Check if a string is substring of another (original) (raw)
Last Updated : 16 Oct, 2024
Given two strings **txt and **pat, the task is to find if **pat is a substring of **txt. If yes, return the index of the first occurrence, else return **-1.
**Examples :
**Input: txt = "geeksforgeeks", pat = "eks"
**Output: 2
**Explanation: String "eks" is present at index 2 and 9, so 2 is the smallest index.**Input: txt = "geeksforgeeks", pat = "xyz"
**Output: -1.
**Explanation: There is no occurrence of "xyz" in "geeksforgeeks"
Table of Content
- Using nested loops - O(m*n) Time and O(1) Space
- Using Pattern Searching Algorithms
- Using in-built library functions
**Using nested loops - O(m*n) Time and O(1) Space
The basic idea is to iterate through a loop in the string **txtand for every index in the string **txt, check whether we can match **pat starting from this index. This can be done by running a nested loop for traversing the string **patand checking for each character of **pat matches with **txt or not. If all character of **pat matches then return the starting index of such substring.
C++ `
// C++ program to check if a string is substring of other // using nested loops
#include using namespace std;
// Function to find if pat is a substring of txt int findSubstring(string &txt, string &pat) { int n = txt.length(); int m = pat.length();
// Iterate through txt
for (int i = 0; i <= n - m; i++) {
// Check for substring match
int j;
for (j = 0; j < m; j++) {
// Mismatch found
if (txt[i + j] != pat[j]) {
break;
}
}
// If we completed the inner loop, we found a match
if (j == m) {
// Return starting index
return i;
}
}
// No match found
return -1;}
int main() { string txt = "geeksforgeeks"; string pat = "eks"; cout << findSubstring(txt, pat);
return 0;}
C
// C program to check if a string is substring of other // using nested loops
#include <stdio.h> #include <string.h>
// Function to find if pat is a substring of txt int findSubstring(char *txt, char *pat) { int n = strlen(txt); int m = strlen(pat);
// Iterate through txt
for (int i = 0; i <= n - m; i++) {
// Check for substring match
int j;
for (j = 0; j < m; j++) {
// Mismatch found
if (txt[i + j] != pat[j]) {
break;
}
}
// If we completed the inner loop, we found a match
if (j == m) {
// Return starting index
return i;
}
}
// No match found
return -1;}
int main() { char txt[] = "geeksforgeeks"; char pat[] = "eks"; printf("%d", findSubstring(txt, pat));
return 0;}
Java
// Java program to check if a string is substring of other // using nested loops
class GfG {
// Function to find if pat is a substring of txt
static int findSubstring(String txt, String pat) {
int n = txt.length();
int m = pat.length();
// Iterate through txt
for (int i = 0; i <= n - m; i++) {
// Check for substring match
int j;
for (j = 0; j < m; j++) {
// Mismatch found
if (txt.charAt(i + j) != pat.charAt(j)) {
break;
}
}
// If we completed the inner loop, we found a match
if (j == m) {
// Return starting index
return i;
}
}
// No match found
return -1;
}
public static void main(String[] args) {
String txt = "geeksforgeeks";
String pat = "eks";
System.out.println(findSubstring(txt, pat));
}}
Python
Python program to check if a string is a substring of another
using nested loops
Function to find if pat is a substring of txt
def findSubstring(txt, pat): n = len(txt) m = len(pat)
# Iterate through txt
for i in range(n - m + 1):
# Check for substring match
j = 0
while j < m and txt[i + j] == pat[j]:
j += 1
# If we completed the inner loop, we found a match
if j == m:
return i
# No match found
return -1if name == "main": txt = "geeksforgeeks" pat = "eks" print(findSubstring(txt, pat))
C#
// C# program to check if a string is substring of other // using nested loops
using System;
class GfG {
// Function to find if pat is a substring of txt
static int findSubstring(string txt, string pat) {
int n = txt.Length;
int m = pat.Length;
// Iterate through txt
for (int i = 0; i <= n - m; i++) {
// Check for substring match
int j;
for (j = 0; j < m; j++) {
// Mismatch found
if (txt[i + j] != pat[j]) {
break;
}
}
// If we completed the inner loop, we found a match
if (j == m) {
// Return starting index
return i;
}
}
// No match found
return -1;
}
static void Main() {
string txt = "geeksforgeeks";
string pat = "eks";
Console.WriteLine(findSubstring(txt, pat));
}}
JavaScript
// JavaScript program to check if a string is a substring of another // using nested loops
// Function to find if pat is a substring of txt function findSubstring(txt, pat) { const n = txt.length; const m = pat.length;
// Iterate through txt
for (let i = 0; i <= n - m; i++) {
// Check for substring match
let j;
for (j = 0; j < m; j++) {
// Mismatch found
if (txt[i + j] !== pat[j]) {
break;
}
}
// If we completed the inner loop, we found a match
if (j === m) {
// Return starting index
return i;
}
}
// No match found
return -1;}
// Driver code const txt = "geeksforgeeks"; const pat = "eks"; console.log(findSubstring(txt, pat));
`
**Time complexity: O(m * n) where **m and **n are lengths of **txt and **pat respectively.
**Auxiliary Space: O(1), as no extra space is required.
Using Pattern Searching Algorithms
There are several Pattern Searching Algorithms like KMP Algorithm, Rabin-Karp Algorithm, Aho-Corasick Algorithm, etc. Please refer to Introduction to Pattern Searching to learn more about them.
Using in-built library functions
This approach uses a built-in function to quickly check if **pattern is part of **text or not. This makes the process simple and efficient.
C++ `
// C++ program to check if a string is substring of other // using in-built functions
#include using namespace std;
int main() { string txt = "geeksforgeeks"; string pat = "eks";
// If pat is found, returns the index of first
// occurrence of pat. Otherwise, returns a special
// constant value std:🧵:npos
size_t idx = txt.find(pat);
if(idx != string::npos)
cout << idx;
else
cout << -1;
return 0;}
C
// C program to check if a string is substring of other // using in-built functions
#include <stdio.h> #include <string.h>
int main() { const char *txt = "geeksforgeeks"; const char *pat = "eks";
// If pat is found, strstr returns a pointer to the first
// occurrence of pat in txt. Otherwise, it returns NULL.
const char *idx = strstr(txt, pat);
if (idx != NULL) {
// Calculate index by subtracting the base address
// of txt from result
printf("%ld", idx - txt);
}
else {
printf("-1");
}
return 0;}
Java
// Java program to check if a string is substring of other // using in-built functions
class GfG { public static void main(String[] args) { String txt = "geeksforgeeks"; String pat = "eks";
// If pat is found, returns the index of first
// occurrence of pat. Otherwise, returns -1
int idx = txt.indexOf(pat);
if (idx != -1)
System.out.println(idx);
else
System.out.println(-1);
}}
Python
Python program to check if a string is substring of other
using in-built functions
if name == "main": txt = "geeksforgeeks" pat = "eks"
# If pat is found, returns the index of first
# occurrence of pat. Otherwise, returns a special
# constant value -1
idx = txt.find(pat)
print(idx)C#
// C# program to check if a string is substring of other // using in-built functions
using System;
class GfG { static void Main() { string txt = "geeksforgeeks"; string pat = "eks";
// If pat is found, returns the index of first
// occurrence of pat. Otherwise, returns a special
// constant value -1
int idx = txt.IndexOf(pat);
Console.WriteLine(idx);
}}
JavaScript
// JavaScript program to check if a string is substring of other // using in-built functions
let txt = "geeksforgeeks"; let pat = "eks";
// If pat is found, returns the index of first // occurrence of pat. Otherwise, returns -1 let idx = txt.indexOf(pat);
console.log(idx);
`
**Note: The time complexity of in-built functions can differ across different languages.