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Illogical Logic what am I doing wrong? [message #1700026]

Mon, 29 June 2015 14:15

Eclipse User

Its a simple problem to describe: when I set diff_1== 0; sometimes it executes the code for diff_1 ==1 huh-? Right? If I isolate the code in another "scratch pad" app to test, it does fine.

diff_1 = 0;
	if (diff_1 == 0.00) {cout<<"flag_1";}
	if (diff_1==1.00) {pennies = pennies + 1;
	cout<<"increment by a penny to "<<pennies<<endl;}

-------------------------------------------------------------
works fine here -- "scratchpad"

#include <iostream>
#include<math.h>

using namespace std;

int main()
{
double difference = 0; // or 1
	if (difference == 0) {cout<<"no adjustment necessary"<<endl;}
		else if ((difference)!=0) {cout<<"adjustment necessary"<<endl;}

}

if I set difference to 1 or 0 it goes where it supposed to.

[Updated on: Mon, 29 June 2015 14:27] by Moderator

Re: Illogical Logic what am I doing wrong? [message #1700092 is a reply to message #1700026]

Tue, 30 June 2015 02:27

Eclipse User

When you use floats and doubles, because of the way they work, the variable does not store the exact value. So 0 (NULL) may not be exactly 0, it might be 0.0000001 or something. This is why inequality and equality comparisons are unreliable with them.

So try to do it this way:

if (diff_1 < .001 ) {
//blah-blah
} else if (diff_1 > 0.99) {
//blah-blah
}

or evet this:

if (diff_1 < 1.0 ) {
//blah-blah
} else {
//blah-blah
}

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