Percentage is calculating as 0 in C++ programe

Question

1.00/5 (1 vote)

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Hi. in program below percentage is showing 0.
I tried all datatypes int float and double but still same problem

#include <iostream>
using namespace std;

int marks=4;
int tmarks=10;
void result()
{
	double per;

	per=(marks/tmarks)*100;
	cout <<"percentage is "<<per<<endl;
	if (per>50)
	{	
	cout <<"pass"<<endl;
	}	
	else
	cout <<"fail"<<endl;
}

int main()
{
    result();
    
}

What I have tried:

#include <iostream>
using namespace std;

int marks=4;
int tmarks=10;
void result()
{
	double per;

	per=(marks/tmarks)*100;
	cout <<"percentage is "<<per<<endl;
	if (per>50)
	{	
	cout <<"pass"<<endl;
	}	
	else
	cout <<"fail"<<endl;
}

int main()
{
    result();
    
}

Posted 5-Jan-22 6:50am

Hamza Jameel

Updated 5-Jan-22 7:31am

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3 solutions

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0x01AA · Accepted Answer · 2022-01-05T07:07:00

Solution 3

Think about ...

per=(marks/tmarks)*100;

... and here especally about

(marks/tmarks)

Both marks and tmarks defined as int

int marks=4;
int tmarks=10;

with plain int 4/10 will always end with result 0.

Think about that:

double marks=4.0;
double tmarks=10.0;

I hope it helps.

Posted 5-Jan-22 7:07am

0x01AA

Comments

Hamza Jameel 5-Jan-22 13:15pm

Hi. Thanks for replying. It works perfectly individually. But putting it along with other functions in my project it still shows 0. double marks has incremenet as user tells right answers.

0x01AA 5-Jan-22 13:17pm

Pls. give an example with 'that other functions' where it fails.

Hamza Jameel 5-Jan-22 13:22pm

http://cpp.sh/2npgr
you can see code here
name and sapid can be anything.
Press 1 to start quiz(only this one works as programe is remaining are yet to complete.

0x01AA 5-Jan-22 13:25pm

int marks=0;
Even it would be double marks=0.0;
0.0 devided by whatever results in 0.0, right?

Hamza Jameel 5-Jan-22 13:28pm

0.0(marks) divided by double total marks
0.0(whatever result) / 10.0

0x01AA 5-Jan-22 13:31pm

0.0 / 10.0 = 0.0 ,right?

Hamza Jameel 5-Jan-22 13:33pm

definitely 0.0/10.0 will show 0
but for example user answers 4 answers correctly the it will be 4.0/10.0. So it should show 0.4 instead of 0 as we are taking double data type :(

0x01AA 5-Jan-22 13:43pm

Have a closer look to this:

using namespace std;
	int  marks=0;
double
 tmarks=10.0;
void PF()
{
	 char choice;
	int  chose, marks, random , i=0 ;

You have a 'global' marks and a local in void PF().

Hamza Jameel 5-Jan-22 13:52pm

Thankyou so much. Solved :)

0x01AA 5-Jan-22 13:57pm

Thank you for accepting and especially also to accept the answer of @ppolymorphe (Patrice T), because his suggestion is very correct in case one will go the 'int' way.

k5054 · Accepted Answer · 2022-01-05T07:31:00

As Solution 1 points out, C++ does integer math when given all integer terms. So even the proposed solution of (marks * 100)/tmarks, may not give you what you expect. If marks = 4 but tmarks = 15, this evaluates to (int)400/(int)15, withe a final value of (int)26.
If, on the other hand you use a floating point constant, then you get 26.6667, which might be closer to what's expected. E.G.

C++

per = (marks * 100.0)/tmarks; // This evaluates to (double)400.0/(int)15
                             // so the whole expression gets promoted to double

Perhaps it might be better to do the whole calculation in floating point by changing the type of marks and tmarks to double.

Patrice T · Accepted Answer · 2022-01-05T06:59:00

Solution 1

try to replace:

C++

per=(marks/tmarks)*100;

with

C++

per=marks*100/tmarks;

In C++, a division of integers id an integer division, and the result have no decimal.

Posted 5-Jan-22 6:59am

Patrice T

Comments

0x01AA 5-Jan-22 13:30pm

To the downvoter: Please explain the downvote. Basically this answer is leading to the right direction.

Patrice T 5-Jan-22 13:37pm

Can someone explain me what is so wrong in my answer that it merit a downvote ?

0x01AA 5-Jan-22 14:17pm

5

Patrice T 5-Jan-22 14:19pm

Thank you.
What happened to your old account ?

0x01AA 5-Jan-22 14:28pm

I deleted it in a fit of annoyance ...
... and now I start again, no big thing ;)

Patrice T 5-Jan-22 14:48pm

Ok :) courage.

0x01AA 5-Jan-22 14:59pm

Juste un forum www, pas vraiment vital :=)