Why bytes(str).length is not enough for getting the length of a string in Solidity, and understanding the strlen method from contracts of ens. In the world of Javascript finding the length of a string is such an easy thing. Just do and that’s all 🤌 str.length But strings are not so friendly to work with, in ❗. In solidity, the string is a group of characters stored inside an array and stores the data in bytes. Solidity There is no length method in string type. I was going through Buildspace’s project and found the link to . I knew to find the length of the string in Solidity we can convert the string into bytes and find its length. So it should have been as easy as doing 🤌 but the method in this util file was a bit different: build-polygon-ens StringUtils.sol bytes(str).length; // SPDX-License-Identifier: MIT // Source: // https://github.com/ensdomains/ens-contracts/blob/master/contracts/ethregistrar/StringUtils.sol pragma solidity >=0.8.4; library StringUtils { /** * @dev Returns the length of a given string * * @param s The string to measure the length of * @return The length of the input string */ function strlen(string memory s) internal pure returns (uint256) { uint256 len; uint256 i = 0; uint256 bytelength = bytes(s).length; for (len = 0; i < bytelength; len++) { bytes1 b = bytes(s)[i]; if (b < 0x80) { i += 1; } else if (b < 0xE0) { i += 2; } else if (b < 0xF0) { i += 3; } else if (b < 0xF8) { i += 4; } else if (b < 0xFC) { i += 5; } else { i += 6; } } return len; } } It had this weird ‘for’ loop in code which I couldn’t understand. So, the developer in me googled it 🕵️♀️, but all the articles I came across did this to find the length of the string I found some similar code on Stackoverflow but no one actually explained what is happening inside. bytes(str).length; for(len = 0; i < bytelength; len++) { bytes1 b = bytes(s)[i]; if(b < 0x80) { i += 1; } else if (b < 0xE0) { i += 2; } else if (b < 0xF0) { i += 3; } else if (b < 0xF8) { i += 4; } else if (b < 0xFC) { i += 5; } else { i += 6; } } After 3 hours of 🐌 self-exploration I was able to figure it out myself (a little slow but I did it 🍾), So I thought let’s write it down so it would be helpful for all the folks like me (not so experienced with bits, bytes 0️⃣1️⃣). Let’s try to Unblock/Decode this How bytes(str).length works When we convert string to bytes this is what Solidity does: // if we do bytes("xyz"), solidity converts it as xyz -> 78 79 7a // 78=x, 79=y, 7a=z ABC -> 41 42 43 // 41=A, 42=B, 43=C Use this for converting strings to bytes website If you see each character generates 1 byte that’s why when we do bytes(””).length we get the length of the string. But there are some characters for which generated bytes are more than one. For example: € -> e2 82 ac For the symbol of the Euro, generated bytes are 3. So if we try to find the length of string which includes the symbol of Euro( ) 🤑 in it, the length returned by will not return the correct string length for this character as there are 3 bytes generated: € bytes(str).length € That’s when that ‘for’ loop we've seen above comes to the rescue ⛑️ Let’s iterate over this e2 82 ac bytes array and check what’s happening inside that loop: for(len = 0; i < bytelength; len++) { bytes1 b = bytes(s)[i]; // b = e2 for first iteration if(b < 0x80) { i += 1; } else if (b < 0xE0) { i += 2; } else if (b < 0xF0) { i += 3; } else if (b < 0xF8) { i += 4; } else if (b < 0xFC) { i += 5; } else { i += 6; } } For the first iteration there is a condition on the following line b=e2 if(b < 0x80) { i += 1; } Let's decode this. This condition will basically compare decimal values of these hexadecimal characters: 0x80 -> 128 // our b is e2 at the moment, decimal value for e2 = 226 0xe2 -> 226 For regular characters, decimal conversion of their hex character will be , like for it is 97. < 128 a So, if we check all conditions like this for(len = 0; i < bytelength; len++) { bytes1 b = bytes(s)[i]; if(b < 0x80) { //0x80 = 128 => 226 < 128 ❌ i += 1; } else if (b < 0xE0) { //0xE0 = 224 => 226 < 224 ❌ i += 2; } else if (b < 0xF0) { //0xF0 = 240 => 226 < 240 ✅ i += 3; } ... } So, if our the condition in ‘for’ loop will be , which is false and the loop will break, and the value of at the moment. i is 3 3<3 len will be 1 And that’s it, it is the correct value for the length of string “ ” € If you want to try some more strings like “ ”, here is a small list of characters that occupies more than 1 byte: € € -> e2 82 ac à -> c3 83 ¢ -> c2 a2 Create, a random string anything like , for example, and try it out. abc¢Ã Ta-Da 🎉, and now it works Connect with me on : Twitter or @pateldeep_eth LinkedIn My DMs are open to any kind of improvement or suggestions Originally published . here
