• Developing Algorithms Using Strings
  • Finding a substring within a string
  • I T E R A T E
    • POPCORN HACK: Run the code.. what happened? How can we fix it?
    • Another issue:
  • I T E R A T E
  • HACKS

Developing Algorithms Using Strings

LEARNING OBJECTIVES: For algorithms in the context of a particular specification that involves String objects:

• identify standard algorithms
• modify standard algorithms
• develop an algorithm

Java has many methods that are helpful when working with strings:

• String .substring –> retrieves portion of a string
• String .equals –> compares two strings
• String .length –> returns length of a string
• for Loop –> iterating through characters of a string

Finding a substring within a string

We can use the “window” method:

A “window” is created the same length as the substring. We then iterate through the main string in sections and compare to the substring

For example:

I T E R A T E

with substring “ERA”

public class StringFinder {
    public static void main(String[] args) {
        String word = "iterate";
        String sub = "xxx";
        boolean found = false; // will be set to true once substring is found

        for (int i = 0; i < word.length(); i++) { //iterating forwards: starting at first index (0) and going to the length of the word.. let's try word.length
            String portion = word.substring(i, i + sub.length());
            if (sub.length() > portion.length()) break;
            if (portion.equals(sub)) { // make sure you use .equals!!
                found = true;
                break;
            }
        }

        if (found)
            System.out.println("substring is found within string!");
        else
            System.out.println("substring is NOT within string");
    }

}


StringFinder.main(null);

POPCORN HACK: Run the code.. what happened? How can we fix it?

Tell us below!

Another issue:

I T E R A T E

What if our substring was the word “RATE”? Note that RATE is at the end of the whole string

HACKS

Create a algorithm similar to the one above. Except this time, use iteration to count the number of vowels within the main string.

HINT: Use the boolean expressions we have learned in previous lessons. Which would you use when comparing your “window” with multiple options of substrings?