Storing an array in C - c

Question

Context: I need to write a program that will accept inputs which will be stored into the array. Before storing in to the array, the inputted number must be checked if it already exists in the array or not. If it does not exist, it is stored into the array. If it exists, another input will be asked.
Now, my code will get inputs from the user, but the code will only work for the first input. It won't work for the second until the last input. Any pointers?
This is my code:
#include<stdio.h>
#define size 5
main()
{
int i;
arr[size];
input;
printf("This program will accept ");
printf("unique inputted numbers that will be stored");
printf(" in an array\n");
for(i = 0;i < size;i++)
{
printf("Enter input: ");
scanf("%d",&input);
if (unique(arr,input,i))
arr[i] = input;
else
i--;
//decrement i because ask for input again
}
for(i = 0;i < size;i++)
printf("%d ",arr[i]);
}
int unique(int arr[],int input,int i)
{
int n, z;
n = 0;
z = 1;
while(i > n)
{
if(arr[n] == input)
{
scanf("%d",&n);
z = 0;
break;
}
else
n=1;
break;
}
return z;
}

Answer 1

Your code is wrong at multiple levels:
The logic in the unique function is wrong.
Doing the scanf in the unique function is extremely bad design. The only thing unique should do is return 0 if input is already in the array.
You have used implicit variable declarations here: arr[size]; input;, it should be int arr[size]; int input;.
You should use descriptive variable names which makes your code easier to understand.
This is a working example (explanations in comments).
#include <stdio.h>
#define SIZE 5 // use capitals for macros (this is a convention)
int unique(int arr[], int value, int arrsize)
{
for (int i = 0; i < arrsize; i++)
{
if (arr[i] == value)
{
return 0; // value found in array
}
}
return 1; // value not found in array
}
void Test(int arr[], int arrsize, int value, int expected)
{
if (unique(arr, arrsize, value) != expected)
printf("Test failed for value %d\n", value);
}
void runtests()
{
int arr[] = { 1,2,3 };
Test(arr, 4, sizeof(arr) / sizeof(*arr), 1);
Test(arr, 1, sizeof(arr) / sizeof(*arr), 0);
Test(arr, 3, sizeof(arr) / sizeof(*arr), 0);
}
#define size 5
int main()
{
int i;
int arr[size]; // declare int variable
int input; // declare int variable
printf("This program will accept unique inputted numbers that will be stored in an array\n");
for (i = 0; i < size; i++)
{
printf("Enter input %d: ", i + 1);
scanf("%d", &input);
if (unique(arr, input, i)) // value already in the array?
arr[i] = input; // no => put it there
else
{ // yes => ask again
printf(" >> %d is already in the array\n");
i--;
}
}
for (i = 0; i < size; i++)
printf("%d ", arr[i]);
}
There are two more functions Test and runtests in this code. They are not called by this code, but they can be very useful for debugging. As an exercise try to understand why they can be useful during the debug phase of your code.

Answer 2

You're close, but overcomplicating it slightly.
Let's take a step back and think about this at a high level. You want to store unique inputs in the array, up to the size of the array. In pseudocode:
while array not full
prompt for and read next input
if input not already in array
store input
else
write a message
end if
end while
What's really key is that you only need one input statement - your unique function should only check for the presence of the input value in the array and return true or false. It shouldn't do any input of its own.
So your main loop is more like
while ( i < size )
{
fputs( "Gimme a number: ", stdout );
/**
* Break out of the loop if there's an error
* on input.
*/
if ( scanf( "%d", &input ) != 1 )
break;
if ( unique( arr, i, input ) )
arr[i++] = input;
else
printf( "%d already exists in the array, try again.\n", input );
}
All your unique function needs to do is cycle through the elements of the array. By calling unique with i instead of size it will only check array elements that have been written to so far and not bother with unassigned elements. This way you don't have to make sure that all of the array elements have been initialized to some known, out-of-band value that's guaranteed to compare unequal to any valid input.
You'll need to compile against C99 or later and include stdbool.h to use the bool type and the true and false constants.
#include <stdbool.h>
...
bool unique( int *arr, size_t size, int input )
{
bool result = true;
for( size_t i = 0; i < size && result; i++ )
if ( arr[i] == input )
result = false;
return result;
}
If you want to get really terse, you could directly assign the result of the Boolean expression to result:
for ( size_t i = 0; i < size && result; i++ )
result = (arr[i] == input);
but people will hit you. It's perfectly valid code, but a little eye-stabby, and most programmers aren't used to seeing Boolean expressions outside of an if, for, while, or switch statement control expression.
Finally, some suggestions:
Fix your formatting. The compiler doesn't care, but it makes it easier for other people to understand what you're trying to do and to spot mistakes.
The presence of main() in your code suggests you're using C89 or K&R C. C99 did away with implicit int declarations. You really should define main as either int main( void ) or int main( int argc, char **argv ). Furthermore, you should move to a compiler that supports later versions of C (C11 or C18).