在C中使用不同的数值获得相同操作的不同长度

Question

I have 2 for-loops which populate arrays with letters from the alphabet. I have a lowercase array set, and an uppercase array set. The problem is when I initialize the arrays with the letters, the lengths are coming back different.

char uppercase[26];
char lowercase[26];
int indexUpper = 0;
int indexLower = 0;


// Get uppercase array:
for(int a = 65; a <= 90; a++){
    uppercase[indexUpper] = a;
    indexUpper++;
}

// Get lowercase array:
for(int b = 97; b <= 122; b++){
    lowercase[indexLower] = b;
    indexLower++;
}

printf("UPPERCASE = %lu\n", strlen(uppercase));
printf("LOWERCASE = %lu\n", strlen(lowercase));

$=> UPPERCASE = 26
$=> LOWERCASE = 27

I apologize if this is a no brainer. I am truly trying to learn and comprehend the C language and its rules. Thanks to all who contribute.

Answer 1

strlen() reads the character array as long until it finds a NUL byte ( '\\0' , numerical value zero). Your arrays don't contain any, since you haven't assigned one there.

That means that strlen will continue reading past the end of the array , which is illegal, and the resulting behaviour is not defined . Getting a 27 is rather mild, you could be getting arbitrary numbers, or your program could crash.

If you want to use strlen() , you should explicitly assign a NUL byte at the end of the string, and of course allocate space for it.

Perhaps something like this:

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

int main(void)
{
    char upper[27];
    int i;
    for (i = 0 ; i < 26; i++) {
        /* This only works in a character set where the letters
           are contiguous */
        upper[i] = 'A' + i;
    }
    /* i == 26 now */
    upper[i] = '\0';
    printf("len: %u\n", (unsigned) strlen(upper));
    return 0;
}

(Though using strlen here at all seems somewhat pointless, since you already know the number of items in those arrays.)

Answer 2

When using strlen the char array must be nul terminated - but yours isn't so you have undefined behavior.

To print the size of the arrays try:

printf("UPPERCASE = %zu\n", sizeof uppercase);
printf("LOWERCASE = %zu\n", sizeof lowercase);