This algorithm finds element in an ordered array which is i=A[i]

findEqualIndexVal.cpp

#include <stdlib.h>
#include <stdio.h>

int findEqualIndex(int arr[],int start,int end);

int main(){

	int arr[6]={-10,-5,-1,0,4,5};
	int arr2[5]={0,2,5,8,7};
	int res = findEqualIndex(arr,0,5);
	printf("Res:%d\n",res);
	return 0;
}

int findEqualIndex(int arr[],int start,int end){

	int mid= (start+end)/2;
	for(int i=start;i<=end;++i){
		printf("%d-",arr[i]);
	}
	printf("\nStart:%d - End%d\n",start,end);
	printf("mid:%d, arr[mid]:%d\n",mid,arr[mid]);

	if(start>end){
		return -1;
	}
	if(mid==arr[mid]){
		printf("mid=arr[mid]\n");
		return mid;
	}else if(mid>arr[mid]){
		printf("mid:%d>arr[mid]:%d\n",mid,arr[mid]);
		return findEqualIndex(arr,mid+1,end);
	}else if(mid<arr[mid]){
		printf("mid:%d<arr[mid]:%d\n",mid,arr[mid]);
		return findEqualIndex(arr,start,mid-1);
	}else return -1;
}