Difference between concatenation of strings using (str += s) and (str = str + s)

A string is a collection of characters. For example, “GeeksforGeeks” is a string. C++ provides primitive data types to create a string. The string can also be initialized at the time of declaration.

Syntax:

string str;

string str = “GeeksforGeeks”

Here, “GeeksforGeeks” is a string literal.

This article shows the difference between the concatenation of the strings using the addition assignment operator (+=) and the addition (+) operator used with strings. Concatenation is the process of joining end-to-end.

Addition assignment (+=) operator

In C++, a string addition assignment operator is used to concatenate one string to the end of another string.

Syntax:

str += value

Here, 
value is a string to be concatenated with str.

It appends the value (literal) at the end of the string, without any reassignment.

Example: Below is the C++ program to demonstrate the addition assignment operator.

C++

// C++ program to implement
// the above approach
#include <iostream>
using namespace std;

// Driver code
int main()
{

    // Declaring an empty string
    string str = "Geeks";

    // String to be concatenated
    string str1 = "forGeeks";

    // Concatenate str and str1
    // using addition assignment operator
    str += str1;

    // Print the string
    cout << str;
    return 0;
}

Java

// Java program to implement
// the above approach
import java.util.*;
class GFG{

// Driver code
public static void main(String[] args)
{

    // Declaring an empty String
    String str = "Geeks";

    // String to be concatenated
    String str1 = "forGeeks";

    // Concatenate str and str1
    // using addition assignment operator
    str += str1;

    // Print the String
    System.out.print(str);
}
}

// This code is contributed by 29AjayKumar

Python3

# Python code for the above approach
# Driver code

# Declaring an empty string
str = "Geeks";

# String to be concatenated
str1 = "forGeeks";

# Concatenate str and str1
# using addition assignment operator
str += str1;

# Print the string
print(str);

# This code is contributed by gfgking

C#

// C# program to implement
// the above approach
using System;

public class GFG{

// Driver code
public static void Main(String[] args)
{

    // Declaring an empty String
    String str = "Geeks";

    // String to be concatenated
    String str1 = "forGeeks";

    // Concatenate str and str1
    // using addition assignment operator
    str += str1;

    // Print the String
    Console.Write(str);
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>
        // JavaScript code for the above approach
        // Driver code

        // Declaring an empty string
        let str = "Geeks";

        // String to be concatenated
        let str1 = "forGeeks";

        // Concatenate str and str1
        // using addition assignment operator
        str += str1;

        // Print the string
        document.write(str);

  // This code is contributed by Potta Lokesh
    </script>

Output

GeeksforGeeks

Addition(+) operator

In C++, a string addition operator is used to concatenate one string to the end of another string. But in this case the after the concatenation of strings, the modified string gets assigned to the string.

Syntax:

str = str + value

Here, 
value is a string to be concatenated with str.

It firstly appends the value (literal) at the end of the string and then reassigns it to str.

Example: Below is the C+ program to demonstrate the above approach.

C++

// C++ program to implement
// the above approach
#include <iostream>
using namespace std;

// Driver code
int main()
{

    // Declaring an empty string
    string str = "Geeks";

    // String to be concatenated
    string str1 = "forGeeks";

    // Concatenate str and str1
    // using addition operator
    str = str + str1;

    // Print the string
    cout << str;
    return 0;
}

Java

// Java program to implement
// the above approach

class GFG{

// Driver code
public static void main(String[] args)
{

    // Declaring an empty String
    String str = "Geeks";

    // String to be concatenated
    String str1 = "forGeeks";

    // Concatenate str and str1
    // using addition operator
    str = str + str1;

    // Print the String
    System.out.print(str);
}
}

// This code is contributed by 29AjayKumar

Python3

# Python program to implement
# the above approach

# Driver code

# Declaring an empty string
str1 = "Geeks"

# String to be concatenated
str2 = "forGeeks"

# Concatenate str and str1
# using addition operator
str1 += str2

# Print the string
print(str1)

# This code is contributed by phasing17

C#

// C# program to implement
// the above approach
using System;
public class GFG {

  // Driver code
  public static void Main(String[] args) {

    // Declaring an empty String
    String str = "Geeks";

    // String to be concatenated
    String str1 = "forGeeks";

    // Concatenate str and str1
    // using addition operator
    str = str + str1;

    // Print the String
    Console.Write(str);
  }
}

// This code is contributed by umadevi9616

Javascript

// JavaScript program to implement
// the above approach

// Driver code
function main() {
    // Declaring an empty string
    let str = "Geeks";

    // String to be concatenated
    let str1 = "forGeeks";

    // Concatenate str and str1
    // using addition operator
    str = str + str1;

    // Print the string
    console.log(str);
}

main();

// Contributed by adityashae15

Output

GeeksforGeeks

Although both operators when used with strings can be used for the concatenation of strings, there are some differences between them:

Factor 1: Assignment of the modified string:

• The addition assignment operator (+=) concatenates two strings by appending one string at the end of another string.
• The addition operator(+) concatenates two strings by appending one string at the end of the original string and then assigning the modified string to the original string.

Example: Below is the C++ program to demonstrate the above approach.

C++

#include <iostream>
using namespace std;

int main()
{

    // Declaring an empty string
    string str = "Geeks";

    // String to be concatenated
    string str1 = "forGeeks";

    // Concatenate str and str1
    // using addition assignment operator
    // Concatenate str1 at the end of str
    str += str1;

    // Print the string
    cout << "Resultant string using += "
         << str << '\n';

    str = "Geeks";

    // Concatenate str and str1
    // using addition operator
    // Concatenate str and str1
    // and assign the result to str again
    str = str + str1;

    // Print the string
    cout << "Resultant string using + "
         << str;
    return 0;
}

Python3

# Python equivalent of above C++ code
# Declaring an empty string
str = "Geeks"

# String to be concatenated
str1 = "forGeeks"

# Concatenate str and str1
# using addition assignment operator
# Concatenate str1 at the end of str
str += str1

# Print the string
print("Resultant string using += ",str)

str = "Geeks"

# Concatenate str and str1
# using addition operator
# Concatenate str and str1
# and assign the result to str again
str = str + str1

# Print the string
print("Resultant string using + ",str)

C#

using System;

class Program {
  static void Main(string[] args) {

    // Declaring an empty string
    string str = "Geeks";

    // String to be concatenated
    string str1 = "forGeeks";

    // Concatenate str and str1
    // using addition assignment operator
    // Concatenate str1 at the end of str
    str += str1;

    // Print the string
    Console.WriteLine("Resultant string using += " + str);

    // Reset str
    str = "Geeks";

    // Concatenate str and str1
    // using addition operator
    // Concatenate str and str1
    // and assign the result to str again
    str = str + str1;

    // Print the string
    Console.WriteLine("Resultant string using + " + str);
  }
}

Java

public class StringConcatenation {
    public static void main(String[] args) {
        // Declaring an empty string
        String str = "Geeks";

        // String to be concatenated
        String str1 = "forGeeks";

        // Concatenate str and str1 using +=
        // Concatenate str1 at the end of str
        str += str1;

        // Print the string
        System.out.println("Resultant string using += " + str);

        // Reset str to its original value
        str = "Geeks";

        // Concatenate str and str1 using +
        // Concatenate str and str1
        // and assign the result to str again
        str = str + str1;

        // Print the string
        System.out.println("Resultant string using + " + str);
    }
}

Javascript

// Declaring an empty string
let str = "Geeks";

// String to be concatenated
let str1 = "forGeeks";

// Concatenate str and str1 using +=
// Concatenate str1 at the end of str
str += str1;

// Print the string
console.log("Resultant string using += " + str);

// Reset str to its original value
str = "Geeks";

// Concatenate str and str1 using +
// Concatenate str and str1
// and assign the result to str again
str = str + str1;

// Print the string
console.log("Resultant string using + " + str);

Output

Resultant string using += GeeksforGeeks
Resultant string using + GeeksforGeeks

Factor 2: Operator overloaded functions used:

• The addition assignment operator (+=) concatenates two strings because the operator is overloaded internally.
• In this case, also, the addition operator (+) concatenates two strings because the operator is overloaded internally.

Factor 3: Number of strings concatenated:

• The addition assignment operator (+=) can concatenate two strings at a time in a single statement.
• The addition operator (+) can concatenate multiple strings by using multiple addition (+) operators between the string in a single statement. For example, str = str₁ + str₂ + str₃ + … + str_n

Example: In this program, three different statements are required to concatenate three strings; str, str1, str2, and str3 using the assignment addition operator (+=) and a single statement is required to concatenate three strings; str, str1, str2, and str3 using the addition operator (+). 

C++

// C++ program to implement
// the above approach
#include <iostream>
using namespace std;

// Driver code
int main()
{

    // Declaring an empty string
    string str = "GeeksforGeeks";

    // String to be concatenated
    string str1 = " GeeksforGeeks";

    // String to be concatenated
    string str2 = " GeeksforGeeks";

    // String to be concatenated
    string str3 = " GeeksforGeeks";

    // Concatenate str, str1, str2 and str3
    // using addition assignment operator
    // in multiple statements
    str += str1;

    str += str2;

    str += str3;

    // Print the string
    cout << "Resultant string using +="
         << str << '\n';

    str = "GeeksforGeeks";

    // Concatenate str, str1, str and str3
    // using addition operator
    // in a single statement
    str = str + str1 + str2 + str3;

    // Print the string
    cout << "Resultant string using + "
         << str;
    return 0;
}

Java

// Java program to implement
// the above approach
import java.io.*;

class GFG {

// Driver code
public static void main (String[] args)
{

    // Declaring an empty string
    String str = "GeeksforGeeks";

    // String to be concatenated
    String str1 = " GeeksforGeeks";

    // String to be concatenated
    String str2 = " GeeksforGeeks";

    // String to be concatenated
    String str3 = " GeeksforGeeks";

    // Concatenate str, str1, str2 and str3
    // using addition assignment operator
    // in multiple statements
    str += str1;

    str += str2;

    str += str3;

    // Print the string
    System.out.println("Resultant string using +="
         +str);

    str = "GeeksforGeeks";

    // Concatenate str, str1, str and str3
    // using addition operator
    // in a single statement
    str = str + str1 + str2 + str3;

    // Print the string
    System.out.print("Resultant string using + "
         + str);
}
}

//this code is contributed by shivanisinghss2110

Output

Resultant string using +=GeeksforGeeks GeeksforGeeks GeeksforGeeks GeeksforGeeks
Resultant string using + GeeksforGeeks GeeksforGeeks GeeksforGeeks GeeksforGeeks

Factor 4: Performance:

• The addition assignment operator (+=) when used for the concatenation of strings gives better efficiency as compared to the addition(+) operator. This is because no reassignment of strings takes place in this case.
• The addition operator (+)  when used for the concatenation of strings, is less efficient as compared to the addition (+=) operator. This is because the assignment of strings takes place in this case.

Example: Below is the program to demonstrate the performance of the += string concatenation method.

C++

// C++ program to calculate
// performance of +=
#include <bits/stdc++.h>
#include <sys/time.h>
using namespace std;

// Function whose time is to
// be measured
void fun()
{
    // Initialize a n empty string
    string str = "";

    // concatenate the characters
    // from 'a' to 'z'
    for (int i = 0; i < 26; i++) {
        char c = 'a' + i;
        str += c;
    }
}

// Driver Code
int main()
{
    // Use function gettimeofday()
    // can get the time
    struct timeval start, end;

    // Start timer
    gettimeofday(&start, NULL);

    // unsync the I/O of C and C++.
    ios_base::sync_with_stdio(false);

    // Function Call
    fun();

    // Stop timer
    gettimeofday(&end, NULL);

    // Calculating total time taken
    // by the program.
    double time_taken;

    time_taken = (end.tv_sec
                  - start.tv_sec)
                 * 1e6;

    time_taken = (time_taken
                  + (end.tv_usec
                     - start.tv_usec))
                 * 1e-6;

    cout << "Time taken by program is : "
         << fixed
         << time_taken << setprecision(6);
    cout << " sec" << endl;
    return 0;
}

Output

Time taken by program is : 0.000046 sec

Example: Below is the program to demonstrate the performance of the + string concatenation method.

C++

// C++ program to calculate
// performance of +
#include <bits/stdc++.h>
#include <sys/time.h>
using namespace std;

// Function whose time is to
// be measured
void fun()
{
    // Initialize a n empty string
    string str = "";

    // concatenate the characters
    // from 'a' to 'z'
    for (int i = 0; i < 26; i++) {

        char c = 'a' + i;
        str = str + c;
    }
}

// Driver Code
int main()
{
    // Use function gettimeofday()
    // can get the time
    struct timeval start, end;

    // Start timer
    gettimeofday(&start, NULL);

    // unsync the I/O of C and C++.
    ios_base::sync_with_stdio(false);

    // Function Call
    fun();

    // Stop timer
    gettimeofday(&end, NULL);

    // Calculating total time taken
    // by the program.
    double time_taken;

    time_taken = (end.tv_sec
                  - start.tv_sec)
                 * 1e6;

    time_taken = (time_taken
                  + (end.tv_usec
                     - start.tv_usec))
                 * 1e-6;

    cout << "Time taken by program is : "
         << fixed
         << time_taken << setprecision(6);
    cout << " sec" << endl;
    return 0;
}

Output

Time taken by program is : 0.000034 sec

S No.	Factor	+= operator	+ operator
1	Assignment	It appends a string at the end of the original string.	It appends a string at the end of the original string and then reassigns the modified string to the original string.
2	Overloaded functions	operator overloaded function used with strings is different from the += operator.	operator overloaded function used with strings is different from the + operator.
3	Number of strings concatenated	It can concatenate two strings at a time in a single statement.	Multiple strings can be concatenated using multiple addition (+) operators between the string. For example, str = str1 + str2 + str3 + … + strn
4	Performance	This operator when used for the concatenation of strings gives better efficiency as compared to the addition(+) operator. This is because no reassignment of strings takes place in this case.	This operator when used for the concatenation is not as efficient as compared to the addition(+=) operator.  This is because reassignment of strings takes place in this case.