How to perform string concatenation in Solidity

Explanation

Line 3: We create a contract named Test.
Line 4: We create a function named concatenate.
Line 5: We call the string.concat() function.

Using the `abi.encodePacked()` function

In the Ethereum community, some ABI-encoded functions are available which can be used without calling another contract. We can concatenate strings with one of the ABI-encoded functions: abi.encodePacked(). This is mostly used for Solidity versions earlier than v0.8.12.

The abi.encodePacked() function returns the answer in byte type, therefore, we’ll cast it into the string type to get the desired output.

Explanation

Line 3: We create a contract named Test.
Line 4: We create a function named concatenate.
Line 5: We call the abi.encodePacked() function.

Using an iterative approach

In this method, we create our own function which performs the concatenation operation.

First, we convert our two strings into byte arrays. These resulting byte arrays contains the ASCII representation of the characters in the string. Then we create a new string that stores the concatenated content. The memory value of this new string will be equal to the sum of the Byte length of the two strings. Next, we convert the new string into byte array. We iterate the byte values of the first string and save its content in the new string byte value. Similarly, we iterate the byte values of the second string and save its content in the new string byte value. As a result, the new string contains all the concatenated data.

The testConcatenate function takes two string inputs (first and second), concatenates them using the concatenate function from the Function library, and returns the result as the output. This provides a convenient way to test the concatenate function and see how it works.

pragma solidity ^0.5.0;
library Function {
   function concatenate(string memory s1, string memory s2) pure internal returns (string memory) {
       bytes memory s1Bytes = bytes(s1);
       bytes memory s2Bytes = bytes(s2);
      
       string memory s3 = new string(s1Bytes.length + s2Bytes.length);
       bytes memory s3Bytes = bytes(s3);
      
       uint i;    
       uint j;
 
       for(i=0;i<s1Bytes.length;i++) {
           s3Bytes[j++] = s1Bytes[i];
       }
       
       for(i=0;i<s2Bytes.length;i++) {
           s3Bytes[j++] = s2Bytes[i];
       }
      
       return string(s3Bytes);
   }
}
contract Test {
  
   using Function for string;
  
   function testConcatenate(string memory first,string memory second) public pure returns (string memory) {
       return first.concatenate(second);
   }
}

Explanation

Line 3: We create a library named Function.
Line 4: We create a function named concatenate.
Line 5: We convert s1 in bytes memory.
Line 6: We convert s2 in bytes memory.
Line 8: We create a new string to store the concatenated data whose memory value is equal to the sum of the byte length of s1 and s2.
Line 9: We convert s3 in bytes memory.
Line 11: We create an unsigned integer, i, used for the iteration of s1 and s2.
Line 12: We create an unsigned variable, j, used for storing values in s3.
Line 14: We iterate byte values of s1.
Line 15: We copy the content of s1 in s3.
Line 18: We iterate byte values of s2.
Line 19: We copy the content of s2 in s3.
Line 22: We cast the byte value of s3 in string and return it.
Line 25: We create a contract named Test.
Line 27: We use Function for string.
Line 29: We create a function named testConcatenate.
Line 30: We return first.concatenate(second).