Weird issue when making an array of random ints in C - arrays

Question

I am currently learning C and I need to write a function to create an array of random integers. I've hit a problem where after creating I try to print and it the first 8 numbers correctly but the rest don't.
int* create(int n) {
int* array = malloc(n);
if (!array) return NULL;
srand(time(NULL));
for (int i = 0; i < n; i++) {
array[i] = rand() % 100 + 1;
printf("num: %i\n", array[i]);
}
for (int i = 0; i < n; i++) {
printf("%i\n", array[i]);
}
return array;
}
Here is my output for this:
num: 39
num: 2
num: 15
num: 74
num: 80
num: 29
num: 14
num: 16
num: 8
num: 11
num: 2
39
2
15
74
80
29
14
16
973747761
909588276
2614

Answer 1

This memory allocation
int* array = malloc(n);
allocates not enough memory for an array with n elements of the type int, You have to write
int* array = malloc( n * sizeof( int ) );
Also the parameter should have unsigned integer type. Otherwise the user can pass a negative integer that will result in undefined behavior.
It is better to declare the parameter as having the type size_t. It is the type of the parameter of the function malloc.
And the function should do one thing: allocate and initialize an array. It is the caller of the function that will decide whether to output the array provided that the function did not return a null pointer.
So the function can look like
int * create( size_t n )
{
const int MAX_VALUE = 100;
int *array = malloc( n * sizeof( int ) );
if ( array != NULL )
{
srand( ( unsigned int )time( NULL ) );
for ( size_t i = 0; i < n; i++ )
{
array[i] = rand() % MAX_VALUE + 1;
}
}
return array;
}
Here is a demonstrative program.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int * create( size_t n )
{
const int MAX_VALUE = 100;
int *array = malloc( n * sizeof( int ) );
if ( array != NULL )
{
srand( ( unsigned int )time( NULL ) );
for ( size_t i = 0; i < n; i++ )
{
array[i] = rand() % MAX_VALUE + 1;
}
}
return array;
}
int main(void)
{
size_t n = 0;
printf( "Enter the size of an array: " );
scanf( "%zu", &n );
int *array = create( n );
if ( array != NULL )
{
for ( size_t i = 0; i < n; i++ )
{
printf( "%d ", array[i] );
}
putchar( '\n' );
}
free( array );
return 0;
}
Its output might look like
Enter the size of an array: 10
75 36 30 75 53 49 42 52 61 9
Though it is better to declare the function such a way that the user can determine the maximum value himself. That is the function can look like
int * create( size_t n, int max_value )
{
int *array = malloc( n * sizeof( int ) );
if ( array != NULL )
{
srand( ( unsigned int )time( NULL ) );
for ( size_t i = 0; i < n; i++ )
{
array[i] = rand() % max_value + 1;
}
}
return array;
}

Answer 2

Issues with your code:
malloc expects a size_t, you are directly giving an int. So you should technically do:
int *array = malloc(sizeof(int) * n);
This basically says that allocate n blocks of sizeof(int) bytes each.
What you are doing is allocating n bytes of data. The program is not guaranteed to run always and you are running out of bounds. If you were lucky you would get a Segmentation Fault in one of the runs.

Answer 3

You aren't allocating enough space. malloc(n) allocates n bytes. You need space for n ints! Use malloc(n * sizeof(int)), or more preferably: malloc(n * sizeof(*array)) (so you don't need to repeat the type) instead.