How to compare number with another number in an Array? [c]

Why it doesn't work and what is wrong with the code?

• You are comparing against just one value rather than all the array elements.
• The value of i after the scanf loop is 10 so arr[i] would exceed the array and could cause Illegal memory access.

Check the comments in the program.

#define ARR_SIZE 10
int main()
{
    int my_arr[ARR_SIZE];  //Use ARR_SIZE because if ARR_SIZE changes, 10 won't causing unforseen errors.
    int secnum = 0;
    int i = 0;

    for (i = 0; i < ARR_SIZE ; i++)
    {
        printf("Enter a number: ");
        scanf("%d",&my_arr[i]);
    }

    printf("Enter the another number");
    scanf("%d",&secnum);

    for (i = 0; i < ARR_SIZE ; i++) // Ensures you are comparing secnum with each array element.
    {
        if(my_arr[i] == secnum)
        {
            printf("an ex"); //Do you wish to break here because one you find a match, the goal is attained :)
        }
    }
}

After the loop, i is equal to ARR_SIZE (10). So you compare my_arr[10] with secnum (0). But my_arr[10], while syntactically correct, points to an undefined value because the size of the array is 10.

#define ARR_SIZE 10

 int main()
{
int my_arr[10];
int secnum = 0;
int i = 0;

for (i=0;i<ARR_SIZE ; i++)
{
    printf("Enter a number: ");
    scanf("%d",&my_arr[i]);
}
printf("Enter the another number");
scanf("%d",&secnum);

for (i=0;i<ARR_SIZE ; i++)
{
    if(my_arr[i]==secnum)
    {
        printf("Given number is in array\n");
        break;
    }
}

}

As pointed by OP in comments to one of the answers, OP apparently need a routine to check for the key in an array.

So once we have stored an array and have accepted a key to search, we need to pass this array and key to a search function which will return true or false depending upon whether the key is present in the array or not.

#include <stdbool.h>    // We need this for `true` and `false` bool values

bool search(int [], int);    // Function declaration

/**** Function Definition *****/
bool search(int numbers[], int key)
{
    int i;

    for(i = 0; i < ARR_SIZE; i++)
        if(numbers[i] == key)
            return true;

    return false;
}  

/** Calling search function from main **/  
...
if(search(my_arr, secnum))
    printf("Number found in array!\n");
else
    printf("Number could NOT be found in array!\n");

To find a value in an array you should iterate through it and compare each value with the wanted number. You should also check the return value of scanf() to control how many item did it actually read. See if this reviewed code is helpfull:

#include <stdio.h>

#define ARR_SIZE 10

int read_numbers(int a[], int size) {
    int i = 0;
    int r = 0;
    while ( i < size ) {
        printf("Please, enter a number (%d of %d): ",i+1,size);
        r = scanf(" %d",&a[i]);         // the space before will ignore any trailing newline
        if ( r == EOF ) break;          // if scanf fails return
        if ( r != 1 ) {                 // if user don't enter a number, repeat
            printf("Wrong input!\n");
            scanf("%[^\n]*");            // will read and ignore everything lweft on stdin till newline
            continue;
        }
        ++i;
    }
    return i;       // return size, unless scanf fails
}

int find_number(int a[], int size, int x) {
    int i = 0;
    while ( i < size  &&  a[i] != x ) ++i;
    return i;   // if x isn't in the array returns size
}

int main()
{
    int my_arr[ARR_SIZE];
    int secnum = 0;
    int i = 0;
    int n = 0;

    n = read_numbers(my_arr, ARR_SIZE);
    if ( n < ARR_SIZE ) {
        printf("Warning, only %d numebers read out of %d!\n",n,ARR_SIZE);
    }  // if happens you have uninitialized elements in array

    printf("Now, enter the value you want to find.\n");
    if ( read_numbers(&secnum, 1) != 1 ) {      // reuse the function
        printf("Sorry, an error has occurred.\n");
        return -1;
    }

    i = find_number(my_arr, n, secnum);     // If all went right n==ARR_SIZE
    if ( i < n ) {              // a match has been found
        printf("Found!\n");
    } else {
        printf("Not found.\n");
    }
    return 0;
}