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Dart: what's the fastest way to check if a particular character in a string is a digit?

Of course I can do this by comparing it to 10 digits or even using regular expressions, but I am looking for the fastest way.

Here's what I came up with now, does that sound reasonable?

int _zero = "0".codeUnits[0];
int _nine = "9".codeUnits[0];
bool isDigit(String s, int idx) => 
 s.codeUnits[idx] >= _zero && s.codeUnits[idx] <= _nine;

I'm a little surprised that I didn't find this method in the standard library, I hope I just missed it.

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3 answers


Try:

bool isDigit(String s, int idx) => (s.codeUnitAt(idx) ^ 0x30) <= 9;


for speed.

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Looks good to me if you are not using Unicode characters.

A similar attempt, but I would not expect this to be faster:



bool isDigit(String s, int idx) =>
  "0".compareTo(s[idx]) <= 0 && "9".compareTo(s[idx]) >= 0;

It would be interesting if the compiler optimizes this duplicate s[idx]

/ s.codeUnits[idx]

, but I would expect it.

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I did a quick micro test for various alternatives and it looks like it is almost a link between the Günter method and brute force testing. I prefer Gunther's method for elegance, but if performance is absolutely critical, it seems like brute force can win by a small margin. The test runs each method once for an index that will return true and once for an index that will return false.

Brute force method:

bool isDigit(String s, int idx) {
  return s[idx] == "0"
      || s[idx] == "1"
      || s[idx] == "2"
      || s[idx] == "3"
      || s[idx] == "4"
      || s[idx] == "5"
      || s[idx] == "6"
      || s[idx] == "7"
      || s[idx] == "8"
      || s[idx] == "9";
}

0.045421617878512024.

Gunther's method:

bool isDigit(String s, int idx) =>
  "0".compareTo(s[idx]) <= 0 && "9".compareTo(s[idx]) >= 0;

0.054188391470161947 us

The way your code is:

int _zero = "0".codeUnits[0];
int _nine = "9".codeUnits[0];
bool isDigit(String s, int idx) => 
 s.codeUnits[idx] >= _zero && s.codeUnits[idx] <= _nine;

0.6344102870896872 us

This can be improved with 2x by storing the results of s.codeUnits [idx] (for some reason the VM cannot optimize it, unlike other methods where repeated calls to s [idx] are optimized and no slower than storing the intermediate result ):

 bool isDigit(String s, int idx) {
     int cuIdx = s.codeUnits[idx];
     cuIdx  >= _zero && cuIdx  <= _nine;
 }

0.29245961607948817 us

Regex method:

RegExp digitRegExp = new RegExp(r'\d');
bool isDigit(String s, int idx) => s[idx].contains(digitRegExp);

4.812064808888846 us

int.parse method (extremely slow):

bool isDigit(String s, int idx) {
  bool isDigit = true;
  try {
    int.parse(s[2]);
  } catch (e) {
    isDigit = false;
  }
  return isDigit;
}

102.48526774276198 us

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