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Reading and writing local variables is cheap, whereas reading and writing state variables that are stored in contract storage is expensive.

function badCode() external {
for(uint256 i; i < myArray.length; i++) { // state reads
myCounter++; // state reads and writes
}
}

function goodCode() external {
uint256 length = myArray.length; // one state read
uint256 local_mycounter = myCounter; // one state read
for(uint256 i; i < length; i++) { // local reads
local_mycounter++; // local reads and writes
}
myCounter = local_mycounter; // one state write
}

Recommendations

before use state var in loop first cache it in local varibel