"If else" conditionals not working as expected

Question

4.00/5 (1 vote)

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Objective-C

#include<stdio.h>

int main(){

float x = 9.81 ; 
if( x == 9.81)
{printf("the value of x is 9.81\n");}
else {printf("No its not 9.81 \n");}
}

In this we expect "if" statement to execute but instead "else" statement get executed.
Why is it so??

Posted 14-Jan-16 7:06am

Dishank Bansal

Updated 14-Jan-16 8:16am

Maciej Los

v2

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KarstenK · Accepted Answer · 2016-01-14T07:21:00

Solution 1

it is common issue that float arent exactly what they seem. ;-)

The reason is there internal representation in the memory. Make an output of it.

The solution is to calculate the difference an accept a small one. Remember that it maybe signed.

C++

float x = 9.81f; 
float d = x - 9.81f;

printf("the diff is %f\n", d);

if( fabs(d) < 0.005/*accepted tolerance*/ )
{printf("the value of x is 9.81\n");}
else {printf("No its not 9.81 \n");}
}

Posted 14-Jan-16 7:21am

KarstenK

Updated 14-Jan-16 7:57am

v3

Comments

Dishank Bansal 14-Jan-16 13:38pm

Still its Executing the "else" statement...
You try yourself once @karstenk

CPallini 14-Jan-16 13:56pm

Because you should call fabs (you have to include math.h) instead of abs. See the updated solution.

Dishank Bansal 14-Jan-16 15:20pm

Thank You...

Dishank Bansal 14-Jan-16 15:24pm

But why is f added after 9.81??

Sergey Alexandrovich Kryukov 14-Jan-16 15:45pm

This is the syntax for the literal of the type float.
—SA

CPallini 14-Jan-16 15:47pm

9.81 is a double literal, 9.81f is a float literal, see
en.cppreference.com/w/cpp/language/floating_literal

Dishank Bansal 15-Jan-16 13:28pm

what is use of using f(float literal) when i have already used "float" data type? What extra it do?

CPallini 15-Jan-16 16:09pm

the data type is for the l-value, the type of the r-value (the expression on the right of the '=' operator) is given by the literal (in this case). If you specify a double literal then a (possibly unintended) implicit cast happens.

CPallini 14-Jan-16 13:57pm

5.

Maciej Los 14-Jan-16 14:17pm

5ed!

Sergey Alexandrovich Kryukov 14-Jan-16 15:41pm

5ed.
Unfortunately, I duplicated very similar code in Solution 3, but I credited yours when I noticed that.
And I also added the relative comparison sample.
—SA

CPallini · Accepted Answer · 2016-01-14T07:59:00

Solution 2

KarstenK already gave you the solution. I suggest you to read: "What Every Computer Scientist Should Know About Floating-Point Arithmetic"[^].

Posted 14-Jan-16 7:59am

CPallini

Comments

Maciej Los 14-Jan-16 14:17pm

5ed!

Sergey Alexandrovich Kryukov 14-Jan-16 15:40pm

My 5. As this article contains a lot of information, I added some more concrete explanation in Solution 3, please see.
—SA

nv3 14-Jan-16 17:13pm

Yes, sir! 5.

Sergey Alexandrovich Kryukov · Accepted Answer · 2016-01-14T09:39:00

Just my 5 cents in addition to other answers:

It's not a good idea to use the operator == with floating-point operands. That's right, I don't know the situations where it can be useful at all. Even with floating-point values representing numbers with zero fractional part, you should not use it; instead you can round a value and typecast to one of the integer types, and even that is rarely needed. Floating-point values represent approximate values. If is impossible to represent "all" real number is such a finite-state machine as computer, even a single real number, generally, contains infinite volume of information. A typical comparison, in case when it makes sense, may look like:

C++

#include <cmath>
double margin = //... some value which is "small enough"
double myValue = //...
double testValue = //...
// let's say, you mean to compare:
// if (myValue == testValue) ... // bad idea
// instead, do this:
if (std::abs(myValue - testValue) < margin) // ...

[EDIT]

After I submitted this post, I found that this code sample is essentially the same as in Solution 1. Crediting that, I decided not to remove mine, maybe it may provide some extra clarification.

Sometimes you need a different criterion, "relative closeness". For example

C++

double margin = 0.001; // say, 0.1%
if (std::abs(myValue - testValue) * 2 <
    margin * (myValue + testValue)) // ...

It is useful if the compared values are supposed to be "big" values with "small" difference between them, so the criterion is to check if the relative difference is "small enough".

[END EDIT]

See also: std::abs(float), std::fabs - cppreference.com[^].

The principles of floating-point calculation have been analyzed and explained to the practical software developers quite deeply in the classical Donald Knuth's book The Art of Computer Programming.

—SA