Defective Binary search in C - c

Question

Currently, I have a program the implements insertion sort and then uses binary search to search it (an array of ints). I currently have a 1 off error it seems.
My insertion sort should sort in descending order. Right now, it seems as if the value stored in the last position is missing.
#include <stdio.h>
void insertionSort(int nums[], int size)
{
int i, key, j;
for (i = 1; i < size; i++)
{
key = nums[i];
j = i - 1;
/* Move elements of arr[0..i-1], that are
greater than key, to one position ahead
of their current position */
while (j >= 0 && nums[j] > key)
{
nums[j + 1] = nums[j];
j = j - 1;
}
nums[j + 1] = key;
}
}
int binarySearch(int nums[], int size, int searchVal)
{
int l = 0, r = size - 1;
while (l <= r)
{
int m = l + (r - l) / 2;
// Check if x is present at mid
if (nums[m] == searchVal)
return m;
// If x greater, ignore left half
if (nums[m] < searchVal)
l = m + 1;
// If x is smaller, ignore right half
else
r = m - 1;
}
// if we reach here, then element was
// not present
return -1;
}
int main()
{
int n;
printf("Enter the number of elements (between 1 and 50) in the array: \n");
scanf("%d", &n);
int i, nums[n];
printf("Enter %d positive integers: \n", n);
for (i = 0; i < n; i++)
{
scanf("%d", &nums[i]);
}
int x = 0;
insertionSort(nums, n);
printf("Enter a positive integer or -1 to quit: \n");
scanf("%d", &x);
do
{
int ind = binarySearch(nums, n, x);
if (ind > 0)
{
printf("Found\n");
}
else
{
printf("Not Found\n");
}
printf("Enter a positive integer or -1 to quit: \n");
scanf("%d", &x);
} while (x != -1);
return 0;
}
Results:
Enter the number of elements (between 1 and 50) in the array:
9
Enter 9 positive integers:
7
4
10
49
6
12
32
17
Enter a positive integer or -1 to quit:
4
Not Found
Enter an positive integer -1 or to quit
12
Found
Enter a positive integer or -1 to quit:
5
Not Found
Enter a positive integer or -1 to quit:
49
Found
Enter a positive integer or -1 to quit:
-1
As you can see everything works but the first test where I test for the number 4. Does anyone know why I'm off by 1?

Answer 1

#include <stdio.h>
void insertionSort (int nums[], int size)
{
int i, key, j;
for (i = 1; i < size; i++) {
key = nums[i];
j = i - 1;
/* Move elements of arr[0..i-1], that are
greater than key, to one position ahead
of their current position */
while (j >= 0 && nums[j] > key) {
nums[j + 1] = nums[j];
j = j - 1;
}
nums[j + 1] = key;
}
}
int binarySearch (int nums[], int size, int searchVal)
{
int l = 0, r = size - 1;
while (l <= r) {
int m = l + (r - l) / 2;
// Check if x is present at mid
if (nums[m] == searchVal)
return m;
// If x greater, ignore left half
if (nums[m] < searchVal)
l = m + 1;
// If x is smaller, ignore right half
else
r = m - 1;
}
// if we reach here, then element was
// not present
return -1;
}
int main ()
{
int n;
printf
("Enter the number of elements (between 1 and 50) in the array: \n");
scanf ("%d", &n);
int i, nums[n];
printf ("Enter %d positive integers: \n", n);
for (i = 0; i < n; i++) {
scanf ("%d", &nums[i]);
}
int x = 0;
insertionSort (nums, n);
printf ("Enter a positive integer or -1 to quit: \n");
scanf ("%d", &x);
do {
int ind = binarySearch (nums, n, x);
if (ind >= 0) {
printf ("Found\n");
} else {
printf ("Not Found\n");
}
printf ("Enter a positive integer or -1 to quit: \n");
scanf ("%d", &x);
} while (x != -1);
return 0;
}
Try this one
just one error that is solve- if(ind>=0)

Answer 2

While you have discovered that checking the (ind > 0) causes failure to find the elements at index 0 and (ind >= 0) provides a solution, let's look at a short test case that validates the binarySearch() finds all elements of your sorted array without the user having to continually enter or redirect information to your program.
Any time you are testing algorithms, provides a simple validation framework that tests all elements, numbers, etc. without a user input requirement. Take your '9' numbers for example. Simply including an initialized array and sorting and then looping over all values will validate whether the binarySearch() is performing as intended. (it will save you no end of grief in the process as well). A short code exercising your search over the complete array can be as simple as:
int main (void) {
int nums[] = { 7, 4, 10, 49, 6, 12, 32, 17, 21 },
n = sizeof nums / sizeof *nums;
insertionSort (nums, n);
for (int i = 0; i < n; i++)
printf (" %2d (%s)\n", nums[i],
binarySearch (nums, n, nums[i]) >= 0 ? "found" : "not found");
}
Example Use/Output
$ ./bin/bsearchtest
4 (found)
6 (found)
7 (found)
10 (found)
12 (found)
17 (found)
21 (found)
32 (found)
49 (found)
Each element is now listed as "found" or "not found" and the code exits on its own.