Noob here, having trouble with simple program

**NoobMan** · 09-19-2016

Hi, new programmer here, having trouble with very simple assignment, everytime i run this program it keeps printing zero as the average.

int main()
{
int numExercises, totalScore = 0, pointsPossible = 0;
bool condition = true;

cout << "Score received for exercise N: ";
cin >> totalScore;
cout << "Total points possible for exercise N: ";
cin >> pointsPossible;

cout.setf(ios::fixed);
cout.setf(ios::showpoint);
cout.precision(4);

cout << "Your average is " << totalScore / pointsPossible;

return 0;

can anyone tell me whats wrong with it?

Guest · 09-20-2016

You can put your code in [code][/code] tags, which makes is easier for others to read.

Your issue comes from the use of integers in the division. C++ will truncate these values to whole numbers, so e.g. 0.76 becomes 0. To get the behavior you want, you can either do:

Code:

double totalScore = 0, pointsPossible = 0;

Or cast (one of) the values to floating point during the calculation:

Code:

static_cast<double>(totalScore) / pointsPossible;

**Xupicor** · 09-20-2016

A way to debug that kind of problem would be to feed the program some data it probably shouldn't see but might help diagnose the issue.

If you feed it "10" and "2", you'll get "5". This tells you that the division part actually does "something"* right when totalScore > pointsPossible. Now feed it "10" and "3" - you'll get "3". But is 10/3 == 3? Yes, if you take into account that we're dealing with integral division here.

Standardese:

5.6 Multiplicative operators [expr.mul] N4582
4 [...] For integral operands the / operator yields the algebraic quotient with any fractional part discarded; [...]

Both operants of / in your case are of integral type, thus integral division.
*) which also means the division part actually did everything right, just not necessarily what you wanted.

**Elysia** · 09-21-2016

Originally Posted by Guest

Or cast (one of) the values to floating point during the calculation:

Code:

static_cast<double>(totalScore) / pointsPossible;

I like to do
1.0 * TotalScore / PointsScore

Guest · 09-21-2016

Oh yes, that's less verbose looking Elysia!

**Elysia** · 09-21-2016

Didn't say it was better/worse, more verbose/less verbose, just that it's another way doing it and I like to do it that way.

**NoobMan** · 09-23-2016

Thank you all so much!

**MutantJohn** · 09-23-2016

I get the point of reinterpret_cast and I guess dynamic_cast is sometimes a thing (though I've seen its use be discouraged) but I really don't get static_cast vs just (double ) my_value.

**Elysia** · 09-24-2016

If my_value is a pointer, (double)my_value will happily work, but static_cast will fail.
If my_value is const, (double)my_value will happily work, but static_cast will fail.
And so on...

(type) is an Evil™ type cast that bundles static_cast, reinterpret_cast and const_cast all into one.

**whiteflags** · 09-24-2016

I like to do
1.0 * TotalScore / PointsScore

Didn't say it was better/worse, more verbose/less verbose, just that it's another way doing it and I like to do it that way.

The cast does make your intentions explicit at a glance though. If I deleted the "1.0 *" thinking it did nothing, the only thing between me and committing the code is a run-time error. You better hope it happens.

Sometimes it's not about how slick you can make something. The point of implicit type promotion is so you don't have to think about it.

**Elysia** · 09-24-2016

To me, the 1.0 is making it explicit, as well.

**whiteflags** · 09-24-2016

I know because you brought it up. I'm just expressing my preferences as well. Code is about communication, bla bla bla.

**Elysia** · 09-24-2016

Yeah I know. You make a good point.

**MutantJohn** · 09-24-2016

Originally Posted by Elysia

If my_value is a pointer, (double)my_value will happily work, but static_cast will fail.
If my_value is const, (double)my_value will happily work, but static_cast will fail.
And so on...

(type) is an Evil™ type cast that bundles static_cast, reinterpret_cast and const_cast all into one.

The pointer conversion is a good point. C++'s type system is pretty strong though so it's easy to understand what you're casting unless you obfuscate behind a bunch of typedefs. Okay, so maybe static_cast should always be used then because any decent library has typedefs out the wazoo.

Also, I didn't know that, is it really that easy to cast away constness using that? I always thought you had to do more nefarious things to effectively undermine const-ness

**Elysia** · 09-24-2016

Yes, a C-cast WILL remove constness.
In C++, you do have to go to pretty great lengths to remove it, that is, if you're using C++ casts. Neither static_cast nor reinterpret_cast will remove const.

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