decimal multiplication?

mseaton (3)
i'm trying to write a program that takes information about a vehicle's engine, such as inch displacement, max rpm, and volumetric efficiency, and output the engine's max cfm, or what size carburetor to use.

the code seems to work fine other than one variable, "veff". it seems to have trouble multiplying by "veff". at first i had called it "ve" and changed to veff because i thought maybe it didn't like 2-char variable names? but the problem didn't stop.

i usually get some sort of scientific formatted number. for example with the first function, cfm. i input 7000 rpm, 355 inches, and .95 volumetric efficiency. the cfm output should be around 685, but the output is "6.8e+002".

i have tried breaking the formulas down piece by piece and it always seems to be when i add the veff to it that it messes up. i've also tried playing with parentheses around different stuff, to no avail. thanks in advance for the help.

-marcus

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#include <iostream>
#include <iomanip>

using namespace std;

int main(int argc, char **argv)
{
	float cfm();
	float rpm();
	float disp();
	float veff();
	int selection;
	char yn;
	
	choose:
	cout << "What would you like me to find? \n 1. Engine CFM \n 
             2. Engine RPM \n 3. Engine Displacement \n 4. Volumetric 
             Efficiency" << endl;
	cin >> selection;
	if (selection<1||selection>4)
	{
	cout << "type 1-4 for choice";
	goto choose;
	}
	
	cout << setprecision(2);
	if (selection==1) cout << cfm() << " cfm" << endl;
	if (selection==2) cout << rpm() << " rpm" << endl;
	if (selection==3) cout << disp() << " cu. inches" << endl;
	if (selection==4) cout << veff() << "% VE" << endl;
	
	cout << "Would you like to go again? y/n:  ";
	cin >> yn;
	if (yn=='y'||yn=='Y') goto choose;
	
	return 0;
}

float cfm()
{
	float rpm;
	float disp;
	float veff;
	
	cout << "Engine RPM:  ";
	cin >> rpm;
	cout << "Engine Displacement:  ";
	cin >> disp;
	cout << "Volumetric Efficiency:  ";
	cin >> veff;
	
	return veff*(disp/1728*(rpm/2));
}

float rpm()
{
	float cfm;
	float disp;
	float veff;
	
	cout << "Engine CFM:  ";
	cin >> cfm;
	cout << "Engine Displacement:  ";
	cin >> disp;
	cout << "Volumetric Efficiency:  ";
	cin >> veff;
	
	return 2*cfm/veff/(disp/1728);
}

float disp()
{
	float cfm;
	float rpm;
	float veff;
	
	cout << "Engine CFM:  ";
	cin >> cfm;
	cout << "Engine RPM:  ";
	cin >> rpm;
	cout << "Volumetric Efficiency:  ";
	cin >> veff;
	
	return 1728*cfm/veff/(rpm/2);
}

float veff()
{
	float cfm;
	float rpm;
	float disp;
	
	cout << "Engine CFM:  ";
	cin >> cfm;
	cout << "Engine RPM:  ";
	cin >> rpm;
	cout << "Engine Displacement:  ";
	cin >> disp;
	
	
	return 100*cfm/(rpm/2)/(disp/1728);
}
Last edited on
mseaton (3)
oh yeah, i'm using geany on windows 7 pro in case that matters. i also have mingw and msys, but haven't tried running this on them. i can't imagine it would make a difference.
and my email is marcus.seaton@gmail.com
-marcus
Stewbond (2827)
Probably something with line 26, not veff. The number is there, but it isn't fully displayed.

Try replacing line 26 with
cout << fixed << setprecision(2);

std::fixed is from <iomanip> and ensures that you don't jump into scientific notation.
shacktar (1187)
the cfm output should be around 685, but the output is "6.8e+002".


"6.8e+002" is around 685:

6.8e+002
= 6.8 x 102
= 680

You can change how cout prints floating point numbers by using the setf function (it uses scientific notation by default).

Try putting this after your call to cout << setprecision(2);:

cout.setf(std::ios::fixed);
mseaton (3)
thanks guys! setting the output to fixed definitely solved my problems with this and a couple other programs. i guess i had forgotten about that. its been a few years.
thanks again!
-marcus
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