Generating Random Values In Dart

In this post, we will learn to generate various kinds of random values in Dart for your Flutter application. As a bonus, we will cover generating a random number including both positive and negative numbers.

Introduction

Often we need to work with random values in our application. Maybe we need to generate a random number to get a random item from a list. Or maybe we want to get a random decimal value. These are quite common scenarios. So how can we get these random values?

We can use the Random class which resides in the dart:math library for this purpose. Let’s play around with some examples.

Getting Random Values With Random Class In Dart

First create an instance of Random object.

import 'dart:math';

void main() {
  var randomGenerator = Random();
}

Now we can use the randomGenerator for generating random values.

Random In Dart
Random In Dart

As shown above, we get various method options to generate random values.

Generating A Random Integer Number In Dart

Use the nextInt function to generate a random number. The max parameter allows to set the restriction for maximum allowed valid value when generating the random number.

randomGenerator.nextInt(10);

The code above will generate a random number from 0 to 10.

Note: The maximum value is not included in the results but minimum is included.

Generating A Random Decimal Value

Similar to integer values, we can also generate random decimal values with the Random class object.

randomGenerator.nextDouble();

Unlike nextInt method, the nextDouble method doesn’t take any parameters. The results range from 0.0 to 1.0 with 1.0 being exclusive.

Generating A Random Boolean Value

Finally, we can use the nextBool method to generate a random boolean value in Dart.

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randomGenerator.nextBool();

Bonus

Generating A Random Positive Or Negative Number In Dart

As you might have noticed, the Random class doesn’t have a direct implementation for generating a random number that includes a negative number. However, we can achieve generating a random that includes negative numbers in the following way:

int _generateRandomIncludingNegative(int max) {
  var randomGenerator = Random();
  
  var positive = randomGenerator.nextBool();
  
  var randInt = randomGenerator.nextInt(max);
  
  var result = positive ? randInt : 0 - randInt;
  return result;
}

The function _generateRandomIncludingNegative uses the combination of nextBool and nextInt to generate a random number including both positive and negative number. The number ranges from negative max to positve max.

For example the code below will return values from -5 to 5 excluding both -5 and 5.

print(_generateRandomIncludingNegative(5));

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