Why does my arithmetic mean function always return 0?

Question

In the code below, the function calculo() doesn't return any values. It was supposed to return the arithmetic mean between cont and aluno variables, but it always returns 0.

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

int i;

int calculo(int alunos, int *p) {
    int cont = 0;
    for (i = 0; i < alunos; i++)
        cont += p[i];
    return cont / alunos;
}

int main()
{
    int *p, alunos, media;

    printf("Digite a quantidade de alunos na turma \n");
    scanf("%d", &alunos);

    p = (int*)calloc(alunos, sizeof(int));

    if (p == NULL) {
        printf("Memória Insuficiente");
        return 1;
    }
    srand(time(NULL));
    for (i = 1; i < alunos + 1; i++) {
        *p = rand() % 10;
        printf("Nota do Aluno [%d]: \t %d \n", i, *p);
    }

    media = calculo(alunos, p);
    printf("A Media Aritimetica da turma e:\t %d", media);
    getche();
    free(p);
    return 0;
}

What am I doing wrong?

What did you observe when inspecting the values using your debugger? You are aware how integer value division works? — πάντα ῥεῖ, Oct 20 '18 at 20:55
Why is this question tagged as "C++", when it doesn't actually have a single line of C++ code, only a bunch of old, crufty, C? — Sam Varshavchik, Oct 20 '18 at 20:56
Take a look at how you're reading in your numbers, and where you store them — 1201ProgramAlarm, Oct 20 '18 at 21:02
The problem is only reading data into one element of `p`. Also be aware that your function does integer division for `cont/alunos`, which ALWAYS rounds toward zero. So the average of `1` and `2` (calculated as `(1 + 2)/2` in your function will give a result of `1`, not `1.5`. — Peter, Oct 20 '18 at 21:14
@LeonardoJob Welcome to StackOverflow. As pointed out, your program is completely in C style and not C++...so the `#include ` is not used. It's very important on this site to distinguish in your questions whether you intend to use C style *(e.g. `printf("...")` and `malloc(...)` and `strcpy(...)`)* or C++ style *(e.g. `std::cout << "..."` and `new ...` and `std::string`)*, because it deeply affects the answers you will get. You should also know that conio.h is very old, [and not standard](https://stackoverflow.com/q/34474627/)--should be avoided in questions here. — HostileFork says dont trust SE, Oct 20 '18 at 21:15
@HostileFork Thanks for all the help, there are so many things wrong that I didnt even know, thanks for pointing it out. — Leonardo Job, Oct 20 '18 at 21:21
@LeonardoJob Sure thing. Be persistent, StackOverflow can sometimes be hard for new users...but it's just about the best way to get better that there is. If you want to learn a language, it can be very useful to monitor a tag and see the other questions go by. You learn fast--not just about the language, but about what makes questions get treated well vs. downvoted and complained about. Like I said, it helps to make a strong decision if you want to be pursuing [tag:c] or [tag:c++], maybe following those tags for a while would help you decide which is more your style. — HostileFork says dont trust SE, Oct 20 '18 at 21:26

score 3 · Accepted Answer · answered Oct 20 '18 at 21:02

3

srand(time(NULL));
for (i = 1; i < alunos + 1; i++) {
    *p = rand() % 10;  // <<=========================== here
    printf("Nota do Aluno [%d]: \t %d \n", i, *p);
}

you are assigning to *p aka p[0] over and over again. What you perhaps want to do is

srand(time(NULL));
for (i = 0; i < alunos; ++i) {
    p[i] = rand() % 10;
    printf("Nota do Aluno [%d]: \t %d \n", i + 1, p[i]);
}

answered Oct 20 '18 at 21:02

Swordfish

12,971
3
21
43

It worked, thank you so much!, But i dont think i get it. If p[i] stores the number inside the array, so what would *p store then?. – Leonardo Job Oct 20 '18 at 21:12
In C and C++ `something[index]` is just syntactic sugar for `*(something + index)` ... so `*p` which is the same as `*(p+0)` could be read as `p[0]`. – Swordfish Oct 20 '18 at 21:13

Why does my arithmetic mean function always return 0?

1 Answers1