how do I add a try catch block inside a for loop to catch an ArrayIndexOutOfBounds exception?

user_5137406 0
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I am new to using try catch blocks and I need some direction. I have a program that counts letters entered in a phrase from user input. The letters 'A' through 'Z' are converted to a number between 0 & 25 by subtracting the corresponding value of the Unicode character set index value of the letter that's being processed from the value of 'Z', which is 90. How would I add a try catch block inside the first for loop that translates the character to the index so that if the phrase contains any characters that are not a letter it would catch it in the ArrayIndexOutOfBoundsException. I then would have a message that prints out the non letter character in a statement to the user. I know what I want to do but, I'm having a little trouble understanding the syntax that I would need to do this. I tried to add if(counts[i] <= 64 || >= 91), but that doesn't seem to work. Do I need to convert it to a string? I know my code needs to be something like:

  try {
     code...
 }
 catch (StringIndexOutOfBoundsException exception)
 {
    System.out.println("Not a letter: " + counts);
 }

Here is my code without the try/catch block:

import java.util.Scanner;
public class CountLetters
{
    public static void main(String[] args)
    {
        int[] counts = new int[26];
        Scanner scan = new Scanner(System.in);
        //get word from user
        System.out.print("\n Enter a single word (letters only): ");
        String word = scan.nextLine();
        scan.close();
 
        //convert to all upper case
        word = word.toUpperCase();
  
        //count frequency of each letter in string
        for (int i=0; i < word.length(); i++)
          counts[word.charAt(i)-'A']++;
 
        //print frequencies
        System.out.println();
        for (int i=0; i < counts.length; i++)
           if (counts [i] != 0)
               System.out.println((char)(i +'A') + ": " + counts[i]);
    }
}
try-catch java
2 Answers

Instead of catching the exception, just check in your loop:

        for (int i=0; i < word.length(); i++) {
            char currentChar = word.charAt(i);
            if (currentChar >= 'A' && currentChar <= 'Z') {
                counts[currentChar -'A']++;
            }
        }

As was mentioned in the comments, your code shouldn't rely on catching out of bounds exceptions.

user_146242350

All you need to do is to put counts[word.charAt(i) - 'A']++ inside a try-catch block as shown below:

import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int[] counts = new int[26];
        Scanner scan = new Scanner(System.in);
        // Get word from user
        System.out.print("Enter a single word (letters only): ");
        String word = scan.nextLine();
        scan.close();// Do not close it.

        // Convert to all upper case
        word = word.toUpperCase();

        // Count frequency of each letter in string
        for (int i = 0; i < word.length(); i++) {
            char ch = word.charAt(i);
            try {
                counts[ch - 'A']++;// Check this for exception
            } catch (ArrayIndexOutOfBoundsException e) {
                System.out.println("'" + ch + "' is non-alphabetic character.");
            }
        }

        // Print frequencies
        System.out.println();
        for (int i = 0; i < counts.length; i++) {
            if (counts[i] != 0) {
                System.out.println((char) (i + 'A') + ": " + counts[i]);
            }
        }
    }
}

A sample run:

Enter a single word (letters only): hello2world
'2' is non-alphabetic character.

D: 1
E: 1
H: 1
L: 3
O: 2
R: 1
W: 1

Some recommendations:

  1. Do not close Scanner(System.in) as it also closes System.in and there is no way to open it again.
  2. Do use {} for a block for loop or if/else/else if even if it just has a single statement.
user_108195730
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