trying to rewrite this so it doesnt violate "prinicples discussed in Code Complete 2nd edition - code-complete

Question

function profit(){
int totalSales=0;
for (int i=0; i<12;i++) // computer yearly sales
totalSales+=montlysales[i];
return get_profit_from_sales(totalsales);
}
So i've already determined that the 12 in the for loop should be a constant instead of just using an integer and that the montlysales should be passed as a parameter into the function so then a check can be run to see if the length of sales is equal to the integer value of months which is also twelve.
I'm not sure if those are all the violations of the princples cause. I feel the last line
return get_profit_from_sales(totalsales)
is wrong and its really bothering me cause I can't seem to figure out why it is in fact bothering me and I think I might have skipped something else.
can anyone help me verify?

Answer 1

Summary - you should refactor out the call to another function and make this function so that it is pure and does only one thing, reducing complexity, and improving your ability to reason abstractly about your program and its correctness.
Your spidey sense is tingling and you should trust it - you are correct, but what is wrong is subtle.
Routines are best when they do one thing, and one thing only. So purity of vision is important in the prime imperative, management of complexity -- it allows our brains to be able to juggle more things because they are simpler. That is, you can just look at the function and know what it does, and you don't have to say, "it totals the sales, but it also calls another function at the end", which sort of clouds its "mission".
This is also part of functional programming and where I feel that languages have to adopt to try to implement the prime imperative spoken of in Code Complete. Functional programming has as one of its tenets, "no side effects", which is similar to "one mission" or "one purpose". What I've done to your code can also be seen as making it more functional, just inputs and outputs and nothing in or out via any other path.
Note also that function get_profit() reads like pseudocode, making it somewhat self-documenting, and you don't have to delve into any of the functions to understand what the function does or how it does it (ideally).
So, here is my idea of what I explained above (loosely coded, and not checked!).
function get_total_sales(int montlysales[]){
int totalSales=0;
for (int i=0; i<12;i++) // computer yearly sales
totalSales+=montlysales[i];
return totalSales;
}
function calc_profit(int all_sales, int all_commissions, int all_losses)(){
// your calculations here
int profit = all_sales - all_commissions - all_losses; // ... etc.
return profit;
}
function get_profit(){
int montlysales[] = read_from_disk();
int total_sales = get_total_sales(montlysales);
int tot_commissions = get_tot_commissions();
int tot_losses = get_tot_losses();
int profit_made = calc_profit(total_sales, tot_commissions, tot_losses);
return profit_made;
}
I read Code Complete about once a year, as coding is truly subtle at times, because it is so multidimensional. This has been very helpful to me. Regards - Stephen