Pattern/Array issue in java

Question

I am studying for an upcoming test next month and looking at some basic problems. This one is a program that requires entering a few sentences and reprinting any that contain a certain string, 'pattern' in this case.

My attempt is below and it compiles however I receive the following error when trying to run it:

 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10
at Grep.main(Grep.java:18)

import java.util.Scanner;
import java.io.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
class Grep {
 public static void main(String[] args) {
 Pattern pattern = Pattern.compile("[Pp]attern");
 String sentences[] = new String[10];
 Scanner scanner = new Scanner(System.in); 
 System.out.println("Please enter some sentences: ");
 for (int i = 0; i <= sentences.length; i++) {
 String s = scanner.next(); 
 sentences[i] = s;
 }
 for (int i = 0; i < sentences.length; i++) { 
 Matcher matcher = pattern.matcher(sentences[i]);
 while (matcher.find()) {
 System.out.println(sentences[i]);
 }
 }
 }
}

Answer 1

for (int i = 0; i <= sentences.length; i++) {

How many items are in the array? What is the last index? What is the last index your loop uses? How many sentances in total does your loop access?

Answer 2

Try this. It works.

Tips: Make sure you use nextLine() so that the input will read full sentences. And I switched your while loop for an if statement within the for loop. No need for two loops there. And I also condensed your first for loop to just one line. No need to create a string variable if you only need it for a second. Just skip that step entirely and get to the point! Good luck, Hope this helps!

Below Is A Program That Mirrors Yours But Now Works

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Grep 
{
 public static void main(String[] args) 
 {
 Pattern pattern = Pattern.compile("[Pp]attern");
 String sentences[] = new String[3];
 Scanner scanner = new Scanner(System.in);
 System.out.println("Please enter some sentences: ");
 for (int i = 0; i < sentences.length; i++) 
 sentences[i] = scanner.nextLine();
 for (int i = 0; i < sentences.length; i++) 
 { 
 Matcher matcher = pattern.matcher(sentences[i]);
 if (matcher.find()) 
 System.out.println(sentences[i]);
 }
 }
} 

Below Is How I Would Write This Same Program. Comments Included For Clarification

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Grep 
{
 public static void main(String[] args) 
 {
 // Initialize and Declare Variables
 Pattern pattern = Pattern.compile("[Pp]attern");
 String sentences[] = new String[3];
 Scanner scanner = new Scanner(System.in);
 int foundCount = 1;
 // Present A Title For The End User
 System.out.println("This Program Will Catch Sentences With The Term Pattern.\n");
 // Read The Inputs From The Users
 for (int i = 0; i < sentences.length; i++)
 {
 System.out.print("Enter Sentence #" + (i+1) + ": ");
 sentences[i] = scanner.nextLine();
 }
 // Line Break
 System.out.println();
 // Write Sentences That Include The Term Pattern
 for (int i = 0; i < sentences.length; i++) 
 { 
 Matcher matcher = pattern.matcher(sentences[i]);
 if (matcher.find())
 {
 System.out.println(foundCount + ") " + sentences[i]);
 foundCount++;
 }
 }
 }
}

• BLL27: Thank you very much for your help. Any tips on how I would invoke the search term from the command line? Eg if I wanted to search for pattern in the input and invoked 'java Grep pattern'.

Answer 3

for (int i = 0; i <= sentences.length; i++) {

The <= needs to be < because you are starting from 0 and you have 10 items, so i has to go from 0 to 9.

• BLL27: Awesome, completely missed that. Thank you.

Answer 4

The problem is in line :18 of your code which is for (int i = 0; i <= sentences.length; i++) it should be for (int i = 0; i < sentences.length; i++)

as you your own in the next for loop in your code has used < instead of <=

• Adriaan Koster: Java generally uses 0-based indexes, so the last element will be at position (size - 1).

Answer 5

Try

for (int i = 0; i < sentences.length; i++)

and you'll be fine :)

• BLL27: Awesome, completely missed that. Thank you.