Find. Often we wish to locate the index of a substring inside another string. In Rust, we can call find() on a String. A find() function is also available on iters.
For strings, the find function searches from the start. And rfind begins at the rightmost position. A function like is_ascii_digit can be used to match character groups.
String find. To start, we use the find() function in its simplest way. Find() works in the same way as other languages like Python. But we must unwrap the result.
Part 1 We use find() and get an option—this helps us determine if a value exists or not. To get the actual value, we use if let Some.
Part 2 After we call find() successfully we can use get() to take a substring at the index returned.
Return The find() function can work exactly like IndexOf in other languages. It returns None if no match is found, not -1.
fn main() {
// Part 1: use find as indexof.
let value = "_xy_xy";
if let Some(result) = value.find("xy") {
println!("INDEXOF: {result}");
}
// Part 2: get substring based on index.
let animals = "cat and dog";
if let Some(result) = animals.find("do") {
if let Some(inner) = animals.get(result..) {
println!("SUBSTRING AT DO: {inner}");
}
}
}INDEXOF: 1
SUBSTRING AT DO: dog
No result. Find() returns an option. What is the best way to handle this? The version 2 in this example is probably the best solution—we can even call find() directly in the if-statement.
Version 1 It is possible to test the result directly against None, but we still need to unwrap() the result in this case.
Version 2 We can test the result of find() in an if-statement, and use Some() to unwrap its result if it is found.
fn main() {
let value = "aa";
// Version 1: if no result is found, find returns None.
let result = value.find("zz");
if result == None {
println!("Returned None");
}
// Version 2: use Some to test, and then get the actual result.
let result2 = value.find("a");
if let Some(m) = &result2 {
println!("{}", m);
}
}Returned None
0
Rfind example. This function does the same thing as find but proceeds from the rightmost position towards the left. It acts in reverse.
And Sometimes we have a string like the one in the example that contains two "xy" substrings.
Detail With the example string, rfind will find the second instance, while find will return the first.
fn main() {
let value = "_xy_xy";
// Use rfind to search from the right.
let result = value.rfind("xy").unwrap();
println!("RFIND: {}", result);
}RFIND: 4
First digit. We can pass a function or closure to the find() function. Suppose we want to find the first digit in a string, and take the remaining part as a substring.
Here We pass is_ascii_digit to find. Then we call get() to extract a substring based on the first digit's position.
fn main() {
let value = "abc123";
// Find index of first digit.
let result = value.find(|c: char| c.is_ascii_digit()).unwrap();
// Take substring of remaining chars.
let numbers = value.get(result..).unwrap();
// Print results.
println!("FIRST DIGIT: {}", result);
println!("FIRST DIGIT START: {}", numbers);
}FIRST DIGIT: 3
FIRST DIGIT START: 123
Iter find. It is possible to call a find function on an iterator as well. With a vector or array, call iter() and then call find.
Here We use find() to get the first element that is greater than or equal to 20—this returns the value 25.
Warning With find() we may need to use the reference operator on the argument twice, which can be confusing at first.
fn main() {
let v = vec![10, 25, 30];
// Use find on an iter() to search a vector for a matching element.
if let Some(result) = v.iter().find(|&&x| x >= 20) {
println!("Greater than or equal to 20: {result}");
}
}Greater than or equal to 20: 25
A review. We search strings with the find function. We can search from start to end, and in the opposite direction. Find() is also a function on iterators.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Nov 22, 2023 (edit).
