C#
IndexOf and LastIndexOf
Learn to search with IndexOf and LastIndexOf.
Searching with IndexOf
The IndexOf method returns the first position of a substring inside a string. If it cannot find it, it returns -1. That return value lets you guard the search with an if.
For example, "alex@codeblanks".IndexOf('@') returns 4 because the '@' sign sits at the fourth index. The search is case-sensitive, so IndexOf('A') and IndexOf('a') may give different results.
Last position with LastIndexOf
The LastIndexOf method returns the last position of a substring inside a string. It is ideal for finding the final folder name in a file path or the last '/' in a URL. If not found, it also returns -1.
Example: "a/b/c.txt".LastIndexOf('/') returns 3. Then Substring(lastSlash + 1) yields just the file name. This is a common pattern in file-handling code.
Providing a start index
IndexOf and LastIndexOf accept a second argument as a starting position. IndexOf('a', 5) begins the search at index 5 and skips earlier matches. This is useful for walking through multiple occurrences in a string.
For example, to visit every space in a text you start each search one past the previous hit. The loop then advances through the string and ends when the call returns -1.
Exercise Find the file name
Get the position of the last '/' in path using LastIndexOf and store it in lastSlash. Then call Substring(lastSlash + 1) to assign the file name to fileName and print it with Console.WriteLine.
Show example solution
Get lastSlash = path.LastIndexOf('/'); then fileName = path.Substring(lastSlash + 1); and print it.
Example solution:
using System;
class Program
{
static void Main()
{
string path = "docs/reports/2024.pdf";
int lastSlash = path.LastIndexOf('/');
string fileName = path.Substring(lastSlash + 1);
Console.WriteLine(fileName);
}
}
Test yourself
-
What does IndexOf return when the substring is missing?
int i = "hello".IndexOf('z');
- 0
- -1
- null
- Throws FormatException
Show answer
-1 — Correct! IndexOf returns -1 when nothing is found; it cannot return null.
-
Position of the last slash
string path = "a/b/c.txt";
int lastSlash = path____;
Console.WriteLine(lastSlash);
Show answer
string path = "a/b/c.txt";
int lastSlash = path.LastIndexOf('/');
Console.WriteLine(lastSlash);
-
Match the call with its result
- "hello".IndexOf('l')
- 2
- "hello".LastIndexOf('l')
- 3
- "hello".IndexOf('z')
- -1
Next lesson: Split and Join
