C: Function to generate random string and add it t

Question

Coming from some experience with Java/C#, I'm struggling to get my head around some parts of C. I have an array of structs. These structs, at the moment, have only one member (I'll be adding others down the track) - an array of chars. This array is a string of 6 characters - two letters, four digits (e.g. XY1234).

The follow code produces the objectCode I'm looking for:

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

#define LETTERS_IN_ALPHABET 26

typedef struct object {
    char objectCode[6];
} object_t;

void GetRandomSeed() {
    srand((unsigned)time(NULL));
}

char RandomLetter() {
    return 'A' + rand() % LETTERS_IN_ALPHABET;
}

int RandomDigit() {
    return rand() % 10;
}

int main() {

    GetRandomSeed();

    object_t object1;

    for (int i = 0; i < 2; i++) {
        object1.objectCode[i] = RandomLetter();
    }

    for (int i = 2; i < 6; i++) {
        object1.objectCode[i] = '0' + RandomDigit();
    }

    // Print objectCode string to screen
    for (int i = 0; i < 6; i++) {
            printf("%c", object1.objectCode[i]);
    }

    printf("\n");

    return 0;
}

I'm trying to wrap the for loops (the ones randomly generating the code) in a function. However, the print out of my attempt to do this is just gibberish:

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

#define LETTERS_IN_ALPHABET 26

typedef struct object {
    char objectCode[6];
} object_t;


void GetRandomSeed() {
    srand((unsigned)time(NULL));
}

char RandomLetter() {
    return 'A' + rand() % LETTERS_IN_ALPHABET;
}

int RandomDigit() {
    return rand() % 10;
}

void GenerateCode(object_t object) {
    for (int i = 0; i < 2; i++) {
        object.objectCode[i] = RandomLetter();
    }

    for (int i = 2; i < 6; i++) {
        object.objectCode[i] = '0' + RandomDigit();
    }
}

int main() {

    GetRandomSeed();

    object_t object1;

    // Print objectCode string to screen
    for (int i = 0; i < 6; i++) {
            printf("%c", object1.objectCode[i]);
    }

    printf("\n");


    return 0;
}

I'm bit of an infant when it comes to C so I really appreciate any help you can give. Thanks.

Answer 1

You're missing the call to GenerateCode on main, that's the reason the print output is gibberish.

However another issue is that on the GenerateCode function, the parameter is by value and the function won't modify the original struct on main. In C, all and every parameters are by value. You should pass a pointer to the struct:

void GenerateCode(object_t* object) {
    for (int i = 0; i < 2; i++) {
        object->objectCode[i] = RandomLetter();
    }

    for (int i = 2; i < 6; i++) {
        object->objectCode[i] = '0' + RandomDigit();
    }
}

int main() {

    // ...
    object_t object1;

    GenerateCode(&object1);
}

Answer 2

Because you pass in an object by value to GenerateCode() the function modifies only the value on the stack (ie: the parameter itself), not the actual object you want to change. use a pointer:

void GenerateCode(object_t* object) {
for (int i = 0; i < 2; i++) {
    object->objectCode[i] = RandomLetter();
}

for (int i = 2; i < 6; i++) {
    object->objectCode[i] = '0' + RandomDigit();
}
}

and call it like:

GenerateCode(&object1);