How to find in a substring starting at a specific index in Rust?

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Is there a find function in Rust's standard library which searches for a substring starting at a given index in the string? Just like indexOf in JavaScript.

rust indexof
2 Answers

You would use str::find on a substr and then add the offset back:

let s = "foobarfoo";
let index: Option<usize> = s[4..].find("foo").map(|i| i + 4);
println!("{:?}", index);

Some(6)
user_21891300

I can think of two ways:

Use the .get() method to safely get a slice of an ASCII string and then apply .find to that.

let s = "foobarfoo";
let res = s.get(4..).and_then(|s| s.find("foo").map(|i| i + 4));

Use match_indices to iterate over matches and their indices and then find_map the one that matches the first condition.

let s = "foobarfoo";
let res = s.match_indices("foo").find_map(|(i, _)| (i >= 4).then(|| i));
  • Both methods will return Some(6).
  • If the index is greater or equal to the length of the string you will get None.
user_11139130
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