PHP Program To Check If A String Is Substring Of Another
Given two strings s1 and s2, find if s1 is a substring of s2. If yes, return the index of the first occurrence, else return -1.
Examples:
Input: s1 = "for", s2 = "geeksforgeeks"
Output: 5
Explanation: String "for" is present as a substring of s2.
Input: s1 = "practice", s2 = "geeksforgeeks"
Output: -1.
Explanation: There is no occurrence of "practice" in "geeksforgeeks"
Simple Approach
The idea is to run a loop from start to end and for every index in the given string check whether the sub-string can be formed from that index. This can be done by running a nested loop traversing the given string and in that loop run another loop checking for sub-string from every index.
For example – consider there to be a string of length N and a substring of length M. Then run a nested loop, where the outer loop runs from 0 to (N-M) and the inner loop from 0 to M. For very index check if the sub-string traversed by the inner loop is the given sub-string or not.
<?php
// PHP program to check if a
// string is substring of other.
// Returns true if s1 is substring
// of s2
function isSubstring($s1, $s2) {
$M = strlen($s1);
$N = strlen($s2);
// A loop to slide
// pat[] one by one
for ($i = 0; $i <= $N - $M; $i++) {
$j = 0;
// For current index i,
// check for pattern match
for (; $j < $M; $j++)
if ($s2[$i + $j] != $s1[$j])
break;
if ($j == $M)
return $i;
}
return -1;
}
// Driver Code
$s1 = "for";
$s2 = "geeksforgeeks";
$res = isSubstring($s1, $s2);
if ($res == -1)
echo "Not present";
else
echo "Present at index " . $res;
?>
Output
Present at index 5
Complexity Analysis:
- Time complexity: O(m * n) where m and n are lengths of s1 and s2 respectively.
A nested loop is used the outer loop runs from 0 to N-M and inner loop from 0 to M so the complexity is O(m*n). - Space Complexity: O(1).
As no extra space is required.
An efficient solution is to use a O(n) searching algorithm like KMP algorithm, Z algorithm, etc. Please refer complete article on Check if a string is substring of another for more details!
