Add element to struct - solidity

Question

I can't use ( push ) because it is used with state variable only
This is the error :
Error message
Is there any alternatives for ( push )
contract m{
struct Message{
address sender;
address receiver;
uint msgContent;
} // end struct
Message[] all;
function get ( address from ) internal
returns ( Message[] subMsgs){
for ( uint i=0; i<all.length ; i++)
{
if ( all[i].sender == from )
{
subMsgs.push (all[i]);
}
}
return subMsgs;
}
} // end contract

Answer 1

You can only use push on dynamic-size arrays (i.e. storage arrays), and not fixed-size arrays (i.e. memory arrays) (see Solidity array documentation for more info).
So to achieve what you want, you'll need to create a memory array with a desired size and assign each element one by one. Here is the example code:
contract m {
struct Message{
address sender;
address receiver;
uint msgContent;
} // end struct
Message[] all;
function get(address from) internal returns (Message[]) {
// Note: this might create an array that has more elements than needed
// if you want to optimize this, you'll need to loop through the `all` array
// to find out how many elements from the sender, and then construct the subMsg
// with the correct size
Message[] memory subMsgs = new Message[](all.length);
uint count = 0;
for (uint i=0; i<all.length ; i++)
{
if (all[i].sender == from)
{
subMsgs[count] = all[i];
count++;
}
}
return subMsgs;
}
} // end contract