Wrong Random Numbers C

Question

I'm building a program that simulates a bus going through bus stops and picking up a random amount of passengers (0-15) the problem is that when i try to print the amount of passengers that got in on a bus stop i get a lot of numbers bigger than 15.

Here's a part of my program:

#include <stdio.h>
#include <stdlib.h>
 
struct Node {
    int data;
    struct Node* next;
};
 

void printList(struct Node* n)
{
    while (n != NULL) {
        printf(" %d ", n->data);
        n = n->next;
    }
}
 
int main()
{
    struct Node*ΤΣ_ΚΤΕΛ = NULL;
    struct Node*ΓΕΦΥΡΑ = NULL;
    ΤΣ_ΚΤΕΛ = (struct Node*)malloc(sizeof(struct Node));
    ΓΕΦΥΡΑ = (struct Node*)malloc(sizeof(struct Node));
    ΤΣ_ΚΤΕΛ->data = rand()%15+1;
    ΤΣ_ΚΤΕΛ->next = ΓΕΦΥΡΑ;

     printList(ΓΕΦΥΡΑ);
 
    return 0;
}

[cannot reproduce](https://godbolt.org/z/73x78vdhq) after seeding the `rand` function and replacing the unknown with `NULL` — yano, Apr 18 '22 at 16:58
I don't see where `ΓΕΦΥΡΑ` is defined. Please provide a full example :) — Aziz, Apr 18 '22 at 16:58
Are you naming variables with non-ascii character? I'd say it's a bad idea. How do you even compile it? — Eugene Sh., Apr 18 '22 at 17:00
This code snippet runs and show the problem im facing , it prints a lot of numbers that are all random and big — Ted, Apr 18 '22 at 17:05
this is not a [mcve]. I should be able to take a MCVE source and compile it. — OldProgrammer, Apr 18 '22 at 17:07
You `malloc` space for `ΓΕΦΥΡΑ` but don't set any of its fields. You're printing an uninitialized `data` field, and who knows if `next` is NULL or not. This invokes undefined behavior. `malloc` simply reserves memory, it's up to you to set its contents. With these issues cleaned up, `rand` performs exactly as it should, your trouble is not in that line. — yano, Apr 18 '22 at 17:09
@EugeneSh.: The C standard explicitly permits identifiers to have “universal character name” characters, as well as implementation-defined characters, in C 2018 6.4.2 1. If somebody is writing for a worldwide community, you might have grounds to advise them to limit the characters they use, but people writing for themselves or their own communities should feel free to use their own alphabets and words without regard to your preferences. — Eric Postpischil, Apr 18 '22 at 17:09
OK, so [Annex D](http://port70.net/~nsz/c/c11/n1570.html#D) is permitting some subset of unicode in identifier names. Still a bad idea though. For example this code can't be compiled with [ideone](https://ideone.com/ij4Y9w) — Eugene Sh., Apr 18 '22 at 17:10
`ΤΣ_ΚΤΕΛ->next = ΓΕΦΥΡΑ;` ==> `ΤΣ_ΚΤΕΛ->next = NULL;` — pmg, Apr 18 '22 at 17:34
Although C does permit universal character names in identifiers, with some limitations, that's a *non-sequitur*, as there are none in evidence in the code presented. It is relevant, however, that implementations may allow additional (multibyte) characters more or less as they choose. — John Bollinger, Apr 18 '22 at 17:34
You only called `rand()` once. How do you expect to have more than 1 random number? **Your output is not showing random numbers: it's showing garbage.** — pmg, Apr 18 '22 at 17:35
@n.1.8e9-where's-my-sharem.: But OP’s code is not written for this community. It would be fine to advise them to change the names when preparing a [mre] for this community, but it is inconsiderate to advise them not to use their own language generally. — Eric Postpischil, Apr 18 '22 at 17:37

score 0 · Accepted Answer · answered Apr 18 '22 at 18:08

Below has some tweaks that fix your basic issues, with explanations in the comments:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>  // you should seed the rand function to get pseudorandom numbers
                   // each time you run
 
struct Node {
    int data;
    struct Node* next;
};
 

void printList(struct Node* n)
{
    while (n != NULL) {
        printf(" %d ", n->data);
        n = n->next;
    }
}
 
int main(void)
{
    // seed the rand function with the current time, this is common
    // see link below for further info
    srand(time(NULL));
    struct Node*ΤΣ_ΚΤΕΛ = NULL;
    struct Node*ΓΕΦΥΡΑ = NULL;
    ΤΣ_ΚΤΕΛ = malloc(sizeof(struct Node));
    ΓΕΦΥΡΑ = malloc(sizeof(struct Node));

    // always check the return value of malloc
    if (ΤΣ_ΚΤΕΛ == NULL || ΓΕΦΥΡΑ == NULL)
    {
        // handle errors how you want
        fprintf(stderr, "out of memory!\n");
        exit(-1);
    }

    ΤΣ_ΚΤΕΛ->data = rand()%15+1;
    ΤΣ_ΚΤΕΛ->next = ΓΕΦΥΡΑ;
    // must fill in data for ΓΕΦΥΡΑ also
    ΓΕΦΥΡΑ->data = rand()%15+1;
    ΓΕΦΥΡΑ->next = NULL;

    // passing in ΤΣ_ΚΤΕΛ will print both structures now
    printList(ΤΣ_ΚΤΕΛ);
 
    return 0;
}

Demonstration.

Also see:

How to generate a random int in C?

Do I cast the result of malloc?

Wrong Random Numbers C

1 Answers1