Expressions and Output: Revision Notes (Computer Science A 1.3)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

• System.out.println(x) displays x and moves to a new line. System.out.print(x) displays x and stays on the same line.
• A string literal is characters in double quotes. Escape sequences start with a backslash: \" is a double quote, \\ is a backslash and \n is a new line. Each is one character.
• The arithmetic operators are +, -, *, / and %.
• Two int operands give an int. At least one double operand gives a double.
• Integer division keeps only the integer part of the quotient: 7 / 2 is 3.
• a % b is the remainder when a is divided by b: 17 % 5 is 2.
• Dividing an int by the integer zero throws an ArithmeticException.

Key relationships

Expression	Value	Reason
7 / 2	3	integer division
7 / 2.0	3.5	one operand is a double
7 % 2	1	7 = 3 * 2 + 1
2 + 3 * 4	14	* before +
(2 + 3) * 4	20	parentheses first
20 / 4 / 2	2	left to right: 5, then 2
7 / 2 * 2.0	6.0	7 / 2 is 3 first, then 3 * 2.0
n % 10	last digit of n	remainder after removing groups of ten
n / 10	n without its last digit	integer division by ten
197 / 60 and 197 % 60	3 and 17	whole hours and leftover minutes

Assumptions and conventions

• % is used with a non-negative first operand and a positive second operand in this course.
• The result type is decided one operation at a time, in evaluation order.
• Expressions that give special double values (such as infinity) are outside the course.
• No calculator is allowed on the exam, so evaluate by hand, one step per line.

Mistakes to avoid

1. Expecting 7 / 2 to be 3.5.
2. Thinking integer division rounds instead of truncating.
3. Evaluating strictly left to right without applying precedence.
4. Forgetting that equal-precedence operators go left to right (10 - 4 - 3 is 3).
5. Writing a lone backslash in a string, or forgetting the closing quote.
6. Using print when println is needed, so the output runs together.
7. Dividing by a variable that holds 0, which compiles but throws an exception.

Quick self-check

1. What does System.out.print("a"); System.out.println("b"); System.out.print("c"); leave on screen? (ab on the first line and c on the second, with the cursor still after the c.)
2. Evaluate 5 + 7 % 3 * 2. (7 % 3 is 1; 1 * 2 is 2; 5 + 2 is 7.)
3. Evaluate 9 / 2 + 9 / 2.0. (4 + 4.5 = 8.5.)
4. What is 345 % 100? (45.)
5. What happens when int n = 0; System.out.println(10 % n); runs? (It compiles; at run time an ArithmeticException is thrown.)

Next: practice questions.