Coding a maths problem to find the difference between two numbers which could be negative

I'm trying to write some c code (objective c) which will take a bank account balance and a desired balance from the user and produce a value which I can add or subtract from the current balance to achieve the desired balance.

I think I'm made things overcomplicated, heres what I have...

//get desired amount to variable dblDesiredBalance
//get balance from database to variable balFromDB

double addAmount = fabs(balFromDB) + fabs(dblDesiredBalance);
double minusAmount = fabs(dblDesiredBalance) - fabs(balFromDB);

// create amount to add to db
if (dblDesiredBalance < 0 ) {
    if (balFromDB < 0 ) {
        dblCommitToDB = balFromDB - minusAmount;
    } else {
        dblCommitToDB = balFromDB - addAmount;
    }
} else {
    if (balFromDB < 0 ) {
        dblCom开发者_如何学编程mitToDB = balFromDB + addAmount;
    } else {
        dblCommitToDB = balFromDB + minusAmount;
    }
}
// update db with dblCommitToDB

Can anyone help me ?

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Why not start from simple algebra:

current_balance + delta = desired_balance

and then solve for delta:

delta = desired_balance - current_balance

This works independent of signs - you know algebraic rules and all.