Text looks full of junk after typed

Question

I started creating a data collection and usage application with the following code:

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

int main() {
    int idade = 0;
    char nome[20];

    printf("Hello! How old are you? ");
    scanf("%d", &idade);
    while (idade == 0) {
        printf("\nAge cannot be 0 nor be decimal. Re-enter your age: ");
        scanf("%d", &idade);
    }
    printf("And what's your name? (write up to 20 characters, only 1st name) ");
    scanf("%[^\n]s", nome);
    printf("\nHello %s, aged %d!\n", nome, idade);

    return 0;
}

But, after compiling with GCC, the output is:

Hello! How old are you? 123
And what's your name? (write up to 20 characters, only 1st name) 
Hello �����dU��m�, aged 123!

What I have to change?

Answer 1

The code has some problems. I’ll solve some, but it’s still not robust code because it doesn’t test other problems, I won’t go into those issues.

One problem is that to read text with white space you need to use fgets(), and needs to treat the line change that must be switched to a terminator. Which leads to another problem: if you want 20 characters you need to reserve 21 bytes of buffer. And the scanf() need to receive a pattern with the new line to leave no dirt on buffer.

At least this is a solution:

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

int main() {
    int idade = 0;
    printf("Hello! How old are you? ");
    scanf("%d\n", &idade);
    while (idade == 0) {
        printf("\nAge cannot be 0 nor be decimal. Re-enter your age: ");
        scanf("%d", &idade);
    }
    char nome[21];
    fgets(nome, 20, stdin);
    nome[strcspn(nome, "\n")] = 0;
    printf("\nHello %s, aged %d!\n", nome, idade);
}

Behold working in the ideone. And in the replit. Also put on the Github for future reference.

It is possible to use a trick with the scanf(), but understand that in real codes this function is rarely used for free data entry:

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

int main() {
    int idade = 0;
    printf("Hello! How old are you? ");
    scanf("%d\n", &idade);
    while (idade == 0) {
        printf("\nAge cannot be 0 nor be decimal. Re-enter your age: ");
        scanf("%d", &idade);
    }
    char nome[21];
    scanf(" %[^\n]s", nome);
    printf("\nHello %s, aged %d!\n", nome, idade);
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Answer 2

Problem

This problem is very common for those who are starting or are not very much on the subject, and is related to the form with the scanf with %d works.

If you repair the scanf with %d does not consume line break, and so allows you to do something like:

int x, y, z;
scanf("%d", &x);
scanf("%d", &y);
scanf("%d", &z);

And then enter the values separated by space, like this:

10 20 30

It’s important to realize that you just pushed Enter once but managed to make 3 readings, for the simple reason that they do not consume you line break.

See this example in Ideone

So in your code, first read age:

scanf("%d", &idade);

But the Enter was in the input buffer and when will read the nome with:

scanf("%[^\n]s", nome);

The first and only thing he gets is Enter, because reading with %[^\n] it’s till I catch \n.

Solution

The solution is to consume the line break. It has at least two simple ways to do it:

• Including a space in the next reading:

  scanf(" %[^\n]s", nome);
  //     ^--- este espaço consome a quebra de linha anterior
  

  See this example in Ideone

• Using fgetc:

  fgetc(stdin); //aqui consome a quebra
  scanf("%[^\n]s", nome); // agora lê normalmente
  

  See also this example in Ideone

Answer 3

Hello, just add this line before the name scanf setbuf(stdin, NULL);

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

int main() {

int idade = 0;
char nome[20];

printf("Hello! How old are you? ");
scanf("%d", &idade);
while (idade == 0) {
    printf("\nAge cannot be 0 nor be decimal. Re-enter your age: ");
    scanf("%d", &idade);
}
printf("And what's your name? (write up to 20 characters, only 1st name) ");
setbuf(stdin, NULL);    
scanf("%[^\n]s", nome);
printf("\nHello %s, aged %d!\n", nome, idade);

return 0;

}