IndexOf

From the start to the end, the IndexOf method in C# searches one string for another. It returns the index of the string part, if one is found.

Return value

If the substring or char is not found, IndexOf returns -1. If we test for positive values, we can tell if the value was present.

An example

We use IndexOf to see if a string contains a word. Usually we want to know the exact result of IndexOf. We can store its result in an int local.

using System;

string value = "cat,dog";

// Part 1: find index of dog.
Console.WriteLine(value.IndexOf("dog"));

// Part 2: test not found value.
if (value.IndexOf("turnip") == -1)
{
    Console.WriteLine("?");
}
4
?

While loop

We often use IndexOf in loops. We have to keep track of several values at once. This prevents us from searching the same parts over and over again.

using System;

// The input string.
string s = "I have a cat";

// Loop through all instances of the letter a.
int i = 0;
while ((i = s.IndexOf('a', i)) != -1)
{
    // Print out the substring.
    Console.WriteLine(s.Substring(i));

    // Increment the index.
    i++;
}
ave a cat
a cat
at

Return value

This is -1 when IndexOf does not find anything, and the index if it does. Often an IndexOutOfRangeException can provoked by using the -1 in other code (like an array access).

using System;

string source = "big dog";
// See result of IndexOf method.
Console.WriteLine("NOT FOUND: {0}", source.IndexOf("cat"));
Console.WriteLine("FOUND: {0}", source.IndexOf("dog"));
NOT FOUND: -1
FOUND: 4

Substring

We can use IndexOf with the Substring method. Here we get the first substring that begins with a certain pattern or character.

using System;

// Input.
const string s = "I have a cat";

// Location of the letter c.
int i = s.IndexOf('c');

// Remainder of string starting at c.
string d = s.Substring(i);
Console.WriteLine(d);
cat

Skip start

Often strings have leading characters that we know cannot contain the searched-for value. We can skip these chars. This will give a performance boost—less searching is needed.

using System;

string value = ":100,200";
// Skip the first character with a startIndex of 1.
int comma = value.IndexOf(',', 1);
Console.WriteLine(comma);
4

IndexOfAny

This method receives a char array argument. It searches for the first index of any of the characters provided in that array.

using System;

var code = "xyz";
// Use IndexOfAny to find an "a" or "z".
int result = code.IndexOfAny(new char[] { 'a', 'z' });
if (result != -1)
{
    Console.WriteLine("a or z found at {0}", result);
}
a or z found at 2

LastIndexOf

This works like IndexOf, but searches in reverse, from the right to the left part of the string. It starts with the last char.

using System;

var code = "cat_dog_?";
// Find last underscore.
int result = code.LastIndexOf('_');
if (result != -1)
{
    Console.WriteLine("_ found at {0}", result);
}
_ found at 7

Contains

This is a wrapper method. It calls IndexOf with StringComparison.Ordinal. It returns true or false, not an integer—so it is a bit simpler to use.

using System;

var name = "red paint";
// Use Contains to see if a string is found in a source string.
if (name.Contains("paint"))
{
    Console.WriteLine("paint found!");
}
paint found!

Benchmark, for-loop

Should we prefer IndexOf over for-loops when-possible? Scanning a string is a common operation. Choosing the fastest code here could help many programs.

using System;
using System.Diagnostics;

const int _max = 1000000;
string s = "abc.123.456.xyz";

// Version 1: use for-loop to count chars.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int c = 0;
    for (int e = 0; e < s.Length; e++)
    {
        if (s[e] == '.')
        {
            c++;
        }
    }
}
s1.Stop();

// Version 2: use IndexOf to count chars.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int c = 0;
    int e = 0;
    while ((e = s.IndexOf('.', e)) != -1)
    {
        e++;
        c++;
    }
}
s2.Stop();

// Result times.
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
18.21 ns:    For-loop
26.78 ns:    IndexOf, while-loop

Benchmark, char argument

Sometimes we are searching for a single character. We can specify this single character as a 1-char string, or a char.

using System;
using System.Diagnostics;

const int _max = 1000000;
string value = "The puppy is adorable.";

// Version 1: use char argument.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int result = value.IndexOf('a');
}
s1.Stop();

// Version 2: use string argument.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int result = value.IndexOf("a");
}
s2.Stop();

// Result times.
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
 13.07 ns:    IndexOf char argument
226.86 ns:    IndexOf string argument

Array.IndexOf

We call IndexOf on strings, but we can also use (different) IndexOf methods on arrays and Lists. These also return -1 if nothing is found.

We commonly use IndexOf to search for chars (or substrings) in strings. For complex problems, a for-loop may be easier to write and faster to execute.