Why can't I return dynamic array in Solidity? - solidity

Question

I am trying to return a list of addresses by declaring an array and pushing to it in a for loop. However, Solidity doesn't like when I try to return a dynamic array.
function listMyPromises() public returns(uint256[] memory ){ //lists all my past promises
uint256[] memory List;
for(uint i=0; i<table.length; i++){
if(table[i].userAddress==msg.sender){
List.push(uint256(table[i].friendAddress));
}
}
return List;
}
Solidity is not allowing me to return a array stating that
TypeError: Data location must be "memory" or "calldata" for return parameter in function, but none was given.
However When I add memory in the return parameter I get the error:
TypeError: Member "push" is not available in uint256[] memory outside of storage
I have no clue what to do.

Answer 1

Solidity doesn't allow resizing (e.g. by using .push()) a memory array.
You need to calculate the size beforehand and then create a fixed-size memory array.
Source: docs
So the result is possibly going to look like this
function listMyPromises() public returns(uint256[] memory ){ //lists all my past promises
uint256 count = getCount();
uint256[] memory List = new uint256[](count);
uint256 j;
for (uint i = 0; i < table.length; i++) {
if(table[i].userAddress==msg.sender){
List[j] = uint256(table[i].friendAddress);
j++;
}
}
return List;
}
function getCount() internal view returns (uint) {
uint count;
for (uint i = 0; i < table.length; i++) {
if (table[i].userAddress == msg.sender) {
count++;
}
}
return count;
}
Depending on the datatypes wrapped in the table[i] struct, you might also run into another error. The friendAddress suggests that it's a type address, but it cannot be converted to uint256.