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# Source file test/fixedbugs/bug120.go

## Documentation: test/fixedbugs

```     1  // run
2
3  // Copyright 2009 The Go Authors. All rights reserved.
4  // Use of this source code is governed by a BSD-style
5  // license that can be found in the LICENSE file.
6
7  package main
8
9  import "os"
10  import "strconv"
11
12  type Test struct {
13  	f   float64
14  	in  string
15  	out string
16  }
17
18  var tests = []Test{
19  	Test{123.5, "123.5", "123.5"},
20  	Test{456.7, "456.7", "456.7"},
21  	Test{1e23 + 8.5e6, "1e23+8.5e6", "1.0000000000000001e+23"},
22  	Test{100000000000000008388608, "100000000000000008388608", "1.0000000000000001e+23"},
23  	Test{1e23 + 8388609, "1e23+8388609", "1.0000000000000001e+23"},
24
25  	// "x" = the floating point value from converting the string x.
26  	// These are exactly representable in 64-bit floating point:
27  	//	1e23-8388608
28  	//	1e23+8388608
29  	// The former has an even mantissa, so "1e23" rounds to 1e23-8388608.
30  	// If "1e23+8388608" is implemented as "1e23" + "8388608",
31  	// that ends up computing 1e23-8388608 + 8388608 = 1e23,
32  	// which rounds back to 1e23-8388608.
33  	// The correct answer, of course, would be "1e23+8388608" = 1e23+8388608.
34  	// This is not going to be correct until 6g has multiprecision floating point.
35  	// A simpler case is "1e23+1", which should also round to 1e23+8388608.
36  	Test{1e23 + 8.388608e6, "1e23+8.388608e6", "1.0000000000000001e+23"},
37  	Test{1e23 + 1, "1e23+1", "1.0000000000000001e+23"},
38  }
39
40  func main() {
41  	ok := true
42  	for i := 0; i < len(tests); i++ {
43  		t := tests[i]
44  		v := strconv.FormatFloat(t.f, 'g', -1, 64)
45  		if v != t.out {
46  			println("Bad float64 const:", t.in, "want", t.out, "got", v)
47  			x, err := strconv.ParseFloat(t.out, 64)
48  			if err != nil {
49  				println("bug120: strconv.Atof64", t.out)
50  				panic("fail")
51  			}
52  			println("\twant exact:", strconv.FormatFloat(x, 'g', 1000, 64))
53  			println("\tgot exact: ", strconv.FormatFloat(t.f, 'g', 1000, 64))
54  			ok = false
55  		}
56  	}
57  	if !ok {
58  		os.Exit(1)
59  	}
60  }
61
```

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