Ensure 4-digit generated numeric value from Set<int> instance with For loop using Dart

Question

I am working with a piece of code snippet where I want to generate a random 4-digit code. But unfortunately, I was not able to get the exact output I desired.

I was using this piece of code snippet:

import 'dart:math';

void _generateCode() {
  Set<int> setOfInts = {};
  setOfInts.add(Random().nextInt(9999));
  for (var num in setOfInts) {
    if (num.toString().length == 4) {
      developer.log('Code: $num');
    } else {
      developer.log('Code: not a 4 digit code');
    }
  }
}

@override
void initState() {
  super.initState();
  _generateCode(); // for testing purposes only
}

The code was mostly generating a 4-digit random code, I thought I could achieve it by including this condition statement,

if (num.toString().length == 4) {
  //fortunately, I could get what I want here
}

But the problem is, according to what is printed on the logs (every time I perform a hot reload):

Restarted application in 218ms.
[log] Code: 8308
Restarted application in 221ms.
[log] Code: 6342
Restarted application in 209ms.
[log] Code: 3404
Restarted application in 208ms.
[log] Code: 9468
Restarted application in 246ms.
[log] Code: 1101
Restarted application in 127ms.
[log] Code: 8230
Restarted application in 133ms.
[log] Code: 4104
Restarted application in 220ms.
[log] Code: 6558
Restarted application in 213ms.
[log] Code: not a 4 digit code 👈 here is the part of the issue it produces
Restarted application in 223ms.
[log] Code: 6234

If I removed the else statement from:

if (num.toString().length == 4) {
  developer.log('Code: $num');
} else {
  developer.log('Code: not a 4 digit code');
}

I wasn't able to get the 4-digit code, I just thought that by using this statement if (num.toString().length == 4), it ensures the generation of the 4-digit code, but it didn't.

How can I resolve it to not generate a 3-digit code value?

Answer 1

If we look at the documentation of Random.nextInt it says:

int nextInt( int max )

Generates a non-negative random integer uniformly distributed in the range from 0, inclusive, to max, exclusive.

So your Random().nextInt(9999) can generate any number from 0 to 9998 (note: not 9999). There is no guarantee that the code will be >999, it may just as well be 1, 2, or 3 digits.

Asimple fix is to just start with 1,000 and then add a number from 0 to 8999 to it:

1000 + Random().nextInt(9000)

Note: 9000 as nextInt will never return the actual max value you pass.