Thread: Checking whether a string is a substring of another string without using indexOf

The following topic has been cross posted here.

Write a method that checks whether a string is a substring of another string. Write a test program that prompts the user to enter two strings, and check whether the first string is a substring of the second. Do not use indexOf or contains methods.

Here is my most up to date version of my code:

import java.util.*;
import java.lang.String;
 
public class CheckingSubstring2 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		System.out.print("Please enter a String: ");
		String string1 = input.next();
		System.out.print("Please enter a second String: ");
		String string2 = input.next();
		if (isSubstring(string1, string2)) {
			System.out.println("The first string is a substring of the second.");
		} else {
			System.out.println("The first string is NOT a substring of the second.");
		}
	}	
	public static boolean isSubstring(String string1, String string2) {
		char c;
		char d;
		boolean match = true;
		for (int i = 0; i < string1.length(); i++) {
			c = string1.charAt(i);
			for (int j = 0; j < string2.length(); j++) {
				d = string2.charAt(j);
				if (d != c) {
					match = false;
					System.out.println("MATCH: N The value of c and d: " + c + " and " + d);	
				} else {
					System.out.println("MATCH: Y The value of c and d: " + c + " and " + d);
				}
				match = true;	
			}
		}
		return match;
	}
}

My program compiles but does not work correctly. I have chosen to cross post this problem because I still have no idea what is wrong with my code. Can someone please tell me the line numbers that are causing the problem to not work properly? Thanks.

After a quick look my suggestion is the inner for loop should only be executed if the current char of String1 is the same as the first char of String2. Then the inner loop should check each subsequent pair of chars and break back to the outer loop as soon as they do not match. If you get to the end of the inner loop then it must be a substring.

@Junky Thanks. I modified my code and it seems like it's working. I've tried using several different strings and they all seem to work. For some reason, I think there might still be something wrong with my code. Does it look okay?

@Mr.777 I eventually realized that when I was working on my code some more so I made lots of changes. When you say "get the index of the string 2", do you mean use the indexOf method? I'm not allowed to use that method so I just used a counter instead. Does my code look okay?

import java.util.*;
import java.lang.String;
 
public class CheckingSubstring2 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		System.out.print("Please enter a String: ");
		String string1 = input.next();
		System.out.print("Please enter a second String: ");
		String string2 = input.next();
		if (isSubstring(string1, string2)) {
			System.out.println("The first string is a substring of the second.");
		} else {
			System.out.println("The first string is NOT a substring of the second.");
		}
	}	
	public static boolean isSubstring(String string1, String string2) {
		char c;
		char d;
		boolean match = true;
		int count = 1;
		int counter = 0;
		boolean matchFound = false;
		for (int i = 0; i < string1.length(); i++) {
			c = string1.charAt(i);
			//Part 2: This part determines if the characters following the 1st matching character are also matches
			if (matchFound == true) {
				for (int j = count + counter; j < string2.length(); j++) {
					d = string2.charAt(j);
					if (d != c) {
						System.out.println("MATCH2: N The value of c and d: " + c + " and " + d);	
						match = false;
					} else {
						counter++;
						System.out.println("MATCH2: Y The value of c and d: " + c + " and " + d + " " + counter);
						matchFound = true;
						break;
					}
				}
			}	
			//Part1: This part determines wheter the 1st character of string1 has a match in string2
			//If a match is found, Part 2 happens
			if (matchFound == false) {
				for (int k = 0; k < string2.length(); k++) {
					d = string2.charAt(k);	
					if (d != c) {
						count ++;
						System.out.println("MATCH: N The value of c and d: " + c + " and " + d + " " + count);	
						matchFound = false;
					} else {
						System.out.println("MATCH: Y The value of c and d: " + c + " and " + d + " " + count);
						matchFound = true;
						break;
					}	
				match = true;
				}	
			}	
		}
			return match;
	}
}

Oh I see. I guess my code isn't working correctly. I also found out I need to put a test to make sure string2 is longer than string1. Thanks.