Java

TOPIC 50 – WALKING THROUGH A STRING

LESSON NOTE

TEMPLATE TO WALK THROUGH A STRING

By “walking through a String”, we mean going through a String one letter at a time. It is very similar as going through an array one element at a time.

There is a general algorithm that allows us to do this:

for (int x = 0; x < str.length(); x++)

{

String c = str.substring(x, x+1);

//Deal with letter stored in c

}

Above, for each iteration, c will get a different letter of the String str. For the first pass, c will get the first letter. For the second pass, c will get the next letter. And so on…

USEFULNESS

Problems dealing with walking through a String are interesting and useful because:

• Their solution can be quite difficult yet very short and elegant.
• They are problems that often come up in real programming.
• They are fun!!! (Right?)

EXAMPLES

Example 1 - Write a program that will get a String from the user and output it one letter at a time.

System.out.println(“Enter a word”);

String word = DummiesIO.getString();

for (int x = 0; x < word.length(); x++)

{

String c = word.substring(x, x+1);

System.out.println(c);

}

As you can see, we have only added a single statement to our template. Getting to full understand the template is very important! This program would be much harder if we didn’t start with the template.

Example 2 – Write a program that will get a String from the user and count the number upper case letters in that String. Results should be outputted to screen.

System.out.println(“Enter a word”);

String str = DummiesIO.getString();

int count = 0;

for (int x = 0; x < str.length(); x++)

{

String c = str.substring(x, x+1);

if (c.equals(c.toUpperCase())) //compare c to itself upperCased

{

count++;

}

}

System.out.println(“Number of upper case letters: “ + count);

Again, we have only added a few lines to the template. The condition is obviously the most challenging part. It verifies to see if c and the uppercase version of c are the same. If they are, then c is already in upper case and we therefore increment the value of count.

Example 3 – Write a program that inverts the order of a String.

System.out.println(“Enter a word”);

String s = DummiesIO.getString();

String invString = “”;

for (int x = 0; x < s.length(); x++)

{

String c = s.substring(x, x+1);

invString = c + invString;

}

System.out.println(invString);

Again, we have only added two lines to our template. Isn’t this great?

Lets look at a trace of the above code. Lets assume that the user entered “hockey”.

Iteration #	Concatenation	InvString value
Before loop	--	“”
1	“h” + “”	“h”
2	“o” + “h”	“oh”
3	“c” + “oh”	“coh”
4	“k” + “coh”	“kcoh”
5	“e” + “kcoh”	“ekcoh”
6	“y” + “ekcoh”	“yekcoh”

It is very important to note that each time invString is given a value, it is a completely new String that is being given. We are not adding to the previous String.