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javascriptcfloating-pointieee-754

IEEE 754: Javascript vs C

From what I understand, the famous

 (0.1 + 0.2) !== 0.3

gotcha is not actually Javascript's fault. Thats just the way the IEEE 754 works. A similar output happens in Python, which also follows the IEEE 754 rules.

Then how come this particular example works as expected in C, sometimes. If I do a direct comparison

printf("%d\n", (0.1+0.2) == 0.3);

I get the (un?)expected output 0, but if I put the values into variables or print them out, I get properly rounded answers.

C Runnable Example

Is the C implementation of IEEE 754 doing something extra? Or is it something completely else that I am missing.

Update

The code sample I posted was broken due to a typo. Try this one Fixed C Runnable Example

But the original Question still remains. 

double d1, d2, d3;
d1 = 0.1;    d2 = 0.2;    d3 = d1 + d2;
printf ("%d\n", ((((double)0.1)+((double)0.2)) == ((double)d3)));
printf ("%.17f\n", d1+d2);
printf ("%d\n", ((d1+d2) == d3));

The output is

1
0.30000000000000004
1

The rephrased question now is:

  1. Why (and when, and how) is the C compiler taking the liberty to say that

    0.3 == 0.30000000000000004

  2. Given all facts, isn't it true that the C implementation is broken, rather than Javascripts'?

Solution

  • Why (and when, and how) is the C compiler taking the liberty to say that

        0.3 == 0.30000000000000004

    Given all facts, isn't it true that the C implementation is broken, rather than Javascripts'?

    It isn't.

    The output given is from this code:

    printf ("%d\n", ((((double)0.1)+((double)0.2)) == ((double)d3)));

    but you wrote:

    d1 = 0.1;    d2 = 0.2;    d3 = d1 + d2;

    so d3 is not 0.3, it's 0.30000000000000004