6

Strings

 

Strings are an indispensable part of any programming language. They are used to input, manipulate and output a collection of characters or words. Since, every program has to deal with a large amount of textual data, the significance of string handling cannot be over-emphasised. The importance of WMLScript will be highlighted in this chapter in the context of using strings. The actual intent behind having WML and WMLScript is to allow a user to enter data, modify it and output the result. The data may be in the form of numbers or characters. Hence, as far as a collection of characters is concerned, a series of functions are mandatory to facilitate string manipulations.

 

In fact, languages like Perl and ASP are preferred by programmers who need to carry out a large number of string manipulations because of the diverse range of number functions that they provide for this purpose. Similarly, WMLScript has a large number of functions that deal exclusively with Strings. WMLScript has a library called String, which contains all the functions that deal with strings. We have already familiarized ourselves with the length function.

 

The first function that we shall look at is isEmpty.

 

Screen 6. 1

aaa.wmls

extern function abc()

{

var i;

i = String.isEmpty("");

if ( i )

Dialogs.alert("true");

else

Dialogs.alert("false");

}

 

 

A string is considered empty if it consists of two double inverted commas with no space between them. If you pass this value to the isEmpty function, it will return true. If you put any text such as "abc" or even a space between the double inverted commas, the function isEmpty will return false.

 

Screen 6. 2

aaa.wmls

extern function abc()

{

var i;

i = String.isEmpty(" ");

if ( i )

Dialogs.alert("true");

else

Dialogs.alert("false");

}

 

 

Remember, all string manipulations require numbers or integers. If you enter a number with a floating point value, Float.int is used to remove the decimal place.

 

We require a function that gives us the position of a specified character within a string. There is a function called charAt which is used for this purpose. In the illustration given below, we have asked the function to convey to us as to which character is available at position 1 in the string "ABC".

 

Screen 6. 3

aaa.wmls

extern function abc()

{

var aa;

aa = String.charAt("ABC",1);

Dialogs.alert(aa);

}

 

 

 

 

 

The length of the string is 3, but since we start counting from 0, the character at position 1 refers to the second character of the string, which happens to be B. The function charAt returns a string of length 1. When you deal with strings, you can have a string consisting of just one character. Such a string will not have a datatype of character.

 

If you ask the fuction charAt to return the character at position 20, it will return an empty string ,since the length of the string is only 3. Therefore, in the following example, where the function charAt has been passed a value of 5 and the string has only 3 characters, the result will be an empty string.

 

Screen 6. 4

aaa.wmls

extern function abc()

{

var aa;

aa = String.charAt("ABC",5);

Dialogs.alert(aa);

}

 

If you pass a value such as -1 to this function, the return value will again be an empty string.

 

 

 

Now, we need to go a step further and look at another function that will return a part of the string that is given as a parameter. This function is called substring. The first parameter to this function is a string; the second one gives the position from where the substring is to be extracted; and the third parameter specifies the number of characters to be extracted. Here also, the function starts counting from 0. The number 2 means that the extraction should start from the third character i.e. C in this example; and the number 3 specifies that 3 characters are to be extracted.

 

Screen 6. 5

aaa.wmls

extern function abc()

{

var aa;

aa = String.subString("ABCDEF",2,3);

Dialogs.alert(aa);

}

 

The value that is returned is "CDE ", which is a substring.

 

 

 

If the start index which is the second parameter, is less than 0, WMLScript will first error check and make it 0.

If the start index is more than the length of the string, the function will return a null string or an empty string.

If the number of characters to be extracted is more than the number of characters available for extraction, this number will get replaced by the length of the string.

 

The next function has been repeated from the third chapter which deals with variables. We have already stated that integers, floats and boolean values get converted to a string. WMLScript does this internally by using the toString function.

 

Screen 6. 6

aaa.wmls

extern function abc()

{

var aa;

aa = String.toString(100);

Dialogs.alert(aa);

}

 

 

 

 

 

If we input the number 100 to toSring, it will return a string "100". However, if we input the number 100.67, it will return a string "100.67". The Boolean values of true and false also get converted to strings, as explained in Chapter 3. Thus, we could have used the function toString instead of using the function format. The functions String.format and String.toString are synonymous with each other and can be used interchangeably.

 

The only difference here is that if you input an invalid value, toString will return the string "invalid". This is the only function that converts the invalid value to a string. And, this happens to be the only variation from WMLScript. It is pertinent to note that "invalid" is a separate data type by itself, and you cannot convert it to anything else.

 

There is another function called compare.

 

Screen 6. 7

aaa.wmls

extern function abc()

{

var i,aa;

i = String.compare("ABC","PQR");

aa = String.format("%d..",i);

Dialogs.alert(aa);

}

 

The function compare takes two strings as parameters. The output can be as follows :

If the two strings are the same, then the function compare returns 0.

If the first string is larger than the second, it returns a value of +1.

If the first string is smaller (which is the case here), it will return a value of -1.

 

For two strings to be considered equal, they have to contain the same number of characters and also the same characters. So, ABCD and ABC are not equal because one has a length of 4 and the other has a length of 3. This function does not carry out case folding. Case folding simply means conversion from the lower case to the upper case, and vice versa. So, this function is case sensitive. Thus, the strings "ABC" and "abc" are not equal.

 

As a programmer, you will need a provision, by means of which, you can remove spaces from a string. For this purpose, a function called squeeze has been provided.

 

Screen 6. 8

aaa.wmls

extern function abc()

{

var aa;

aa = String.squeeze(" ABC DEF ");

Dialogs.alert(aa);

}

 

 

 

 

 

Squeeze removes all the spaces in the string, whether they are at the beginning, in the middle, or at the end of the string. There is another function called trim that removes the leading and trailing spaces.

 

Screen 6. 9

aaa.wmls

extern function abc()

{

var aa;

aa = String.trim(" ABC DEF ");

Dialogs.alert(aa);

}

 

 

 

 

 

Next, we shall look at the function String.find that lets you search for a substring within a string.

 

Screen 6. 10

aaa.wmls

extern function abc()

{

var aa,i;

i = String.find("ABCDEF","CD");

aa = String.format("%d..",i);

Dialogs.alert(aa);

}

 

 

 

 

For example, if you search for the substring "CD" in the string "ABCDEF", the return value of the function will be 2, as the function will find it at the third location. Remember, the function starts counting from 0.

 

If you change CD to CDF, you will get -1 as the answer because this substring is not found in the string. The function find does not look for a part of the substring. It either finds the whole substring or none at all.

 

The next function is String.replace that enables you to replace one substring with another within a string.

 

Screen 6. 11

aaa.wmls

extern function abc()

{

var aa;

aa = String.replace("ABCD AB AD ","AB","XYZ");

Dialogs.alert(aa);

}

 

 

 

 

 

In the original string, we wish to find a string called AB and replace it with XYZ. First, the substring AB which is part of ABCD gets replaced with XYZ. Then, the next occurrence of AB gets replaced by XYZ. But, the substring AD does not get replaced because it does not match the substring AB. All the occurrences of the substring get replaced within the main string.

 

The function explored below is String.elements.

 

Screen 6. 12

aaa.wmls

extern function abc()

{

var i,aa;

i = String.elements("hi,bye,",",");

aa = String.format("%d..",i);

Dialogs.alert(aa);

}

 

 

 

 

Now, we shall cover an interesting function called String.Elements. WMLScript has a concept of arrays. The second symbol that we pass as a parameter is a separator, which is a single character. In the example above, it is a comma. The string has 3 components :

 

The first component is upto the first comma.

The second one is the text bye.

The third one is a null.

Since the string has 3 elements, the function string.elements will return a value of 3.

 

If there are 2 commas and the 3rd element is null, string.element will still return 3. Now, suppose we had used a comma and a semicolon as separators, it is smart enough to recognize only the first one, which is a comma. So, do not put multiple separators because they will all get ignored and only the first one will be accepted.

 

An empty string is also valid. The function string.element can never return a value of 0 or a negative value. It will return a value of 1 even for an empty string ,since an empty string is also considered a separator.

 

If the first parameter is an empty string and the second parameter is a comma, you will get an empty string. If both are null, you will still get the value of 1, because an empty string is a valid element.

 

So, we have the function string.elementAt at this point.

 

Screen 6. 13

aaa.wmls

extern function abc()

{

var aa;

aa = String.elementAt("My name is vijay",1," ");

Dialogs.alert(aa);

}

 

 

 

 

 

The above example is interesting. It is because there are 4 elements and the separator is a space. We have specified the second parameter of the function String.elementAt as 1. This represents the second element of the string, which is "name". Had we given a value of 0, the answer would have been "My".

 

There are few cardinal rules that you need to know :

The index of the first element is 0.

If you give an index that is larger than the actual number of elements, the function will return the last element.

If the first parameter is an empty string, the function will return an empty string.

If you give a separator like a comma, which is not present in the string, it will return the entire string.

 

The next function is string.RemoveAt. It removes the element at a specified position in the string.

 

Screen 6. 14

aaa.wmls

extern function abc()

{

var i,aa;

aa = String.removeAt("My name is Vijay",1," ");

Dialogs.alert(aa);

}

 

 

 

 

 

Even the separator, which immediately follows the specified element, is removed by this function. So, in the above example, the element number 1 is "name". When the removeAt function executes, it removes the element "name" and the space that immediately follows this element. The string now becomes "My is Vijay". All the cardinal rules given above are applicable here also.

 

We shall now consider the function String.replaceAt.

 

Screen 6. 15

aaa.wmls

extern function abc()

{

var i,aa;

aa = String.replaceAt("My name is Vijay","boss",1,"");

Dialogs.alert(aa);

}

 

 

 

 

 

ReplaceAt simply removes the element at the specified position and replaces it with the word specified as the second parameter. Here, we are interested in the second element. The separator is a space, and the new word to be inserted is "boss". This function will replace the element "name" with "boss", and the resultant string will now become "My boss is Vijay". The same cardinal rules are applicable here also.

 

The next function of interest is String.InsertAt.

 

Screen 6. 16

aaa.wmls

extern function abc()

{

var i,aa;

aa = String.insertAt("My name is Vijay","boss",1,"");

Dialogs.alert(aa);

}

 

 

 

 

 

You would have noticed that the parameters in this case, are similar in structure to the replaceAt function. In this example, the function adds the word "boss" as the second element of the string. The resultant string now becomes "My boss name is Vijay". If the value of 1 is replaced with 7, and, since the string does not have 7 elements, the word "boss" gets added after the last word. So, the resultant string now becomes "My name is Vijay boss".

 

We have already used the function String.format. This function is very similar to the printf function in the C programming language. A format specifier of "%9.8d",10.6 will show the number 10 within 9 spaces/zeroes, and the decimal number 6 will be within 8 spaces/zeroes . The precision rules remain the same. Within the double quotes, you can use either of the following :

d for integer values

f for decimal values

s for character strings .

 

Screen 6. 17

aaa.wmls

extern function abc()

{

var i,aa;

i=10.6;

aa = String.format("%2.2f",i);

Dialogs.alert(aa);

}

 

 

 

 

 

 

Screen 6. 18

aaa.wmls

extern function abc()

{

var i,aa;

i="hello";

aa = String.format("%s",i);

Dialogs.alert(aa);

}