd> %
<Q,X,W> *

I'm trying something like this

#include <stdio.h>

int ret = 0;
char str[8];
ret = scanf("%8[^\n]",str);

but str ends up with garbage. I just don't know how to read it and how to get only capital letters.

Thanks

Sorry.A string has the format: <LET,LET,LET> op where LET is a capital letter and op has to be '+' or '%'. — Telefang
– Telefang, Commented Apr 27, 2016 at 4:40
Your example has + and * as op, and all three are numbers in the bracket. — fluter
– fluter, Commented Apr 27, 2016 at 4:42

BLUEPIXY · Accepted Answer · 2016-04-27 04:53:10Z

4

try this:

#include <stdio.h>

int main(void) {
    char let1[2], let2[2], let3[2], op[2];
    char line[80];
    while(fgets(line, sizeof line, stdin)){
        if(sscanf(line, "<%1[A-Z], %1[A-Z], %1[A-Z]> %1[+%]", let1, let2, let3, op) == 4)
            puts("valid");
        else
            puts("invalid");
    }

    return 0;
}

answered Apr 27, 2016 at 4:53

BLUEPIXY

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5 Comments

Telefang Over a year ago

Why line is [80] and why each let is [2]?

BLUEPIXY Over a year ago

[80] is my console's size of width. Please change to your required size.(However, in 8 it is small) [2] : one character + null-terminator.

Telefang Over a year ago

This works great. But it doesnt work if I ad a space BEFORE a comma. Should I add a space after the comma on each %1[A-Z], ?

BLUEPIXY Over a year ago

If the compiler does not allow A-Z, replace it with ABCDEFGHIJKLMNOPQRSTUVWXYZ.

BLUEPIXY Over a year ago

@Telefang Try adding a space before and after the comma. (E.g %1[A-Z] , %1[A-Z]) It seems to work for me.

chux · Accepted Answer · 2016-04-27 22:15:38Z

1

This method also detects trail garbage on the line.

"%n" saves the offset of the current scan. Only if n is non-zero and indexes '\0' was there success.

By using string literal concatenation, code can clearly show what format is being used for the various parts of the scan.

#include <stdio.h>

#define F_LET " %1[A-Z]"
#define F_SEP " ,"
#define F_OP  " %1[+%]"

int main(void) {
  char buf[100];
  while (fgets(buf, sizeof buf, stdin)) {
    char let[3][2];
    char op[2];
    int n = 0;
    sscanf(buf, " <" F_LET F_SEP F_LET F_SEP F_LET " >" F_OP " %n", 
        let[0], let[1], let[2], op, &n);

    puts((n && buf[n] == '\0') ? "Success" : "Fail");
  }
}

answered Apr 27, 2016 at 22:15

chux

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Comments

dreamlax · Accepted Answer · 2016-04-27 05:57:26Z

An alternative to BLUEPIXY's perfectly good answer, is to simply use char, and scan the input using %c instead, e.g.:

#include <stdio.h>
#include <ctype.h>

int main()
{
    char let1, let2, let3, op;
    char line[80]; // or some other suitable size

    while (fgets(line, sizeof line, stdin))
    {
        if (sscanf(line, "< %c , %c , %c > %c", &let1, &let2, &let3, &op) == 4 && (op == '+' || op == '%') && isupper(let1) && isupper(let2) && isupper(let3))
        {
            printf("valid!\nlet1 = %c\nlet2 = %c\nlet3 = %c\nop = %c\n", let1, let2, let3, op);
        }
        else
        {
            puts("invalid");
        }
    }
}

Note that the spaces before and after the %c are important as they allow sscanf to gobble up all whitespace before trying to match the %c or the comma.

Unlike BLUEPIXY's answer however, we have to check that op matches + or % manually.

EDIT: I missed the part where the LET characters must be uppercase. I've fixed the answer, but of course, it is now more complicated.

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