How do I add leading zeroes to the new C# interpolated strings?

Question

3.50/5 (2 votes)

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Hi,

I am struggling to work out how to pad an integer field with leading zeroes in the new C# interpolated strings.

According to this article C#6: String Interpolation, "The same composite formatting power is there to specify things like significant figures and leading zeroes."

In this same article is a link to this page: Standard Numeric Format Strings[^]. According to this page, something like "D2" should use two digits for an integer and pad with zeroes. When I try it, it seems to do nothing.

E.g.

C#

int x = 3;
Console.WriteLine( $"Integer padded to two places: {x:D2}." );

This prints "3", not "03".

Does anyone know how to do this?

Kind wishes ~ Patrick

PS - Judging by discussion below, it seems I have not been 100% clear about what I am trying to achieve.

I know the numbers will always be in the range 0 <= x <= 99. When I get an input in the range 0-9, I want it to add one leading zero (e.g. 4 prints "04", but 14 prints "14"). Hope this clarifies the question.

What I have tried:

In addition to the code shown above, I have tried "##", "DD" and a couple of other variants that I have forgotten.

Posted 30-Mar-16 6:15am

Patrick Skelton

Updated 28-Apr-17 9:33am

v2

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Comments

Richard Deeming 30-Mar-16 12:21pm

Your code prints 03 for me.

What output do you get using the older syntax?
Console.WriteLine("X = {0:D2}", x);

Mario Z 30-Mar-16 13:03pm

Same as Richard, I get the expected "Integer padded to two places: 03."
What version of VS are you using? It could be a version issue, are you perhaps using VS 2013 and you downloaded the Roslyn by yourself, in which case it's possible you're using an older compiler.

Patrick Skelton 30-Mar-16 13:35pm

Mmm... maybe it is a compiler issue. I am actually in a Xamarin.iOS application, which I think may well use the Roslyn compiler.

If I try this:

int x = 3;
string s = String.Format( "{0:D2}", x );
Console.WriteLine( s );

I still don't get a padded integer.

Richard Deeming 30-Mar-16 16:57pm

Sounds like it's a bug / "feature" in Xamarin.iOS; your code works fine in a standard .NET application.

3 solutions

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RickZeeland · Accepted Answer · 2016-03-30T07:21:00

Solution 1

Not exactly C# 6, but simple:

string x = "3";
Console.WriteLine(x.PadLeft(2, '0'))

Posted 30-Mar-16 7:21am

RickZeeland

Comments

Patrick Skelton 30-Mar-16 13:39pm

This pads the integer, but using a space character, not a leading zero (for me anyway - see comment about compiler above).

Sergey Alexandrovich Kryukov 30-Mar-16 16:38pm

This solution is so wrong!
I want you to understand it.
Say, you have number 3 and 13. You will have
"003" and
"0013", output of different width. But it's pretty obvious that the inquirer really needs fixed-width output:
"003"
"013", and so on...
—SA

RickZeeland 31-Mar-16 2:12am

Strange, when I test with .NET 4.52 it works as it should, read about it here if you don't believe it: https://msdn.microsoft.com/en-us/library/92h5dc07(v=vs.110).aspx

Sergey Alexandrovich Kryukov 31-Mar-16 2:23am

You don't understand. There is no need to test, your method works as you expected, but what you expected is not what is required.

The requirement is padding, which means, for example (pseudo-code):

if input in 0..9, pad 3 '0'
if input in 10..99, pad 2 '0'
if input in 100...999 pad 1 '0'
else no padding

Can you see the idea? In all ranges, the length should be the same, 4 digits (for example). And this is solved by just one string.Format, using Custom Format with '0' placeholders, for example.

—SA

RickZeeland 31-Mar-16 3:04am

Just test it, you will see the length of the string will stay the same.

Sergey Alexandrovich Kryukov 31-Mar-16 10:39am

Ah... my mistake, sorry.
My 5. Thank you for proving your point.
—SA

Patrick Skelton 31-Mar-16 4:24am

I've up-voted this to 5* because this discussion really got me thinking about this, and by playing about with the options above, I discovered that part of the problem was a bug I was introducing. One of the pitfalls of trying to post the smallest code example that demonstrates the problem. Think I need to put my hands up and say that I must not have actually run the _exact_ same code I posted above. Sorry, guys, if I have wasted anyone's time.

So, to be clear, I can get $"x:00" to work as expected using the Xamarin.iOS compiler. And the tool suggested above (https://code.msdn.microsoft.com/net-framework-4-formatting-9c4dae8d) is a god-send for playing with this stuff quickly.

Thank you very much for the detailed assistance. - Patrick

Sergey Alexandrovich Kryukov 31-Mar-16 10:43am

Right. Pay attention: I admitted my mistake above.
—SA

Sergey Alexandrovich Kryukov · Accepted Answer · 2016-03-30T10:47:00

Solution 2

"D" or "d" in specifier found in "Standard Numeric Format Strings" is not it. It is not leading/padding, this is "Precision specifier: Minimum number of digits". Can you see the difference? Here is an additional explanation: Standard Numeric Format Strings[^].

For leading digits or padding, you need "Custom Numeric Format Strings": Custom Numeric Format Strings[^].

Pay attention for the first line, the specifier "0", see also "#". Play with it a bit; and you will quickly see what it really means.

—SA

Posted 30-Mar-16 10:47am

Sergey Alexandrovich Kryukov

Comments

Matt T Heffron 30-Mar-16 17:34pm

From the Standard Numeric Format Strings link you provided, "D2" will pad 0-9 with a leading 0, >= 10 will not pad.
If the OP knows that their values are all <= 99, then "D2" is a perfectly valid way to accomplish output with a fixed width of 2 chars.

Sergey Alexandrovich Kryukov 30-Mar-16 20:58pm

You are right, but the inquirer is not happy, and usually it happens when people want exact fixed length of output string for certain range of numbers giving different lengths otherwise...
—SA

Kristian Lindberg Vinther · Accepted Answer · 2017-04-28T09:33:00

Solution 3

A concrete example:

C#

var number = 123;
Console.WriteLine($"{number:000000}");

// 000123

Posted 28-Apr-17 9:33am

Kristian Lindberg Vinther