C++ Program to Concatenate Two Strings

Here you will learn and get code on string concatenation in C++. The program to concatenate string is created in following ways:

• String Concatenation without using Library Function strcat()
• Using strcat()
• Using + operator
• Using Pointer

String Concatenation without strcat()

This program concatenates or appends second string into first without using any library function like strcat():

#include<iostream>
#include<stdio.h>
using namespace std;
int main()
{
    char strOne[100], strTwo[50], i, j=0;
    cout<<"Enter the First String: ";
    gets(strOne);
    cout<<"Enter the Second String: ";
    gets(strTwo);
    for(i=0; strOne[i]!='\0'; i++)
        j++;
    for(i=0; strTwo[i]!='\0'; i++)
    {
        strOne[j] = strTwo[i];
        j++;
    }
    strOne[j] = '\0';
    cout<<"\nString after Concatenation: "<<strOne;
    cout<<endl;
    return 0;
}

This program was build and run under Code::Blocks IDE. Here is its sample run:

Now supply any string as input say codes and then another string as input say cracker. Press ENTER key to add the second string cracker to first one codes as shown in the output given below:

Here is another sample run with user input, this is as first string and codescracker as second string:

Note - Don't forgot to enter space after this is or before codescracker while entering the first or second string. Otherwise, both the string gets concatenated without space like this iscodescracker.

The following two statements:

strOne[j] = strTwo[i];
j++;

can also be replaced with:

strOne[j++] = strTwo[i];

That is, a post increment to j is used, that increments its value by 1 after evaluating the statement:
strOne[j] = strTwo[i];

If user enters first string as codes, then it gets initialized to strOne in a way that:

• First character (c) gets initialized to strOne[0]
• Similarly, strOne[1]=o, strOne[2]=d, strOne[3]=e, strOne[4]=s

And if user enters cracker as second string, then it also gets initialized to strTwo in similar way. Therefore with this two string input, the dry run of above program goes like:

• At first execution of for loop, 0 gets initialized to i and checks the condition, strOne[i]!='\0' or strOne[0]!='\0' or c!='\0'.
• The condition evaluates to be true. Because, c is not equal to a null terminated character
• Therefore program flow goes inside the loop and increments the value of j
• Then program flow goes back to update part of for loop, increments the value of i and checks for the condition again with new value of i
• This process continues until the condition evaluates to be true
• Because the first string is codes, therefore at fifth time, condition evaluates to be false, and program flow exits the loop, and goes to second for loop. Now the value of j is 5
• Therefore, from 5^th index, all the characters (one by one) of second string gets initialized to the first string in a way that:
  • First character of second string (c) gets initialized to strOne[5]
  • Second character of second string (r) gets initialized to strOne[6]
  • Similarly, strOne[7]=a, strOne[8]=c, strOne[9]=k, strOne[10]=e, strOne[11]=r
• And after exiting the loop, \0 gets initialized to strOne[12]
• Print the value of strOne that equals codescracker

String Concatenation using strcat()

The function strcat() takes two strings as argument. The value of second string gets appended or concatenated to first string. For example, if there are two string say strOne and strTwo. And the value of both strings are:

strOne = "codes";
strTwo = "cracker";

Then,

strcat(strOne, strTwo);

Means now strOne = codescracker. That is, the value of second string concatenated to first one. This function is defined in string.h library

#include<iostream>
#include<stdio.h>
#include<string.h>
using namespace std;
int main()
{
    char strOne[100], strTwo[50];
    cout<<"Enter the First String: ";
    gets(strOne);
    cout<<"Enter the Second String: ";
    gets(strTwo);
    strcat(strOne, strTwo);
    cout<<"\nString after Concatenation: "<<strOne;
    cout<<endl;
    return 0;
}

This program will produce the same output as of previous one.

String Concatenation using + Operator

The question is, write a program in C++ to concatenates two strings using + operator. The answer to this question is given below:

#include<iostream>
using namespace std;
int main()
{
    string strOne="", strTwo="", strThree="";
    cout<<"Enter the First String: ";
    cin>>strOne;
    cout<<"Enter the Second String: ";
    cin>>strTwo;
    strThree = strOne + strTwo;
    cout<<"\nString after Concatenation: "<<strThree;
    cout<<endl;
    return 0;
}

Here is its sample run with user input, codes and cracker as first and second string:

Note - Previous program will only work with string input without any space.

String Concatenation using Pointer

This is the last program, that concatenates two strings using pointer.

#include<iostream>
#include<stdio.h>
using namespace std;
int main()
{
    char strOne[100], strTwo[50];
    char *firstString, *secondString;
    cout<<"Enter the First String: ";
    gets(strOne);
    cout<<"Enter the Second String: ";
    gets(strTwo);
    firstString = strOne;
    secondString = strTwo;
    while(*firstString)
        firstString++;
    while(*secondString)
    {
        *firstString = *secondString;
        firstString++;
        secondString++;
    }
    *firstString = '\0';
    cout<<"\nString after Concatenation: "<<strOne;
    cout<<endl;
    return 0;
}

Here is its sample run with user input, C++ Program to as first string and Concatenate Two Strings as second string:

The address of both the string strOne and strTwo gets initialized to its corresponding pointers, say firstString and secondString. And based on the address, we've concatenates the second string's value to the first one. To learn more about pointer in C++, you can follow its separate tutorial

Note - The * is called as value at operator

Same Program in Other Languages

• C Concatenate String
• Java Concatenate String
• Python Concatenate String

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