5.3 String values

In the first version of class Account, the owner of the account was represented by a String data item.

class Account(owner: var String):
   balance: var float
   interestRate: var float
   -"- 

The specification var String in the declaration of owner, specifies that owner may hold string values, and String is the type of owner.

The type String is usually not considered a primitive value type. The reason is that the primitive values may be represented in a fixed number of bytes in the computer, whereas a String value may need a variable number of bytes for its representation.

A string value is a sequence of characters as opposed to char values, which are single characters. A string value may thus consist of zero or more characters. In the program text, a string value is enclosed in double quotes (“) and special characters are preceded by a slash (\). Example are shown below:

"John Smith"       
"Hello world\n".          -- a newline is represented by \n
"a + b * (c +111)"
"He said: \"Go away!\""   -- the character " is represented by \"

The method length of a String returns the number of characters in the String, and the method get returns the character at a given position in a String.

The following example shows how to get the characters in a given String and print them on the console. s: val "Hello world" specifies that s has the value "Hello world" during the whole program execution.

s: val "Hello world"
for (1):to S.length :repeat
   console.print(S.get[inx])

The default value for a String variable is the empty string ("").

The get method

The next example shows how to search for an occurrence of a given character in a String. The method assumes that the aName holds a name consisting of a first name and a surname, separated by blanks

aName: val "John Smith"
firstSurname:
   i: var integer
   i := 1
   while (aName.get[i] <> ' ') :repeat
       console.print(aName.get[i])
       i := i + 1

Here we use a while-loop which executes a sequence of statements as long as the condition is True.

The expression aName.get[i] <> ' ' is the condition that is evaluated before each iteration of the loop. As long as this condition is true, the statements

console.print(aName.get[i])
i := i + 1

are executed. The operator <> means “not equal”, which means that the expression aName.get[i] <> ' ' is true if aName.get[i] is not equal to the character ' ' (blank).

As can been seen, i = 1 in the first iteration and the condition is thus aName.get[1] <> ' ', which evaluates to true since aName[1] = 'J' (the first char in aName) which is not equal to the the char ' '.

In the second iteration, i = 2 and the condition again evaluates to true since aName[2] = 'o'.

Finally when i = 5, the condition evaluates to False, since aName[5] = ' ', and the while-loop terminates.

String operators

A number of operators are available on String values.

The operator '+' concatenates two String values and return a new String value. This is illustrated by the following example:

S1,S2: var String
S1 := "John" + " Smith"
S2 := "Hello " + S1

The expression "John" + " Smith" evaluates to the String "John Smith", which is then assigned to S1.

The expression "Hello " + S1 evaluates to the String "Hello John Smith", which is assigned to S2.

The operator '=' compares two String values, and evaluates to true if the two Strings are identical and false if they differ. Consider the following example:

S1,S2: var String
S1 := "John"
S2 := "Liza"
S1 = S2  -- false
S1 <> S2 -- true
S1 = "John" -- true
S1 = "john" -- false

Note that S1 = "john" evaluates to false since the upper-case letter 'J' differs from the lower-case letter 'j'.

The operator '<=' compares two String values according to the numerical value of the characters in the strings.

Immutability

A String value is immutable, which means that it cannot be changed. There is thus no put method on String values.