Merge sort is a divide and conquer comparison-based sorting algorithm.

Algorithm:

1. If the list is of length 0 or 1, then it is already sorted. Otherwise:
2. Divide the unsorted list into two sublists of about half the size.
3. Sort each sublist recursively by re-applying the merge sort.
4. Merge the two sublists back into one sorted list.

#include<stdio.h>

void merge(float arr[], int lo, int mid, int hi){	
	int i;
	int j;
	int k;
	float temp[hi];

	// copy arr to temp
	for (i = lo; i <= hi; i++)
		temp[i] = arr[i];

	i = lo;
	j = mid + 1;
	k = lo;
    
	while (i <= mid && j <= hi)
		if (temp[i] <= temp[j])
			arr[k++] = temp[i++];
		else
			arr[k++] = temp[j++];

	// copy back remaining elements of first half (if any)
	while (i <= mid)
		arr[k++] = temp[i++];
}


void mergesort(float arr[], int lo, int hi){
	int mid;

	if (lo < hi){	
		mid = (lo + hi) / 2;
		mergesort(arr, lo, mid);
		mergesort(arr, mid + 1, hi);
		merge(arr, lo, mid, hi);
	}
}


int main(){
	int i;
	int n;
	float arr[30];

	//number of items to be sorted
	printf("How many items to be sorted:");
	scanf("%i", &n);

	//items to be sorted
	for(i = 0; i < n; i ++){
		printf("Item %i: ", i+1);	
		scanf("%f", &arr[i]);
	}

	mergesort(arr, 0, n - 1);

	for (i = 0; i < n; i++)
		printf("%.2f  ", arr[i]);

	return 0;
}