#Lecture 101 — Dart Libraries

print, List and DateTime work in every file, yet sqrt and Random are "not found". They live in libraries you have to import — and one import line unlocks them.

Contents

Prerequisites · Connection · Objectives · Lesson · Code Examples · Common Mistakes · Best Practices · Mini Challenge · Practice Questions · Answers · Takeaways

Every output and error message in this lecture is real. Each one was produced by running the code shown on Dart 3.12.2. Programs that use Random() print different numbers on every run; the others use a fixed seed so your output matches.

#Prerequisites

Students should already understand, from Lectures 035–090:

• passing a function as a tear-off, and reduce on a list (Lectures 035 and 044)
• importing another file with import 'bank_account.dart'; (Lecture 057)
• that privacy with _ is per file (Lecture 057)
• extensions and their names (Lecture 069)
• import 'dart:async'; for TimeoutException (Lecture 090)

#Connection With Previous Lectures

In Lecture 044 we found the biggest mark with reduce, because lists have no .max. Now Sara wants the square root of a number:

void main() {
  print(sqrt(16));
}

Error: Method not found: 'sqrt'.
  print(sqrt(16));
        ^^^^

Dart has a sqrt function — it just isn't visible in this file. To use it, we need to know where Dart keeps its tools and how to bring them in.

#Learning Objectives

By the end of this lecture you will be able to:

• Explain what a library is in Dart
• Recognize the core libraries dart:core, dart:math, dart:async, dart:io, dart:convert and dart:collection
• Import a library with import 'dart:math';
• Use sqrt, pi, max, min, pow and Random
• Read a name-clash error and fix it with an as prefix
• Limit what an import brings in with show and hide
• Recognize the error messages Dart produces for common import mistakes

#1. What Is a Library?

A library is a collection of code — functions, classes, constants — that other code can use. In Dart, every .dart file is a library. That is why privacy with _ works per file (Lecture 057): a private name stays inside its own library.

Analogy — think of a school with many rooms. Each room (library) has its own tools. To use the science lab's microscope, you first go to the science lab: that is an import.

#2. dart:core Is Always There

Dart ships with core libraries whose names start with dart:. One of them, dart:core, is imported into every file automatically. It holds print, int, String, List, Map, DateTime and everything you have used so far:

void main() {
  var exam = DateTime(2026, 10, 17);
  var marks = <int>[80, 75];
  print(exam.year);
  print(marks.length);
  print(int.parse('42') + 1);
}

2026
2
43

No import line — yet every name works.

#3. The Core Libraries

Everything else needs an import. These are the core libraries you will meet most:

#4. Importing dart:math

Write import with the library name in quotes, at the very top of the file:

import 'dart:math';

void main() {
  print(sqrt(16));
  print(pi);
}

4.0
3.141592653589793

sqrt and pi are now visible. sqrt always returns a double, so 16 gives 4.0.

#5. max, min and pow

dart:math also compares and raises numbers:

import 'dart:math';

void main() {
  print(max(80, 95));
  print(min(80, 95));
  print(pow(2, 10));
}

95
80
1024

max and min take exactly two numbers. For a whole list, pass max as a tear-off to reduce (Lectures 035 and 044):

import 'dart:math';

void main() {
  var marks = [72, 95, 64, 88];
  print(marks.reduce(max));
  print(marks.reduce(min));
}

95
64

A ready-made .max and .sum on lists comes from a package, which we meet in Lecture 103 — Dart Packages.

#6. Random Numbers

Random makes random numbers. nextInt(6) gives 0 to 5, so add 1 for a dice roll:

import 'dart:math';

void main() {
  var random = Random();
  var dice = random.nextInt(6) + 1;
  print('Ali rolled $dice');
}

Ali rolled 2

Run it again and you get another number. Give Random a seed — Random(42) — and it produces the same sequence every run, which is useful for testing:

import 'dart:math';

void main() {
  var random = Random(42);
  print(random.nextInt(6) + 1);
  print(random.nextInt(6) + 1);
  print(random.nextInt(6) + 1);
}

2
1
1

#7. Other Core Libraries

Several imports can sit together. Here are three more core libraries at work:

import 'dart:collection';
import 'dart:convert';
import 'dart:io';

void main() {
  print(jsonEncode({'name': 'Sara', 'marks': 88}));

  var line = Queue<String>();
  line.add('Ali');
  line.add('Sara');
  line.addFirst('Zayan');
  print(line);

  stdout.write('Hello, ');
  stdout.writeln('Dart!');
}

{"name":"Sara","marks":88}
{Zayan, Ali, Sara}
Hello, Dart!

jsonEncode turns a map into JSON text, a Queue adds and removes at both ends, and stdout.write prints without starting a new line. We will study JSON in Lecture 108 and files with dart:io in Lecture 106.

#8. When Two Libraries Use the Same Name

Ali and Sara each wrote a helper file, and both named their function describe:

// ali_tools.dart
String describe(int marks) => marks >= 50 ? 'Pass' : 'Fail';

// sara_tools.dart
String describe(int marks) => 'Marks: $marks';

extension ShoutTools on String {
  String shout() => toUpperCase();
}

Import both, and Dart cannot know which describe you mean:

import 'ali_tools.dart';
import 'sara_tools.dart';

void main() {
  print(describe(72));
}

Error: 'describe' is imported from both 'ali_tools.dart' and 'sara_tools.dart'.
  print(describe(72));
        ^^^^^^^^

Writing helper files like these is the topic of Lecture 102 — Creating Your Own Libraries.

#9. Fixing a Clash with as

as gives a library a prefix. Its names are then reached through that prefix, like a surname:

import 'ali_tools.dart';
import 'sara_tools.dart' as sara;

void main() {
  print(describe(72));
  print(sara.describe(72));
}

Pass
Marks: 72

Many programmers prefix dart:math too, so math.max reads clearly:

import 'dart:math' as math;

void main() {
  print(math.max(72, 95));
  print(math.sqrt(81));
}

95
9.0

#10. show and hide

show imports only the names you list. hide imports everything except them:

import 'ali_tools.dart';
import 'sara_tools.dart' hide describe;

void main() {
  print(describe(72));
  print('sara'.shout());
}

Pass
SARA

hide describe removed the clash, while ShoutTools still arrived. Names work for extensions too — this is how you hide an extension (Lecture 069): import 'sara_tools.dart' hide ShoutTools;. With show, a name you did not list is simply missing:

import 'dart:math' show sqrt, pi;

void main() {
  print(sqrt(25));
  print(pi);
}

5.0
3.141592653589793

Calling max here would give Error: Method not found: 'max'.

#11. Deferred Imports (A Quick Look)

You may see import 'reports.dart' deferred as reports;. It loads a library only when the program asks for it, mainly so web apps start faster. You won't need it in this course — just recognize it.

#12. A Real-World Example

A class result summary that uses a prefix, show, and a seeded lucky draw:

import 'dart:convert' show jsonEncode;
import 'dart:math' as math;

void main() {
  var marks = {'Ali': 72, 'Sara': 95, 'Zayan': 64, 'Hina': 88};
  var values = marks.values.toList();

  var highest = values.reduce(math.max);
  var lowest = values.reduce(math.min);
  var names = marks.keys.toList();
  var winner = names[math.Random(7).nextInt(names.length)];

  print('Highest: $highest, Lowest: $lowest');
  print('Lucky draw: $winner');
  print(jsonEncode({'highest': highest, 'lowest': lowest, 'winner': winner}));
}

Highest: 95, Lowest: 64
Lucky draw: Ali
{"highest":95,"lowest":64,"winner":"Ali"}

math. shows at a glance which names come from dart:math, and show jsonEncode brings in just the one function we need.

#Code Examples

#Example 1 — dart:core Needs No Import

void main() {
  var name = 'Zayan';
  var age = int.parse('6');
  print('$name is $age');
  print(name.toUpperCase());
}

Zayan is 6
ZAYAN

#Example 2 — Area of a Circle

import 'dart:math';

void main() {
  var radius = 7;
  var area = pi * radius * radius;
  print('Area: ${area.toStringAsFixed(2)}');
}

Area: 153.94

#Example 3 — max, min, pow

import 'dart:math';

void main() {
  print(max(-3, 8));
  print(min(4.5, 2.5));
  print(pow(5, 3));
}

8
2.5
125

#Example 4 — Seeded Dice

import 'dart:math';

void main() {
  var random = Random(3);
  for (var i = 1; i <= 3; i++) {
    print('Roll $i: ${random.nextInt(6) + 1}');
  }
}

Roll 1: 2
Roll 2: 6
Roll 3: 5

#Example 5 — An as Prefix

import 'dart:math' as math;

void main() {
  var side = math.sqrt(49);
  print('Side: $side');
  print('Bigger: ${math.max(side, 6.5)}');
}

Side: 7.0
Bigger: 7.0

#Example 6 — show Only What You Need

import 'dart:math' show pow, sqrt;

void main() {
  var a = 6;
  var b = 8;
  var c = sqrt(pow(a, 2) + pow(b, 2));
  print('Hypotenuse: $c');
}

Hypotenuse: 10.0

#Example 7 — Student Marks

import 'dart:math' as math;

void main() {
  var marks = [55, 91, 78, 66];
  var best = marks.reduce(math.max);
  var worst = marks.reduce(math.min);
  print('Best: $best');
  print('Worst: $worst');
  print('Gap: ${best - worst}');
}

Best: 91
Worst: 55
Gap: 36

How it works

1. math.max is passed to reduce as a tear-off — no parentheses.
2. reduce keeps the bigger of each pair, ending with 91.
3. The same works with math.min for the lowest mark.

#Example 8 — Shopping Total as JSON

import 'dart:convert' show jsonEncode;

void main() {
  var cart = {'Notebook': 500, 'Pen': 50, 'Bag': 1500};
  var total = cart.values.reduce((a, b) => a + b);
  print(jsonEncode({'items': cart.length, 'total': total}));
}

{"items":3,"total":2050}

How it works

1. dart:core gives the map and reduce; no import is needed for them.
2. show jsonEncode imports a single function from dart:convert.
3. jsonEncode writes keys in double quotes, as JSON requires.

#Example 9 — Bank Balance with Interest

import 'dart:math' as math;

void main() {
  var balance = 1000;
  var rate = 0.05;
  var years = 3;
  var result = balance * math.pow(1 + rate, years);
  print('After $years years: ${result.toStringAsFixed(2)}');
}

After 3 years: 1157.63

How it works

1. Each year multiplies the balance by 1.05; three years is 1.05 to the power 3.
2. math.pow(1.05, 3) is about 1.157625.
3. toStringAsFixed(2) rounds to cents: 1157.63.

#Example 10 — Last Three Temperatures

import 'dart:collection';

void main() {
  var readings = Queue<double>();
  for (var t in [29.5, 31.0, 30.5, 32.0]) {
    readings.add(t);
    if (readings.length > 3) readings.removeFirst();
  }
  print(readings);
}

{31.0, 30.5, 32.0}

How it works

1. Queue comes from dart:collection.
2. Each new reading joins at the back with add.
3. Once there are more than three, removeFirst drops the oldest, so only the latest three remain.

#Common Beginner Mistakes

1. Forgetting the import

void main() {
  // print(sqrt(16)); // ✗ without import 'dart:math'
}

Error: Method not found: 'sqrt'.

2. Putting import below other code

void main() {
  print('x');
}

// import 'dart:math'; // ✗ must be at the top

Error: Directives must appear before any declarations.
Try moving the directive before any declarations.

3. Leaving out the quotes

// import dart:math; // ✗

Error: Expected a String, but got 'dart'.

The library name is a string: import 'dart:math';.

4. Misspelling the library

// import 'dart:maths'; // ✗

Error: Dart library 'dart:maths' is not available on this platform.

5. Adding a prefix, then forgetting it

import 'dart:math' as math;

void main() {
  // print(sqrt(4)); // ✗
  print(math.sqrt(4));
}

Error: Method not found: 'sqrt'.

With as math, every name needs math. in front.

6. Expecting your own name to clash with dart:math

If your file marks_tools.dart defines a max and you also import dart:math, Dart does not report a clash — your max quietly wins over the dart: one. A clash error appears only between two of your own libraries (or two dart: ones). Use a prefix to be clear about which one you call.

#Best Practices

• Put all imports at the top, dart: imports first, in alphabetical order.
• Import only the libraries you use; dart analyze warns about unused imports.
• Prefix libraries whose names are short or common, such as import 'dart:math' as math;.
• Use show when you need one or two names from a big library.
• Use hide to remove a single clashing name.
• Seed Random while testing; drop the seed for real randomness.

#Mini Challenge

Zayan rolls two dice with Random(7) from a prefixed dart:math. Print both rolls, their total, and the higher roll using max.

Your program should print exactly:

Rolls: 5 and 6
Total: 11
Higher: 6

Try it yourself before opening the solution.

import 'dart:math' as math;

void main() {
  var random = math.Random(7);
  var first = random.nextInt(6) + 1;
  var second = random.nextInt(6) + 1;
  print('Rolls: $first and $second');
  print('Total: ${first + second}');
  print('Higher: ${math.max(first, second)}');
}

Rolls: 5 and 6
Total: 11
Higher: 6

#Practice Questions

Work through these in order — they run from beginner to application to challenge.

Beginner

1. Import dart:math and print the square root of 144.
2. Print pi rounded to two decimal places.
3. Print the larger and the smaller of 15 and 40 using max and min.

Application

4. Print the area of a circle with radius 10, rounded to two decimals.

5. Import dart:math with the prefix math and print 3 to the power 4.

6. Import only sqrt with show, and print the square root of 2.25.

7. For the marks [48, 83, 97, 61], print the highest and lowest with reduce.

8. This program does not compile. Fix it in two different ways:

   dartCopy

   import 'dart:math' as math;
   
   void main() {
     print(sqrt(49));
   }

Challenge

9. Sara saves 2000 at 10% interest per year for 2 years. Use pow to print Balance: 2420.00.
10. Using Random(42), draw five lucky numbers from 1 to 100 into a list. Print the list and the biggest number.

#Practice Answers

#Question 1 — Solution

import 'dart:math';

void main() {
  print(sqrt(144));
}

12.0

#Question 2 — Solution

import 'dart:math';

void main() {
  print(pi.toStringAsFixed(2));
}

3.14

#Question 3 — Solution

import 'dart:math';

void main() {
  print(max(15, 40));
  print(min(15, 40));
}

40
15

#Question 4 — Solution

import 'dart:math';

void main() {
  var area = pi * 10 * 10;
  print(area.toStringAsFixed(2));
}

314.16

#Question 5 — Solution

import 'dart:math' as math;

void main() {
  print(math.pow(3, 4));
}

81

#Question 6 — Solution

import 'dart:math' show sqrt;

void main() {
  print(sqrt(2.25));
}

1.5

#Question 7 — Solution

import 'dart:math';

void main() {
  var marks = [48, 83, 97, 61];
  print(marks.reduce(max));
  print(marks.reduce(min));
}

97
48

#Question 8 — Solution

Either add the prefix to the call, or remove the prefix from the import:

import 'dart:math' as math;

void main() {
  print(math.sqrt(49));
}

import 'dart:math';

void main() {
  print(sqrt(49));
}

Both print:

7.0

#Question 9 — Solution

import 'dart:math';

void main() {
  var balance = 2000 * pow(1.10, 2);
  print('Balance: ${balance.toStringAsFixed(2)}');
}

Balance: 2420.00

#Question 10 — Solution

import 'dart:math';

void main() {
  var random = Random(42);
  var lucky = [for (var i = 0; i < 5; i++) random.nextInt(100) + 1];
  print(lucky);
  print('Biggest: ${lucky.reduce(max)}');
}

[88, 59, 5, 86, 78]
Biggest: 88

The seed makes the draw repeatable; without it, every run gives new numbers.

#Quick Revision

1. What is a library in Dart?
2. Which core library is imported automatically?
3. Where must import lines go?
4. Which library holds sqrt, pi and Random?
5. How many numbers do max and min take?
6. What does as do in an import?
7. What is the difference between show and hide?
8. Why would you give Random a seed?

#Key Takeaways

• Every Dart file is a library; privacy with _ is per library.
• dart:core is imported automatically; other dart: libraries need import.
• dart:math gives sqrt, pi, max, min, pow and Random.
• Imports go at the top of the file, with the library name in quotes.
• Two imports with the same name cause an error; an as prefix separates them.
• show imports only the listed names; hide imports all but them — extensions included.
• deferred as loads a library later; you only need to recognize it.

#What's Next?

Lecture 102 — Creating Your Own Libraries. Ali's and Sara's helper files were libraries too. Next we will build our own multi-file libraries on purpose — splitting code into files, choosing what each file makes public, and importing them cleanly.