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Fix the Substring Check

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containsWord(text, word) returns true when word occurs anywhere in text (case-sensitive), and false otherwise.

A user reports that searching for "Error" in "Error: disk full" returns false. Find the bug and fix it. The same bug exists in every language version.

Example 1

Input: text = "disk full: Error", word = "Error"
Output: true

Example 2

Input: text = "Error: disk full", word = "Error"
Output: true
Explanation: The buggy version returns false.

The buggy code

function containsWord(text, word) {
  if (text.indexOf(word)) return true;
  return false;
}

function containsWord(text: string, word: string): boolean {
  if (text.indexOf(word)) return true;
  return false;
}

def contains_word(text: str, word: str) -> bool:
    if text.find(word):
        return True
    return False

<?php

function containsWord(string $text, string $word): bool
{
    if (strpos($text, $word)) {
        return true;
    }
    return false;
}

class Solution {
    public boolean containsWord(String text, String word) {
        if (text.indexOf(word) > 0) return true;
        return false;
    }
}

Hints

Hint 1

What does indexOf / strpos / find return when the match starts at position 0?

Hint 2

And what do they return when there is no match at all?

Find and fix the bug

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Solution and explanation

Show the bug and the fix

Time O(n x m) · Space O(1)

The bug: the position of the match is used as a boolean. A match at position 0 is falsy, so it reads as "not found". In JavaScript and Python the no-match value (-1) is truthy, so the check is also backwards for missing words; in PHP, strpos returns false for no match and 0 for a match at the start, and both look false to if. Java's version compares > 0, which has the same off-by-one.

The fix: use the API that returns a boolean (includes, in, str_contains, contains), or compare explicitly against the not-found value (!== -1, !== false, >= 0).

This is one of the most common PHP interview gotchas, and the PHP manual has a warning box about it on the strpos page.

Fixed version

function containsWord(text, word) {
  return text.includes(word); // or: text.indexOf(word) !== -1
}

function containsWord(text: string, word: string): boolean {
  return text.includes(word); // or: text.indexOf(word) !== -1
}

def contains_word(text: str, word: str) -> bool:
    return word in text  # or: text.find(word) != -1

<?php

function containsWord(string $text, string $word): bool
{
    return str_contains($text, $word); // PHP 8; before: strpos($text, $word) !== false
}

class Solution {
    public boolean containsWord(String text, String word) {
        return text.contains(word); // or: text.indexOf(word) >= 0
    }
}

Common mistakes

PHP: if (strpos(...) != false) with loose comparison: 0 == false is true, so it is still wrong.

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