C: Stop Condition Loop For

Question

Hello, could someone please explain to me what is happening in this tie stop condition for?

#include <stdio.h>

int main(void) { 

    char frase[] = "Linguagem C";
    
    for(int i = 0; frase[i]; i++) { //frase[i]?
        printf("%c", frase[i]);
    } 
    
    return 0;
}

The output is: C language
I’ve used the for several other times, but I’ve never seen a condition like this.
In the example below, it is displayed in addition to the int vector content:

#include <stdio.h>

int main(void) { 

    int vetor[] = {34, 42, -12, 984, 86, 14};
    
    for(int i = 0; vetor[i]; i++) { //A condição de parada é vetor[i]
        printf("%d\t", vetor[i]);
    }  
    
    return 0;
}

The output is: 34 42 -12 984 86 14 1 7 136112
I believe the values from 14 are probably "junk in memory". But why does it happen?

Answer 1

The syntax of a for loop has the following definition:

for( inicialização ; condição_de_saída ; incremento ) {}

Your code is:

for( inicialização ; variável ; incremento ) {}

The definition of a for loop is flexible and allows until expressions are omitted, but placing a variable where a conditional expression is expected generates undefined behavior. In your case the conditional was implicitly set, but even to iterate on a char[] it is wrong pq just a user enter a string that does not end in NULL to happen a segmentation failure by the unauthorized memory access.

Answer 2

In C, there is by default the logical type/Boolean, C uses the number 0 for false and any other value other than 0 with being true. In its first case, the char vector, '0' which is the "string" terminator in C is set to 0, so when i=11, vector[11] will be equal to 0 and the for is terminated. In his second example, the code is more obscure because it can give different results, but the idea of the for will go through the data in memory until finding a 0.