C Programming Yellow Book

Simple Data Processing
Writing a Program
2.3.5 Neater Printing
Note that the way that a
number is printed does not
affect how it is stored by the
program, it just tells the
printing method how it is
supposed to be printed.
If you have run any of the above programs you will by now have discovered that the
way in which numbers are printed leaves much to be desired. Integers seem to come
out OK, but floating point numbers seem to have a mind of their own. To get around
this C# provides a slightly different way in which numbers can be printed. This
provides more flexibility, and is also somewhat easier to use if you are printing a
large number of values.
Using Placeholders in Print Strings
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 99 th 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.
Adjusting real number precision
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.
Specifying the number of printed digits
I can specify a particular number of digits by putting in a given number of zeroes:
int i = 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.
Really Fancy Formatting
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 );
C# Programming © Rob Miles 2015 46