Why start + (end – start)/2 is preferable method for calculating middle of an array over (start + end)/2 ?

I am very sure that everyone is able to find middle index of array once you know start index and end index of array, but there are certain benefits of using start + (end – start)/2 over (start + end)/2, which are described below :

The very first way of finding middle index is 

mid = (start + end)/2

But there is a problem with this approach, what if the value of start or end or both is INT_MAX, it will cause integer overflow.
The better way of calculating mid-index is : 

mid = start + (end - start)/2        //mid-index

Let’s try these both methods in different programming languages :  

// C++ program for calculating mid of array
#include <bits/stdc++.h>
using namespace std;

// driver program
int main(){
    int start = INT_MAX, end = INT_MAX;
      cout<<"start = "<<start<<endl;
      cout<<"end = "<<end<<endl;

    // method 1
    int mid1 = (start + end) / 2;
      cout<<"mid using (start + end)/2 = "<<mid1<<endl;

    // method 2
    int mid2 = start + (end - start) / 2;
      cout<<"mid using start + (end - start)/2 = "<<mid2<<endl;
    return 0;
}

// program for calculating mid of array
#include <stdio.h>
#include <limits.h>
int main()
{
    int start = INT_MAX, end = INT_MAX;
    printf("start = %dn", start);
    printf("end = %dn", end);

    // method 1
    int mid1 = (start + end) / 2;
    printf("mid using (start + end)/2 = %dn", mid1);

    // method 2
    int mid2 = start + (end - start) / 2;
    printf("mid using start + (end - start)/2 = %dn", mid2);
    return 0;
}

/*package whatever //do not write package name here */

import java.io.*;

class GFG {
    public static void main (String[] args) {
        int start = Integer.MAX_VALUE;
        int end = Integer.MAX_VALUE;
        
        System.out.println("start = " + start);
        System.out.println("end = " + end);
        
        // Method 1
        int mid1 = (start + end) / 2;
        System.out.println("mid using (start + end) / 2 = " + mid1);
        
        // Method 2
        int mid2 = start + (end - start) / 2;
        System.out.println("mid using start + (end - start) / 2 = " + mid2);

    }
}

# Function to find the midpoint using two different methods
def find_midpoint(start, end):
    print("start =", start)
    print("end =", end)

    # Method 1: Using (start + end) / 2
    mid1 = (start + end) // 2  # Using integer division to ensure the result is an integer
    print("mid using (start + end) / 2 =", mid1)

    # Method 2: Using start + (end - start) / 2
    mid2 = start + (end - start) // 2  # Using integer division to ensure the result is an integer
    print("mid using start + (end - start) / 2 =", mid2)

# Main function
def main():
    start = 2147483647  # Maximum value for int in Python
    end = 2147483647  # Maximum value for int in Python
    find_midpoint(start, end)

# Execute main function
if __name__ == "__main__":
    main()

using System;

class Program
{
    static void Main()
    {
        int start = int.MaxValue, end = int.MaxValue;
        Console.WriteLine("start = " + start);
        Console.WriteLine("end = " + end);

        // method 1
        int mid1 = (start + end) / 2;
        Console.WriteLine("mid using (start + end)/2 = " + mid1);

        // method 2
        int mid2 = start + (end - start) / 2;
        Console.WriteLine("mid using start + (end - start)/2 = " + mid2);
    }
}
// This code is contributed by Yash Agarwal(yashagarwal2852002)

// JavaScript program for calculating mid of array

// Driver program
function main() {
  let start = 2147483647;
  let end = 2147483647;

  console.log("start = " + start);
  console.log("end = " + end);

  // method 1
  let mid1 = Math.floor((start + end) / 2);
  console.log("mid using (start + end)/2 = " + mid1);

  // method 2
  let mid2 = start + Math.floor((end - start) / 2);
  console.log("mid using start + (end - start)/2 = " + mid2);
}

// Calling the main function
main();

start = 2147483647
end = 2147483647
mid using (start + end)/2 = -1
mid using start + (end - start)/2 = 2147483647

The time complexity of this program is O(1) as it only performs a fixed number of arithmetic operations and print statements, regardless of the input size.

The space complexity of this program is also O(1) as it only uses a fixed number of integer variables and constant string literals for the print statements, regardless of the input size.

Note : (end – start) may overflow if end < 0 or start < 0
If you see the output, by using second method you get correct output and first method fails to calculate mid and if you use this index (-1 in this case), it can cause segmentation fault because of invalid index of array.

start + (end – start)/2 also works even if you are using pointers : 

Example : 
Method 1  

#include <iostream>

int main() {
    int s = 2, e = 3;
    int* start = &s;
    int* end = &e;
    int* mid = start + (end - start) / 2;
    
    // Output
    std::cout << "Start: " << *start << std::endl;
    std::cout << "End: " << *end << std::endl;
    std::cout << "Mid: " << *mid << std::endl;

    return 0;
}

#include <stdio.h>

int main() {
    int s = 2, e = 3;
    int* start = &s;
    int* end = &e;
    int* mid = start + (end - start) / 2;
    
    // Output
    printf("Start: %d\n", *start);
    printf("End: %d\n", *end);
    printf("Mid: %d\n", *mid);

    return 0;
}

public class Main {
    public static void main(String[] args) {
        int s = 2, e = 3;
        int[] start = new int[]{s};
        int[] end = new int[]{e};
        int[] mid = new int[]{start[0] + (end[0] - start[0]) / 2};

        // Output
        System.out.println("Start: " + start[0]);
        System.out.println("End: " + end[0]);
        System.out.println("Mid: " + mid[0]);
    }
}

class Main:
    @staticmethod
    def main():
        s = 2
        e = 3
        start = [s]
        end = [e]
        mid = [start[0] + (end[0] - start[0]) // 2]

        # Output
        print("Start:", start[0])
        print("End:", end[0])
        print("Mid:", mid[0])

# Run the main method
Main.main()

let s = 2, e = 3;
let start = s;
let end = e;
let mid = Math.floor((start + end) / 2);

// Output
console.log("Start: " + start);
console.log("End: " + end);
console.log("Mid: " + mid);

Start: 2
End: 3
Mid: 2

Output : 

error: invalid operands of types ‘int*’ and ‘int*’ to binary ‘operator+’
     int *mid = (start + end)/2;

Method 2  

#include <iostream>

int main() {
    // Initializing start and end pointers
    int s = 2, e = 3;
    int* start = &s;
    int* end = &e;

    // Calculating mid pointer using pointer arithmetic
    int* mid = start + (end - start) / 2;

    // Displaying the values
    std::cout << "start = " << *start << std::endl;
    std::cout << "end = " << *end << std::endl;
    std::cout << "mid = " << *mid << std::endl;

    return 0;
}

int s = 2, e = 3;
int* start = &s;
int* end = &e;
int* mid = start + (end - start) / 2;

public class Main {
    public static void main(String[] args) {
        // Initializing start and end pointers
        int s = 2, e = 3;
        int[] start = {s};
        int[] end = {e};

        // Calculating mid pointer using pointer arithmetic
        int[] mid = {start[0] + (end[0] - start[0]) / 2};

        // Displaying the values
        System.out.println("start = " + start[0]);
        System.out.println("end = " + end[0]);
        System.out.println("mid = " + mid[0]);
    }
}
//this code is contributed by Monu.

# Initializing start and end pointers
s = 2
e = 3
start = s
end = e

# Calculating mid pointer
mid = start + (end - start) // 2

# Displaying the values
print("start =", start)
print("end =", end)
print("mid =", mid)
#this code is contributed by monu.

using System;

class Program
{
    static void Main()
    {
        // Initializing start and end pointers
        int s = 2;
        int e = 3;

        // Initializing start and end pointers
        int start = s;
        int end = e;

        // Calculating mid pointer
        int mid = start + (end - start) / 2;

        // Displaying the values
        Console.WriteLine("start = " + start);
        Console.WriteLine("end = " + end);
        Console.WriteLine("mid = " + mid);
    }
}

// Main function
function main() {
    // Initializing start and end pointers
    let s = 2, e = 3;
    let start = [s];
    let end = [e];

    // Calculating mid pointer using pointer arithmetic
    let mid = [start[0] + (end[0] - start[0]) / 2];

    // Displaying the values
    console.log("start = " + start[0]);
    console.log("end = " + end[0]);
    console.log("mid = " + mid[0]);
}

// Call the main function
main();

start = 2
end = 3
mid = 2

Explanation: pointer addition is not supported in C while pointer subtraction is supported, the reason being the result of subtraction is the difference (in array elements) between the operands. The subtraction expression yields a signed integral result of type ptrdiff_t (defined in the standard include file STDDEF.H)(in short subtraction gives memory distance), but addition of two pointers in not meaningful, that’s why not supported

References : 
1) Additive Operators: + and – 
2) why-prefer-start-end-start-2-over-start-end-2-when-calculating-the 
3) pointer-addition-vs-subtraction

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Article Tags :

• Arrays
• DSA

Practice Tags :

• Arrays