2.11. Substitutions in Console.WriteLine

2.11.1. Output With +

An elaboration of a “Hello, World” program, could greet the user, after obtaining the user’s name. If the user enters the name Elliot, the program could print

Hello, Elliot!

This is a very simple input-process-output program (in fact with almost no “process”). Think how would you code it.

You need to obtain a name, remember it and use it in your output. A solution is in the next section.

2.11.2. String Format Operation

A common convention is fill-in-the blanks. For instance,

Hello, _____!

and you can fill in the name of the person greeted, and combine given text with a chosen insertion. C# has a similar construction, better called fill-in-the-braces, that can be used with Console.WriteLine.

Instead of inserting user input with the + operation as in hello_you1/hello_you1.cs:

using System;

class HelloYou1
{
   static void Main ()
   {
      Console.WriteLine ("What is your name?");
      string name = Console.ReadLine ();
      Console.WriteLine ("Hello, " + name + "!");
   }

}

look at a variation, hello_you2/hello_you2.cs, shown below. Both programs look exactly the same to the user:

using System;

class HelloYou
{
   static void Main ()
   {
      Console.WriteLine ("What is your name?");
      string name = Console.ReadLine ();
      Console.WriteLine ("Hello, {0}!", name);
   }

}

All the new syntax is in the line:

Console.WriteLine ("Hello, {0}!", name);

Console.WriteLine actually can take parameters after an initial string, but only when the string is in the form of a format string, with expression(s) in braces where substitutions are to be made, (like in fill-in-the-blanks). Here the format string is "Hello, {0}!".

The remaining parameters, after the initial string, give the values to be substituted. To know which further parameter to substitute, the parameters after the initial string are implicitly numbered, starting from 0. Starting with 0 is consistent with other numbering sequences in C#. So here, where there is just one value to substitute (name), it gets the index 0, and where it is substituted, the braces get 0 inside, to indicate that parameter with index 0 is to be substituted.

Everything in the initial string that is outside the braces is just repeated verbatim. In particular, if the only parameter is a string with no braces, it is printed completely verbatim (reducing to the situations where we have used Console.WriteLine before).

A more elaborate silly examples that you could test in csharp would be:

string first = "Peter";
string last = "Piper";
string what = "pick";
Console.WriteLine("{0} {1}, {0} {1}, {2}.", first, last, what);

It would print:

Peter Piper, Peter Piper, pick.

where parameter 0 is first (value "Peter"), parameter 1 is last ( value "Piper"), and parameter 2 is what (value "pick").

Make sure you see why the given output is exactly what is printed.

Or try in csharp:

int x = 7;
int y = 5;
Console.WriteLine("{0} plus {1} is {2}; {0} times {1} is {3}.", x, y, x+y, x*y);

and see it print:

7 plus 5 is 12; 7 times 5 is 35.

Note the following features of the parameters after the first string:

  • These parameters can be any expression, and the expressions get evaluated before printing.
  • These parameters to be substituted can be of any type.
  • These parameters are automatically converted to a string form, just as in the use of the string + operation.

In fact the simple use of format strings shown so far can be completed replaced by long expressions with +, if that is your taste. We later discusses fancier formatting in Tables, that cannot be duplicated with a simple string + operation. We will use the simple numbered substitutions for now just to get used to the idea of substitution.

A technical point: Since braces have special meaning in a format string, there must be a special rule if you want braces to actually be included in the final formatted string. The rule is to double the braces: "{{" and "}}". The fragment

int a = 2, b = 3;
Console.WriteLine("The set is {{{0}, {1}}}.", a, b);

produces

The set is {2, 3}.

Note: Braces only get their special meaning if there are at least two parameters (forcing the first parameter to be a format string). If there is just a single parameter, braces are interpreted as regular characters.

2.11.2.1. Format Reading Exercise

What is printed?

Console.WriteLine("{0}{1}{1}{2}", "Mi", "ssi", "ppi");

Check yourself.

2.11.2.2. Exercise for Format

Write a program, quotient_format.cs, that behaves like Exercise for Quotients, but generate the sentence using Console.WriteLine with a format string and no + operator.

2.11.2.3. Madlib Exercise

Write a program, my_mad_lib.cs, that prompts the user for words that fit specified gramatical patterns ( a noun, a verb, a color, a city....) and plug them into a multiline format string so they fit grammatically, and print the usually silly result. If you are not used to mad libs, try running (not looking at the source code) the example project mad_lib, and then try it again with different data. If this exercise seems like too big of a challenge yet, see our example source code, mad_lib/mad_lib.cs, and then start over on your own.

2.11.3. Overloading

The WriteLine function can take parameters in different ways:

  • It can take a single parameter of an type (and print its string representation).
  • It can take a string parameter followed by any number of parameters used to substitute into the initial format string.
  • It can take no parameters, and just advance to the next line (not used yet in this book).

Though each of these uses has the same name, Console.WriteLine, they are technically all different functions: A function is not just recognized by its name, but by its signature, which includes the name and the number and types of parameters. The technical term for using the same name with different signatures for different functions is overloading the function (or method).

This only makes practical sense for a group of closely related functions, where the use of the same name is more helpful than confusing.