String Format C# Small Negative Values

Question

Why does the following program not output the negative sign for the second line?

var smallpos = 3.65433E-005;
var smallneg = -3.65433E-005;

Console.WriteLine("{0} in F4 format with a width of 8 characters {1}",
                  smallpos,
                  smallpos.ToString("F4").PadLeft(8).Substring(0, 8));

Console.WriteLine("{0} in F4 format with a width of 8 characters {1}",
                  smallneg,
                  smallneg.ToString("F4").PadLeft(8).Substring(0, 8));

Using VS 2017 Professional 15.8.2 C# 7.2

What is the output you are getting and what you were expected? — sujith karivelil
– sujith karivelil, Commented Aug 31, 2018 at 4:43

galdin · Accepted Answer · 2018-08-31 05:14:26Z

3

-3.65433E-005 represents -0.0000365433.

The issue here is with smallneg.ToString("F4"). It only considers the first 4 places after the decimal point; since they're all 0 the negative sign is left out as -0 wouldn't make much sense.

answered Aug 31, 2018 at 5:14

galdin

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1 Comment

Fildor Over a year ago

You could enforce it with a custom format, if I remember correctly, though.

Kjartan · Accepted Answer · 2018-08-31 06:32:30Z

1

Once you've eliminated enough digits from the back, the remaining number will have a value of zero, at which point the existence of a "-" has no real meaning.

This can be understood quite intuitively by running the snippet below, which has a decreasing number provided for the fixed-point specifier.

In the last line, that specifier is omitted, at that point NumberFormatInfo.NumberDecimal decides the number of decimal places used (depending on the culture used):

var smallneg = -3.65433E-005;

Console.WriteLine(smallneg.ToString("F10")); // -0,0000365433
Console.WriteLine(smallneg.ToString("F9"));  // -0,000036543
Console.WriteLine(smallneg.ToString("F8"));  // -0,00003654
Console.WriteLine(smallneg.ToString("F7"));  // -0,0000365
Console.WriteLine(smallneg.ToString("F6"));  // -0,000037
Console.WriteLine(smallneg.ToString("F5"));  // -0,00004
Console.WriteLine(smallneg.ToString("F4"));  // 0,0000 <-- Zero --> (-0) == 0

Console.WriteLine(smallneg.ToString("F"));   // 0,00

answered Aug 31, 2018 at 6:32

Kjartan

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3 Comments

Fildor Over a year ago

"at which point the existence of a "-" has no real meaning." I disagree with that. If you are aware that the displayed number has been rounded, then it would indicate the original (exact) number was < 0 ... - I don't know if there are in fact real life applications, where this would be interesting or important, but there is always a first time for everything.

Kjartan Over a year ago

@Fildor I see your point, but note that the resulting output of smallneg.Tostring() is not a number, but a string, and in the context of that string, any information about eliminated digits has already been lost.

Fildor Over a year ago

Of course. Just saying it could have a meaning if it was possible to have it. Though I admit in those cases you'd probably rather have the exact value than just a "hint" that the exact value was a tiny bit smaller than 0 ...

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