How to pop from (decrease the length of) a memory array in Solidity

Question

I have a function that gets an address array from another contract, conditionally deletes the msg.sender from the array, then saves the new array back to that contract.

Because the array backerList is in memory, i can't do backerList.length--;

But I cannot declare backerList as a storage array because memory array can't be converted to a storage one.

I am stuck here, what should I do?

 address[] memory backerList = syndicate.getBackerList();

 if(syndicate.individualTotalBacking(msg.sender) == 0){

        uint index;

        for(uint i=0; i<backerList.length; i++){
            if (backerList[i] == msg.sender){
                index = i;
            }

        //shifting array

            for(uint k=index; k<backerList.length-1; k++){
                backerList[k]=backerList[k+1];
            }

        }

        backerList.length--;
        syndicate.setBackerList(backerList);
    } else {}

Answer 1

Here's a way to do backerList.length--; on an address[] memory backerList by using inline assembly:

assembly { mstore(backerList, sub(mload(backerList), 1)) }

Some important points to remember:

• Make sure this assembly code never runs when backerList.length == 0 (don't allow the array length to underflow)

• Don't try to use this to increase the size of an array (by replacing sub with add)

• Only use it on variables with a type like ...[] memory (for example, don't use it on a address[10] memory or address)

Disclaimer: The use of inline assembly is usually not recommended. Use it with caution and at your own risk :)

Answer 2

Yes you are correct, you can't reduce size of memory array explicitly, it's only applicable for storage array. But you can use delete operator to clear memory, which will save your gas.

However the size of the array will not change, it will remain the same. You need to create extra variable to track size of array.