strange c array

I wrote a small snippet of C code. Ignore the strange "sentence" name. My question is, why is it that when I call strlen on this array, it prints 5, instead of 2? Thanks.

Code:

int main(){
char sentence [2];
sentence[0]= 'y';
sentence[1]='o';
int len= strlen(sentence);
printf("%d", len);
}

Because you didn't leave room for the null terminator, leading to a buffer overflow.

Code:

int main(){
    char sentence [3];
    sentence[0]= 'y';
    sentence[1]= 'o';
    sentence[2]= 0;
    int len= strlen(sentence);
    printf("%d", len);
    return 0;
}

Strings end with a null byte. Your code needs to be changed to:

Code:

int main(void){
    char sentence [3];
    size_t len;
    sentence[0] = 'y';
    sentence[1] = 'o';
    sentence[2] = '\0';
    len= strlen(sentence);
    printf("%u", len);
    return 0;
}

Before fixing it, change your program to this to see what you get:

Code:

int main(){
unsigned char sentence [2];
printf("1: sen[0]=%01x, sen[1]=%01x, sen[2]=%01x\n", sentence[0], sentence[1], sentence[2]) ;
sentence[0]= 'y';
sentence[1]='o';
printf("2: sen[0]=%02x, sen[1]=%02x, sen[2]=%02x\n", sentence[0], sentence[1], sentence[2]) ;
int len= strlen(sentence);
printf("%d", len);
}

You'll note that storage is not initialized, unless you specifically tell the compiler (or runtime) to initialize it.

use %zu to print size_t, or %lu and cast to unsigned long in C89.