#Lecture 17 — Arithmetic & Assignment Operators

We have used + and * since Lecture 001, but we could not split 125 minutes into hours and minutes, or update a balance without writing its name twice. Dart's full set of arithmetic and assignment operators does both.

Contents

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

Every error message in this lecture is real. Each one was produced by running the code shown on Dart 3.12.2, so you will recognize it when you meet it yourself.

#Prerequisites

Students should already understand, from Lectures 001–016:

• declaring variables with a type, with var, and with final and const
• the number types int, double, and num, and why a double prints .0
• string interpolation with $name and ${expression}
• the null-aware assignment ??= from Lecture 014

#Connection With Previous Lectures

Since Lecture 001 we have added, multiplied, and divided numbers. But try a few everyday questions:

void main() {
  var minutes = 125;
  print(minutes / 60);
}

2.0833333333333335

Nobody says a film lasts "2.0833 hours". We want 2 hours and 5 minutes, and we have no way to get the "5". And every time a balance changed, we wrote balance = balance - 250, typing the name twice. This lecture fills both gaps.

#Learning Objectives

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

• Explain what an operator and an operand are
• Use the arithmetic operators +, -, *, /, ~/, and %
• Predict whether a calculation gives an int or a double
• Split a quantity into whole groups and a leftover with ~/ and %
• Use unary minus to flip the sign of a value
• Update variables with =, +=, -=, *=, /=, ~/=, and %=
• Recognize which compound assignments an int variable rejects, and why
• Recognize the error messages Dart produces for common operator mistakes

#1. What Is an Operator?

An operator is a symbol that does something with values. The values it works on are its operands. In 7 + 3, the operator is + and the operands are 7 and 3.

Analogy — think of the buttons on a calculator. The numbers are what you type in; the operator buttons decide what happens to them.

void main() {
  var result = 7 + 3;
  print(result);
}

10

Most operators sit between two operands. This lecture covers the ones that calculate and the ones that store results.

#2. Addition, Subtraction, and Multiplication

+ adds, - subtracts, and * multiplies. They work exactly as on paper:

void main() {
  var apples = 12;
  var oranges = 5;
  print(apples + oranges);
  print(apples - oranges);
  print(apples * oranges);
}

17
7
60

Two int operands give an int result, so there is no .0 in sight.

#3. Division with / Always Gives a double

The / operator divides — and its answer is always a double, even when the division comes out even:

void main() {
  print(10 / 2);
  print(10 / 4);
}

5.0
2.5

Dart cannot know in advance whether a division will come out even, so / always answers with the type that can hold a fraction.

#4. Integer Division with ~/

Sometimes you only want the whole part. The ~/ operator divides and throws away the fraction, giving an int:

void main() {
  var sweets = 17;
  var children = 5;
  print(sweets ~/ children);
  print(7.5 ~/ 2);
}

3
3

Each child gets 3 whole sweets. Even 7.5 ~/ 2 (which is 3.75) gives the int 3 — ~/ always returns an int, like truncate() from Lecture 009.

#5. Remainder with %

The % operator gives the remainder: what is left over after dividing into whole groups.

void main() {
  var sweets = 17;
  var children = 5;
  print(sweets % children);
  print(20 % 5);
}

2
0

17 sweets shared by 5 children: each gets 3, and 2 are left over. A remainder of 0 means the division comes out even. (For negatives, Dart's % never goes below zero: -7 % 3 is 2. You will rarely need this.)

#6. Using ~/ and % Together

~/ counts the whole groups and % counts what is left. Together they answer the question from the start of this lecture:

void main() {
  var minutes = 125;
  var hours = minutes ~/ 60;
  var rest = minutes % 60;
  print('$hours h $rest min');
}

2 h 5 min

The same pair converts seconds to minutes, items into boxes, or cents into whole coins.

#7. Even, Odd, and the Last Digit

A number is even when dividing by 2 leaves nothing over, so % 2 gives 0 for even numbers and 1 for odd ones. % 10 gives the last digit:

void main() {
  var ticket = 4827;
  print(ticket % 2);
  print(ticket % 10);
}

1
7

This is how isEven and isOdd from Lecture 009 work inside. Turning ticket % 2 == 0 into a true/false answer needs comparison operators — Lecture 018.

#8. Unary Minus

Placed in front of a single value, - flips its sign. Because it has only one operand, it is called unary minus:

void main() {
  var temperature = 8;
  var debt = -250;
  print(-temperature);
  print(-debt);
}

-8
250

-debt is "minus minus 250", which is 250. Unary minus does not change the variable; it gives a new value. Remember Lecture 009's warning: -7.abs() means -(7.abs()), so wrap a negative literal in brackets before calling a method on it.

#9. int or double? The Result Type

The operator and the operands together decide the type of the result:

void main() {
  print(5 + 2.0);
  print(6 / 3);
  print(7.5 % 2);
}

7.0
2.0
1.5

One double operand is enough to make the whole result a double. This matters as soon as you store the result in a typed variable:

void main() {
  int half = 10 / 2;
  print(half);
}

Error: A value of type 'double' can't be assigned to a variable of type 'int'.
  int half = 10 / 2;
                ^

Use ~/ when you want an int, or declare the variable as double.

#10. Which Operator Goes First?

As in school maths, *, /, ~/, and % are worked out before + and -. Brackets go first of all:

void main() {
  print(2 + 3 * 4);
  print((2 + 3) * 4);
}

14
20

When in doubt, add brackets — they cost nothing and make your intent clear. We will study the full order of all operators in Lecture 021 — Operator Precedence & Custom Operators.

#11. The Assignment Operator =

= is an operator too. It works out the right-hand side first, then stores the result in the variable on the left:

void main() {
  var balance = 1000;
  balance = balance - 250;
  print(balance);
}

750

Read = as "becomes", not "equals": balance becomes balance minus 250. Checking whether two values are equal uses a different operator, ==, in Lecture 018.

#12. Compound Assignment: += and -=

"Take the variable, change it, store it back" is so common that Dart has a shortcut. balance -= 250 means exactly balance = balance - 250:

void main() {
  var balance = 1000;
  balance -= 250;
  print(balance);
  balance += 100;
  print(balance);
}

750
850

These are compound assignment operators: an arithmetic operator joined to =. The name is written once, so there is less to type and less to get wrong.

#13. *=, /=, ~/=, and %=

Every arithmetic operator has a compound version:

void main() {
  var price = 200.0;
  price *= 3;
  print(price);
  price /= 4;
  print(price);

  var items = 47;
  items ~/= 10;
  print(items);

  var seconds = 135;
  seconds %= 60;
  print(seconds);
}

600.0
150.0
4
15

items ~/= 10 keeps the number of whole tens; seconds %= 60 keeps only the leftover seconds. You already know one more member of this family: ??= from Lecture 014, which assigns only when the variable is null.

#14. Compound Assignment Keeps the Variable's Type

A compound assignment still stores into the same variable, so the result must fit its type. An int variable cannot take the double that /= produces:

void main() {
  int total = 10;
  total /= 4;
  print(total);
}

Error: A value of type 'double' can't be assigned to a variable of type 'int'.
  total /= 4;
        ^

The same happens with total += 2.5. Use ~/= to stay whole, or make the variable a double from the start.

#15. += with Strings

+ joins strings (Lecture 007), so += works on a String variable too — it adds text to the end:

void main() {
  var name = 'Sara';
  name += ' Ahmed';
  print(name);
}

Sara Ahmed

The other compound operators have no meaning for text. For building messages, interpolation is still the clearer choice.

#16. A Real-World Example

void main() {
  const pencilsPerBox = 12;
  const boxPrice = 150.0;
  var pencils = 100;

  var boxes = pencils ~/ pencilsPerBox;
  var loose = pencils % pencilsPerBox;

  var bill = boxes * boxPrice;
  bill += 25;
  bill -= bill * 0.1;

  print('Full boxes: $boxes');
  print('Loose pencils: $loose');
  print('Bill: ${bill.toStringAsFixed(2)}');
}

Full boxes: 8
Loose pencils: 4
Bill: 1102.50

~/ and % split the pencils into full boxes and leftovers. Because boxPrice is a double, bill is a double, so -= with a 10% discount is allowed. The bill is 1200, plus 25 delivery, minus 10%.

#Code Examples

#Example 1 — The Basic Four

void main() {
  var a = 20;
  var b = 8;
  print(a + b);
  print(a - b);
  print(a * b);
  print(a / b);
}

28
12
160
2.5

#Example 2 — / Versus ~/

void main() {
  print(9 / 3);
  print(9 ~/ 3);
  print(10 / 3);
  print(10 ~/ 3);
}

3.0
3
3.3333333333333335
3

#Example 3 — Whole Groups and Leftovers

void main() {
  var seconds = 250;
  var minutes = seconds ~/ 60;
  var left = seconds % 60;
  print('$minutes min $left sec');
}

4 min 10 sec

#Example 4 — Even or Odd

void main() {
  print(14 % 2);
  print(21 % 2);
  print(0 % 2);
}

0
1
0

#Example 5 — Unary Minus

void main() {
  var profit = 300;
  var loss = -profit;
  print(loss);
  print(-loss);
}

-300
300

#Example 6 — All Six Compound Operators

void main() {
  var value = 50.0;
  value += 10;
  value -= 5;
  value *= 2;
  value /= 4;
  print(value);

  var count = 50;
  count ~/= 3;
  count %= 7;
  print(count);
}

27.5
2

How it works

1. value goes 50.0 → 60.0 → 55.0 → 110.0 → 27.5.
2. value is a double, so /= is allowed.
3. count is an int: 50 ~/ 3 is 16, and 16 % 7 is 2.

#Example 7 — Student Marks

void main() {
  var student = 'Ali';
  var total = 0;
  total += 78;
  total += 91;
  total += 84;
  var average = total / 3;
  print('Student: $student');
  print('Total: $total');
  print('Average: ${average.toStringAsFixed(1)}');
  print('Whole average: ${total ~/ 3}');
}

Student: Ali
Total: 253
Average: 84.3
Whole average: 84

How it works

1. total starts at 0 and collects each mark with +=.
2. total / 3 gives the double 84.33333333333333, formatted to one decimal.
3. total ~/ 3 gives the whole-number average, 84, as an int.

#Example 8 — Shopping Total

void main() {
  var total = 0.0;
  total += 350 * 2;
  total += 120.5;
  total *= 0.85;
  print('Total after 15% off: ${total.toStringAsFixed(2)}');
}

Total after 15% off: 697.42

How it works

1. total starts as 0.0, so it is a double and can take any price.
2. Two items at 350 and one at 120.5 make 820.5.
3. *= 0.85 keeps 85% of the total — a 15% discount: 697.425, shown as 697.42.

#Example 9 — Bank Balance

void main() {
  var owner = 'Sara';
  var balance = 5000.0;
  balance -= 1200;
  balance += 750;
  balance *= 1.02;
  print('Owner: $owner');
  print('Balance: ${balance.toStringAsFixed(2)}');
}

Owner: Sara
Balance: 4641.00

How it works

1. A withdrawal of 1200 and a deposit of 750 leave 4550.0.
2. *= 1.02 adds 2% interest: 4550 × 1.02 is 4641.
3. balance must be a double for *= 1.02 to compile.

#Example 10 — Game Score

void main() {
  var player = 'Zayan';
  var score = 0;
  score += 40;
  score += 35;
  score *= 2;
  score -= 15;
  var stars = score ~/ 50;
  var toNextStar = 50 - score % 50;
  print('Player: $player');
  print('Score: $score');
  print('Stars: $stars');
  print('Points to next star: $toNextStar');
}

Player: Zayan
Score: 135
Stars: 2
Points to next star: 15

How it works

1. score goes 40 → 75 → 150 → 135.
2. Every 50 points earns a star: 135 ~/ 50 is 2.
3. 135 % 50 is 35 points towards the next star, so 15 more are needed.

#Common Beginner Mistakes

1. Storing a / result in an int

void main() {
  // int half = 10 / 2; // ✗
}

Error: A value of type 'double' can't be assigned to a variable of type 'int'.

/ always gives a double. Use ~/ for a whole answer.

2. Using /= or += 2.5 on an int variable

void main() {
  var total = 10;
  // total += 2.5; // ✗
}

Error: A value of type 'double' can't be assigned to a variable of type 'int'.

var total = 10 made total an int. Write var total = 10.0; if it will hold decimals.

3. Writing =+ instead of +=

void main() {
  var count = 5;
  // count =+ 2; // ✗
}

Error: '+' is not a prefix operator.
Try removing '+'.

The arithmetic symbol comes first: +=, -=, *=.

4. Dividing by zero with ~/ or %

void main() {
  print(10 / 0);
  print(10 ~/ 0);
}

Infinity
Unhandled exception:
IntegerDivisionByZeroException

/ by zero gives the special double value Infinity, but ~/ and % by zero stop the program while it is running. (Dart also prints the file locations, left out here.) We will learn to handle such errors in Category 14 — Error & Exception Handling.

5. Compound-assigning a final variable

void main() {
  final total = 10;
  // total += 5; // ✗
}

Error: Can't assign to the final variable 'total'.

+= is still an assignment, and a final variable cannot be assigned again. Use var.

6. Compound-assigning a variable with no value

void main() {
  int score;
  // score += 5; // ✗
}

Error: Non-nullable variable 'score' must be assigned before it can be used.

score += 5 must read score first. Start a running total at 0.

7. Forgetting that * and / go before + and -

void main() {
  var average = 80 + 90 / 2;
  print(average);
}

125.0

No error — just a wrong answer. Only 90 was halved. Write (80 + 90) / 2 to get 85.0.

#Best Practices

• Use / when you need a fraction and ~/ when you need whole groups.
• Pair ~/ with % to split a quantity into groups and a leftover.
• Prefer compound operators (total += price) over repeating the variable name.
• Start a running total at 0 — or 0.0 if decimals may be added.
• Make a variable a double from the start if /= or decimal values will reach it.
• Add brackets whenever the order of a calculation is not obvious.
• Never ~/ or % by a value that could be zero.

#Mini Challenge

Sara has 1000 rupees. A notebook costs 135. Work out how many notebooks she can buy, how much she spends, and her change.

Your program should print exactly:

Notebooks: 7
Spent: 945
Change: 55

Try it yourself before opening the solution.

void main() {
  var money = 1000;
  var price = 135;
  var notebooks = money ~/ price;
  var spent = notebooks * price;
  var change = money % price;
  print('Notebooks: $notebooks');
  print('Spent: $spent');
  print('Change: $change');
}

Notebooks: 7
Spent: 945
Change: 55

#Practice Questions

Work through these in order — they run from beginner to application to challenge. Every question gives you exact values, so you can compare your output with the answers.

Beginner

1. Print the results of 15 + 4, 15 - 4, 15 * 4, and 15 / 4, each on its own line.
2. Print the results of 23 ~/ 4 and 23 % 4, each on its own line.
3. Store temperature = 12 and print it with its sign flipped, using unary minus.

Application

4. Store number = 37. Print its remainder when divided by 2, then its last digit.

5. Store score = 10. Using only compound operators, add 5, multiply by 3, then subtract 9. Print the final score.

6. Store seconds = 3725 and print it as 1 h 2 min 5 sec.

7. This program does not compile. Fix it by changing one operator, so that it prints 81:

   dartCopy

   void main() {
     var total = 245;
     int average = total / 3;
     print(average);
   }

8. Ali's balance is 5000. Using compound operators, withdraw 1200, then deposit 750, and print Balance: 4550.

Challenge

9. A farm packs 100 eggs into trays of 12. Print the number of full trays, the eggs left over, and how many more eggs are needed to fill one more tray: Full trays: 8, Left over: 4, Needed: 8.
10. Ali, Sara, and Zayan share a bill of 1000 in whole rupees. Each pays the same whole amount, and Ali also pays whatever is left over. Print Each pays: 333 and Ali pays: 334.

#Practice Answers

#Question 1 — Solution

void main() {
  print(15 + 4);
  print(15 - 4);
  print(15 * 4);
  print(15 / 4);
}

19
11
60
3.75

#Question 2 — Solution

void main() {
  print(23 ~/ 4);
  print(23 % 4);
}

5
3

4 fits into 23 five whole times (20), leaving 3.

#Question 3 — Solution

void main() {
  var temperature = 12;
  print(-temperature);
}

-12

#Question 4 — Solution

void main() {
  var number = 37;
  print(number % 2);
  print(number % 10);
}

1
7

A remainder of 1 when dividing by 2 means the number is odd.

#Question 5 — Solution

void main() {
  var score = 10;
  score += 5;
  score *= 3;
  score -= 9;
  print(score);
}

36

10 → 15 → 45 → 36.

#Question 6 — Solution

void main() {
  var seconds = 3725;
  var hours = seconds ~/ 3600;
  var minutes = seconds % 3600 ~/ 60;
  var rest = seconds % 60;
  print('$hours h $minutes min $rest sec');
}

1 h 2 min 5 sec

seconds % 3600 is the 125 seconds left after the full hour; ~/ 60 turns them into 2 minutes. % and ~/ have the same priority, so they are worked out left to right.

#Question 7 — Solution

Change / to ~/:

void main() {
  var total = 245;
  int average = total ~/ 3;
  print(average);
}

81

/ would give 81.66666666666667, a double. ~/ gives the whole part as an int.

#Question 8 — Solution

void main() {
  var balance = 5000;
  balance -= 1200;
  balance += 750;
  print('Balance: $balance');
}

Balance: 4550

#Question 9 — Solution

void main() {
  var eggs = 100;
  var perTray = 12;
  var trays = eggs ~/ perTray;
  var left = eggs % perTray;
  var needed = perTray - left;
  print('Full trays: $trays');
  print('Left over: $left');
  print('Needed: $needed');
}

Full trays: 8
Left over: 4
Needed: 8

The partly filled tray already holds 4 eggs, so it needs 12 - 4 more.

#Question 10 — Solution

void main() {
  var bill = 1000;
  var people = 3;
  var share = bill ~/ people;
  var aliPays = share;
  aliPays += bill % people;
  print('Each pays: $share');
  print('Ali pays: $aliPays');
}

Each pays: 333
Ali pays: 334

1000 ~/ 3 is 333, and 1000 % 3 is the 1 rupee nobody has paid yet. Ali adds it with +=: 333 + 333 + 334 is exactly 1000.

#Quick Revision

1. In 8 * 5, which part is the operator and which are the operands?
2. Which type does / always return?
3. What do 17 ~/ 5 and 17 % 5 give?
4. How can % tell you whether a number is even?
5. What does unary minus do to -250?
6. What type is 5 + 2.0, and why?
7. What does price *= 3 mean, written without the shortcut?
8. Why does total /= 4 fail when total is an int?

#Key Takeaways

• An operator works on operands: in 7 + 3, + is the operator.
• +, -, and * behave as on paper; / divides and always returns a double.
• ~/ divides and keeps only the whole part, as an int; % gives the remainder.
• Together, ~/ and % split a quantity into whole groups and a leftover.
• Unary minus (-x) flips a value's sign without changing the variable.
• Two ints give an int (except with /); one double operand makes the result a double.
• = works out the right side first, then stores it; compound operators (+=, -=, *=, /=, ~/=, %=) combine a calculation with that assignment.
• The result of a compound assignment must still fit the variable's type, so /= is not allowed on an int.

#What's Next?

Lecture 18 — Comparison & Equality. We can now calculate answers, but we cannot yet ask questions about them: is the balance more than the price? Is ticket % 2 equal to 0? Next we will meet ==, !=, <, >, <=, and >=, which compare two values and answer with a bool.