Printing types in C#

Printing types in C#

A placeholder just marks the place where the value is to be printed. Consider:

int i=150;
double f=1234.56789;
Console.WriteLine("i:{0} f:{1}",i,f);
Console.WriteLine("i:{1} f:{0}",f,i);

This would print out:
i: 150 f: 1234.56789
i: 150 f: 1234.56789

The {n} part of the string says “parameter number n, counting from 0”. In the second write statement I have swapped the order of the numbers, but since I've swapped the order of the parameters too the output is the same.

If I do something mad, for example use {99} to try and get the 99th parameter, the WriteLine method will fail with an error. This error will not be picked up by the compiler however, the program will fail when it runs.

Placeholders can have formatting information added to them:

int i=150;
double f=1234.56789;
Console.WriteLine("i:{0:0} f:{1:0.00}",i,f);

This would print out:
i: 150 f: 1234.57

The 0 characters stand for one or more digits. When placed after a decimal point they can be used to control the number of decimal places which are used to express a value. Note that doing this means that if the number is an integer it is printed out as 12.00.

I can specify a particular number of digits by putting in a given number of zeroes:

inti=150;
double f=1234.56789;
Console.WriteLine("i:{0:0000}f:{1:00000.00}",i,f);

This would print out:
i: 0150 f: 01234.57

Note that if I do this I get leading zeroes printed out, which is useful if you are printing things like cheques.

If you want really fancy levels of control you can use the # character. A # in the format string means “put a digit here if you have one”:

int i=150;
double f=1234.56789 ; Console.WriteLine("i:{0:#,##0} f: {1:##,##0.00}",i,f);

I have used the # character to get my thousands printed out with commas:
i: 150 f: 1,234.57

Note that the formatter only uses the # characters and commas that it needs. The value 150 does not have a thousands digit so it and the comma are left out. Note also though that I have included a 0 as the smallest digit. This is so that when I print the value 0 I actually get a value printed, otherwise when I print zero I get nothing on the page.

Finally I can add a width value to the print layout information. This is very useful if you want to print material in columns:

int i=150;
double f=1234.5678;
Console.WriteLine("i:{0,10:0} f:{1,15:0.00}",i,f);
Console.WriteLine ("i:{0,10:0} f: {1,15:0.00}",0,0);

This would produce the output:
i: 150 f: 1234.57
i: 0 f: 0.00

The integer value is printed in a column 10 characters wide, and the double is printed in a 15 character wide column. At the moment the output is right justified, if I want the numbers left justified I make the width negative:

int i=150;
double f=1234.56789;
Console.WriteLine("i:{0,-10:0} f:{1,-15:0.00}",i,f); Console.WriteLine("i:{0,-10:0} f:{1,-15:0.00}",0,0);

This would produce the output:
i: 150 f: 1234.57
i: 0 f: 0.00

Note that this justification would work even if you were printing a string rather than a number, so if you want to print columns of words you can use this technique to do it. You can specify the print width of any item, even a piece of text, which makes printing in columns very easy.