Some Instance Methods and the Length Property

Strings are a special type in C#. We have used string literals as parameters to functions and we have used the special concatenation operator +. Thus far we have not emphasized the use of objects, or even noted what is an object. In fact strings are objects. Like other objects, strings have a general notation for functions that are specially tied to the particular type of object. These functions are called instance methods. They always act on an object of the particular class, but a reference to the object is not placed inside the parameter list, but before the method name and a dot as in:

csharp> string s = "hello";
csharp> s.ToUpper();
"HELLO"

ToUpper (the method converting to upper case) does a particular action that makes sense with strings. It takes s (the string object reference before the dot in this example) and returns a new string in upper case, based on s. Since this action depends only on the string itself, no further parameters are necessary, and the parentheses after the method name are empty. The general method syntax is

object-reference.methodName (further-parameters )

More string methods are listed below, some with further parameters.

Data can also be associated with object properties. A property of a string is its Length (an int). References to property values use dot notation but do not have a parameter list in parentheses at the end:

csharp> string s = "Hello";
csharp> s.Length;
5
csharp> "".Length;
0

Be careful: Though 5 is the length of s in the example above, the last character in s is s[4]. Using s[5] would generate an IndexOutOfRangeException.

String objects have associated string methods which can be used to manipulate string values. There are an enormous number of string methods, but here are just a few of the most common ones to get you started. The string object to which the method is being applied is referred to as this string in the descriptions. After the methods, the length property is also listed. In the heading this object is not shown explicitly, so be careful when applying these methods and the length property: In actual use in your programs they must be preceded by a reference to a string, followed by a dot, as shown in all the examples. The reference to this string can be a variable name, a literal, or any expression evaluating to a string.

Summary of String Length and Some Instance Methods

int IndexOf(string target)

Returns the index of the beginning of the first occurrence of the string target in this string object. Returns -1 if target not found. Examples:

csharp> string greeting = "Bonjour", part = "jo";
csharp> greeting.IndexOf(part);
3
csharp> greeting.IndexOf("jot");
-1

string Substring(int start)

Returns the substring of this string object starting from index start through to the end of the string object. Example:

csharp> string name = "Sheryl Crow";
csharp> name.Substring(7);
"Crow"

string Substring(int start, int len)

Returns the substring of this string object starting from index start, including a total of len characters. Example:

csharp> string name = "Sheryl Crow";
csharp> name.Substring(3,5);
"ryl C"

Java programmers: Note the second parameter is not the same as in Java.

string ToUpper()

Return a string like this string, except all in upper case. Example:

csharp> "Hi Jane!".ToUpper();
"HI JANE!"

string ToLower()

Return a string like this string, except all in lower case. Example:

csharp> "Hi Jane!".ToLower();
"hi jane!"

int Length

Property referring to the length of this string object. Example:

csharp> string greeting = "Bonjour";
csharp> greeting.Length;  //no parentheses
7

Warning

All of these methods that return a string return a new string. No string method alters the original string. Strings are immutable: They are objects that cannot be changed after they are first produced. This is a common source of errors.

csharp> string s = "Hello";
csharp> s.ToUpper()
"HELLO"
csharp> s
"Hello"
csharp> s = s.ToUpper();
csharp> s
"HELLO"

See that you need an explicit assignment if you want the variable associated with the original string to change.

Further string methods are introduced in More String Methods.

Time to reflect, thinking back to Learning to Solve Problems. Without forcing all the code details on yourself, how can you concisely say what powers you have with strings so far? Remember that kernel.

With strings you can: Index characters, find a part; extract a part; convert case; determine length. These may not be evocative phrases for you. Find your own.

When we get to loops, we will find this is useful.

Here is a brief example of a function using several of these methods,

parenthesized/parenthesized.cs:

using System;

class Parenthesized
{
   static void Main()
   {
      string s = "What (really) happened?";
      Console.WriteLine ("Original: " + s);
      Console.WriteLine ("In parentheses: " + InsideParen(s));
   }

   /// Assume s contains parentheses.
   /// Return the substring between the first parentheses.
   static string InsideParen(string s)
   {
      int first = s.IndexOf('(') + 1;
      int past = s.IndexOf(')');
      int len = past - first;
      return s.Substring(first, len);
   }
}

It is a silly assumption, but until we get to Decisions, we will have to assume there is a parenthesized expression in the parameter string.

String Methods Exercise

1. What is printed by this fragment?

   string w = "quickly";
   Console.WriteLine(w.Length);
   Console.WriteLine(w[w.Length-2]);
   Console.WriteLine(w.Substring(3, 2));
   Console.WriteLine(w.Substring(2));
   Console.WriteLine(w.IndexOf("ti"));
   Console.WriteLine(w.IndexOf("ick"));
   int k = w.IndexOf("c");
   Console.WriteLine("{0} {1} {2} {3}",
      k, w[k], w[k-3], w.Substring(k));
   

2. What is printed by this fragment?

   string s = "HELLO!", t = s.ToLower();
   Console.WriteLine(s+t);
   

Play with csharp: Declare other strings and make up string expressions with these methods for which you predict the value and then test.