How to get the last n-characters in a string in Dart?

Question

How do I get the last n-characters in a string?

I\'ve tried using:

var string = \'Dart is fun\';
var newString = string.substring(-5);

Answer 1

While @Alexandre Ardhuin is correct, it is important to note that if the string has fewer than n characters, an exception will be raised:

Uncaught Error: RangeError: Value not in range: -5

It would behoove you to check the length before running it that way

String newString(String oldString, int n) {
  if (oldString.length >= n) {
    return oldString.substring(oldString.length - n)
  } else {
    // return whatever you want
  }
}

While you're at it, you might also consider ensuring that the given string is not null.

oldString ??= '';

If you like one-liners, another options would be:

String newString = oldString.padLeft(n).substring(max(oldString.length - n, 0)).trim()

If you expect it to always return a string with length of n, you could pad it with whatever default value you want (.padLeft(n, '0')), or just leave off the trim().

---

At least, as of Dart SDK 2.8.1, that is the case. I know they are working on improving null safety and this might change in the future.

Answer 2

var newString = string.substring((string.length - 5).clamp(0, string.length));

note: I am using clamp in order to avoid Value Range Error. By that you are also immune to negative n-characters if that is somehow calculated.

In fact I wonder that dart does not have such clamp implemented within the substring method.

If you want to be null aware, just use:

var newString = string?.substring((string.length - 5).clamp(0, string.length));

Answer 3

Create an extension:

extension E on String {
  String lastChars(int n) => substring(length - n);
}

Usage:

var source = 'Hello World';
var output = source.lastChars(5); // 'World'

Answer 4

var newString = string.substring(string.length - 5);