Understanding why it does not find duplicates in array - c

Question

I wrote the following function in C:
int last(long arr[], int length) {
for (int i = 0; i < length-1; i++)
if (*(arr+i) == *(arr + length - 1))
return 1;
return 0;
}
it checks if the last value of the array was used more than once. In the main:
int *arr = malloc(length*sizeof(int));
for (int i = 0; i < length; i++)
scanf("%d", ++arr);
printf(last((long *) arr, length);
For some reason for the array [1,2,2,3] it returns that the last element was used multiple times and I'm not sure why. I think that is because of scanf("%d", ++arr); but I don't know how to fix it.
My goal is that it will return 1 for [1,3,2,3] and 0 for [1,2,2,3]. What could be the problem?

Answer 1

You should use scanf("%d", &arr[i]);. Using ++arr causes the array to be incremented before you pass it to last, and also reads into data beyond arr, which is undefined behavior.
Another one of the issues in this is the cast to long *.
You should use %ld in scanf and long *arr = malloc(length*sizeof(*arr));.
Also make sure to check for NULL. You never know when malloc is going to fail or someone's going to pass bad data.
Full example:
#include <stdio.h>
#include <stdlib.h>
int last(long arr[], int length) {
if(!arr) return -1;
for (int i = 0; i < length-1; i++)
{
if (arr[i] == arr[length-1])
return 1;
}
return 0;
}
int main(void)
{
long *arr = malloc(4*sizeof(*arr));
if(!arr) return 1;
for (int i = 0; i < 4; i++)
scanf("%ld", &arr[i]);
printf("%d\n", last(arr, 4));
}

Answer 2

Several problems in your code:
Look at this statement:
scanf("%d", ++arr);
^^^^^
In the last iteration of loop, the pointer arr will be pointing to one element past end of array arr (due to pre-increment) and it is is passed to scanf(). The scanf() will access the memory location pointed by the pointer which is an invalid memory because your program does not own it. This is undefined behavior. Note that a pointer may point to one element past the end of array, this is as per standard but dereferencing such pointer will lead to undefined behavior.
Once the main() function for loop finishes the arr pointer pointing to location past the end of memory allocated to arr and just after this you are passing arr to last() function. So, you are passing an invalid memory reference to last() function and then accessing that memory in last() function - one more undefined behavior in your program.
Probably you should take another pointer and point it to arr, so that arr keep pointing to allcoated memory reference returned by malloc().
Note that if you want to read the input the way you are doing then use the post-increment operator in scanf(), like this:
int *arr = malloc(length*sizeof(int));
if (arr == NULL)
exit(EXIT_FAILURE);
int *ptr = arr;
for (int i = 0; i < length; i++)
scanf("%d", ptr++);
but the more appropriate and readable way is - scanf("%d", &arr[i]).
Another big problem in your code is accessing the int values as long type.
The last() function parameter arr type is long and you are passing it int pointer typecasted to long *.
Note that the size of long and int may be different based on the platform. You cannot assume them to be of same size on all platforms.
Assume the case where int size is 4 bytes and long size is 8 bytes.
In this case, when accessing an int pointer using long type pointer then every object will be considered as 8 byte long and when you do arr+1 in last(), the pointer will be advance by 8 bytes and you will never get correct result.
Compiler must be throwing warning message on this statement:
printf(last((long *) arr, length);
because the printf() expects first argument as const char * and you are passing it int (return type of last()). You should give the first argument to printf() a string which contains appropriate format specifier('s).
Putting these altogether:
#include <stdio.h>
#include <stdlib.h>
int last(int arr[], int length) {
if (arr == NULL) {
return 1;
}
for (int i = 0; i < length - 1; i++) {
if (arr[i] == arr[length - 1]) {
return 1;
}
}
return 0;
}
int main(void) {
int length = 4;
int *arr = malloc (length * sizeof (*arr));
if (arr == NULL) {
exit(EXIT_FAILURE);
}
printf ("Enter %d numbers:\n", length);
for (int i = 0; i < length; i++) {
scanf ("%d", &arr[i]);
}
printf ("Duplicate found: %s\n", last (arr, length) == 1 ? "Yes" : "No");
return 0;
}