What can I do to compare two variables of type float accurately in C++, one being initialized at the time of declaration and the other being computed through the program?

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Hope your well.
When I am comparing two variables having value either 0.5 or 0.25, the program is showing the desired output but when I am comparing two variables having value either 0.333333 or 0.666667, the program is not showing the desired output. Will you please tell me where I am going wrong? Please help me in writing the correct piece of code. I am a novice and it will be a great experience to learn from you. Thank you for your concern.Here is the code with some of the outputs:

C++

#include

OUTPUTS:

0.333333 0.333333
UNEQUAL

0.666667 0.666667
UNEQUAL

0.25 0.25
EQUAL

0.5 0.5
EQUAL

Regards,
Prateek Senapati

What I have tried:

I have tried to enter different decimal values but did not get expected results in all the cases.

Posted 8-Oct-16 22:29pm

Prateek Senapati

Updated 8-Oct-16 23:35pm

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nv3 · Answer 1 · 2016-10-08T23:35:00

Just consider that the floating point representation cannot precisely express numbers like 1/3. The floating point number format uses a fractional representation in base 2, i.e. it can represent numbers like 1/2, 1/4, 1/8, 1/16 etc. correctly, but not 1/3 or 1/5. For those it uses the nearest approximation. Now, when you do computations in floating point format the approximation error gets bigger and bigger with every single computation. Hence, the representation of 0.3333... as constant in your program might not exactly be equal to your computation result.

The solution: Don't compare for equality, but always allow for a small approximation error.

Jochen Arndt · Answer 2 · 2016-10-08T23:28:00

Floating point numbers can not represent most real numbers exactly. The numbers 0.25 and 0.5 can be represented exactly but not 0.33 and 0.66.

This is sourced by how these numbers are represented internally (see IEEE Floating Point - Wikipedia[^].

To check if floating points numbers are nearly equal, special checks must be performed. See for example The Floating-Point Guide - Comparison[^].

Patrice T · Answer 3 · 2016-10-08T22:34:00

Solution 1

We do not do your HomeWork.
HomeWork is not set to test your skills at begging other people to do your work, it is set to help your teacher to check your understanding of the courses you have taken and also the problems you have at applying them.
Any failure of you will help your teacher spot your weaknesses and set remedial actions.
So, give it a try, reread your lessons and start working. If you are stuck on a specific problem, show your code and explain this exact problem, we might help.

Posted 8-Oct-16 22:34pm

Patrice T

Comments

Prateek Senapati 9-Oct-16 4:39am

Actually, I could not post my question correctly. My question included the code and some of the outputs as well. I can't figure out how to edit it. I'm really sorry for the inconvenience caused.

Patrice T 9-Oct-16 4:40am

Use Improve question to update your question.

Prateek Senapati 9-Oct-16 4:47am

Here is the code:
#include

Prateek Senapati 9-Oct-16 4:48am

I am not able to enter the piece of code after #include

What can I do to compare two variables of type float accurately in C++, one being initialized at the time of declaration and the other being computed through the program?

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