Strange output from printf

Question

I have met a strange 9(unexpected) output from printf in C.

The code is:

 1  #include <stdio.h>
 2
 3  int
 4  main(void) {
 5      printf("%lf", 3.14 - 1e20 + 1e20);
 6  }

However, the output is 0.000000.

I do not know how it works.

Can anyone help?

What did you expect for the output? – machine_1 Dec 21 '19 at 05:11 — machine_1, Dec 21 '19 at 05:11

score 3 · Accepted Answer · answered Dec 21 '19 at 04:21

3

The double you're trying to print has 53 bits of precision and 11 bits of exponent. 1e20 (base 10) is 101 0110 1011 1100 0111 0101 1110 0010 1101 0110 0011 0001 0000 0000 0000 0000 0000 in binary (56BC75E2D63100000 in hex). Your 3.14 is too small to be represented once added to a number of that magnitude. Change the order of operations if you want to work around that. This code:

    printf("%f\n", -1e20 + 1e20 + 3.14);

Prints 3.140000.

answered Dec 21 '19 at 04:21

Carl Norum

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4

*prints 3.140000 on your system – M.M Dec 21 '19 at 04:23
2

@M.M - would you expect different behaviour in some case? – Carl Norum Dec 21 '19 at 04:24
1

@M.M, Operator associativity is defined for addition in C (left to right) – ikegami Dec 21 '19 at 04:34
Associativity is not the same as order of evaluation. – Jonathan Leffler Dec 21 '19 at 04:55
Depending on `FLT_EVAL_METHOD` and other factors, `3.14 - 1e20 + 1e20` may also print `3.140000` or a nearby value. `3.14 - 1e20 + 1e20` may evaluate using `long double`. IOWs, the value being printed has 53 bits of precision, yet the addition may have been done with more. – chux - Reinstate Monica Dec 21 '19 at 04:55

Strange output from printf

1 Answers1