[Ethereum] How to concatenate strings in solidity

contract-developmentsoliditystring

I'm trying to concatenate two strings in solidity.

I tried with + or . but neither option works. There is not much documentation on string concatenation yet. Is it even possible?

If not, can I insert variables into strings?

Best Answer

As of Feb 2022, in Solidity v0.8.12 you can now concatenate strings in a simpler fashion!

string.concat(s1, s2)

Taken directly from the solidity docs on strings and bytes:

Solidity does not have string manipulation functions, but there are third-party string libraries. You can also compare two strings by their keccak256-hash using keccak256(abi.encodePacked(s1)) == keccak256(abi.encodePacked(s2)) and concatenate two strings using string.concat(s1, s2).

contract Poem {
    string public text;

    function addLine(string memory added_text) public {
           text = string.concat(text,added_text);
    }
}

The above code snippet is provided by Hannu-Daniel Goiss on his article about string concatenation.

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[Ethereum] How to serialize an array of strings in Solidity

This is not going to work in Solidity. The += operator is not implemented for strings, you need to use a library that allows to concatenate strings; I think the most popular is this one. There is an example of how to use concatenation here.

I'm not sure what you're trying to do, but that kind of concatenation is expensive. Wouldn't it be enough to have two methods:

getStringsLength() returns (uint) and getStringByIndex(uint index) returns (string), and retrieve the strings in a loop? You can then do the concatenation outside of the contract (probably cheaper).

Solidity:

string[] myArray;

function getStringsLength() returns (uint) {
    return myArray.length;
}

function getStringByIndex(uint index) returns (string) {
    if (index < myArray.length){
        return myArray[index];
    }
    return "";
}

Solidity-consumer pseudocode:

strings_to_serialize = []
for (int i=0; i<contract.getStringsLength(); i++)
    strings_to_serialize.append(contract.getStringByIndex(i))
# process the strings here

Best Answer

Related Solutions

[Ethereum] How to serialize an array of strings in Solidity

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