Expressions, Operators, Precedence and Assignment (ISC Class 12 Computer Science)

Understand Expressions, Operators, Precedence and Assignment in Java

Expressions, Operators, Precedence and Assignment in Java: key facts

Precedence order

• •Postfix ++ --
• •Prefix ++ -- ! unary
• •multiply divide remainder
• •+ -
• •< <= > >=
• •== !=
• •&&
• •||
• •?:
• •assignment

Highest first; same level goes left to right.

Increment forms

• •a++ uses the old value, then adds 1
• •++a adds 1, then uses the new value
• •Same for decrement

Trace each use separately.

Remainder and division

• •(a / b) times b + a % b = a for ints
• •The sign of a % b follows a
• •-17 % 5 = -2
• •17 / 5 = 3

Integer division drops the fraction.

Short-circuit

• •false && x skips x
• •true || x skips x
• •& and
• •always evaluate both sides

Used to guard a division by zero.

How to answer Expressions, Operators, Precedence and Assignment in Java questions

1. 1Write the current value of every variable in a small table.
2. 2Fully bracket the expression using precedence and associativity.
3. 3Evaluate the innermost bracket first; for a++ and ++a update the variable at the right moment.
4. 4Apply integer division when both operands are int.
5. 5For && and ||, stop as soon as the result is decided and do not execute the remaining side effects.
6. 6Update the table and write the printed output exactly.

Expressions, Operators, Precedence and Assignment in Java in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Expressions, Operators, Precedence and Assignment in Java

Short memory aids. Say each one aloud once, then use it on a question the same day.

• MD before AS

  Multiply, divide, remainder before add, subtract; brackets beat all.

  Use it when: Arithmetic precedence.

• a++ after, ++a before

  The sign after the name acts after use; the sign before acts first.

  Use it when: Increment tracing.

• AND stops on false, OR stops on true

  False && anything is false; true || anything is true.

  Use it when: Short-circuit questions.

• LHS holds, RHS computes

  The right side makes a value; the left side is the place it goes.

  Use it when: Assignment questions.

Expressions, Operators, Precedence and Assignment in Java: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

int a = 5, b = 3;
int r = a++ + ++b * 2;
System.out.println(r + " " + a + " " + b);

int x = 10;
x += x++ + ++x;
System.out.println(x);

int a = 4, b = 0;
boolean r1 = (b != 0) && (a / b > 1);
boolean r2 = (b == 0) || (a / b > 1);
boolean r3 = (a > 3) & (b++ == 0);
System.out.println(r1 + " " + r2 + " " + r3 + " " + b);

final double RATE = 0.25;
double amt = 500;
double tax = amt * RATE;
System.out.println(tax + " " + (amt + tax));
System.out.println(1 + 2 + "A" + 1 + 2);

Where students lose marks in Expressions, Operators, Precedence and Assignment in Java

The slips examiners see most often, and what to write instead.

• ✗Mistake: Treating a++ as ++a

  The old value is used in the expression when the sign follows the name.

  Wrong working

  a = 5; r = a++ + 1 gives 7

  Right working

  a = 5; r = a++ + 1 gives 6 and a becomes 6

  ✓Do this: Use the value, then update.

  Usually costs: 1 mark

• ✗Mistake: Left-to-right surprise with strings

  Numbers before the first String are added, not joined.

  Wrong working

  1 + 2 + "A" gives 12A

  Right working

  1 + 2 + "A" gives 3A

  ✓Do this: Evaluate from the left.

  Usually costs: 1 mark

• ✗Mistake: Forgetting that a final variable is fixed

  Any later assignment is a compile-time error.

  Wrong working

  final int N = 5; N = 6;

  Right working

  final int N = 5; int m = N + 1;

  ✓Do this: Write the value once at declaration.

  Usually costs: 1 mark

Practice questions on Expressions, Operators, Precedence and Assignment in Java

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   What is the value of -17 % 5 in Java?
   (a) 2 (b) -2 (c) 3 (d) -3

   Hint

   The sign follows the left operand.

   Answer

   (b) -2

2. Q2 · 2 marks · Short answerBoard level

   Explain the difference between the left-hand side and the right-hand side of an assignment, and say why 5 = x; is invalid.

   Hint

   One is a place, the other a value.

   Answer

   In an assignment the right-hand side is an expression evaluated to a value; the left-hand side must be a variable (a storage location) that receives it. 5 = x; is invalid because 5 is a constant value, not a variable that can store something.

3. Q3 · 3 marks · Short answerHarder board level

   Write the output:

   int p = 7, q = 2;
   double r = p / q * 2.0 + p % q;
   p -= q * 2 + 1;
   System.out.println(r + " " + p);

   Hint

   p / q is integer division.

   Answer

   p / q = 3 (integer), 3 * 2.0 = 6.0, p % q = 1, so r = 7.0. Then p = 7 - (2 * 2 + 1) = 2. Output: 7.0 2

4. Q4 · 3 marks · Short answerHarder board level

   Write the output:

   int a = 8, b = 3;
   int m = a > b ? a - b : b - a;
   boolean t = a % 2 == 0 && b % 2 != 0 || a < b;
   int z = a - b - 2;
   System.out.println(m + " " + t + " " + z);

   Hint

   Bracket by precedence: && before ||.

   Answer

   a > b is true, so m = 8 - 3 = 5. t = ((true) && (true)) || false = true. z = (8 - 3) - 2 = 3. Output: 5 true 3

5. Q5 · 4 marks · Case-basedHOTS / competency

   A student swaps a and b without a third variable: a = a + b; b = a - b; a = a - b;. (i) Trace it for a = 6, b = 9. (ii) Trace it for a = 2000000000, b = 1000000000, where a + b overflows an int, and say whether the swap still works.

   Hint

   int arithmetic wraps by 4294967296.

   Answer

   (i) a = 15; b = 15 - 9 = 6; a = 15 - 6 = 9. Swapped. (ii) a + b = 3000000000, which wraps to -1294967296. b = -1294967296 - 1000000000 = -2294967296, which wraps to 2000000000. a = -1294967296 - 2000000000 = -3294967296, which wraps to 1000000000. The swap still works because int arithmetic wraps consistently, but the intermediate overflow is a risk and a temporary variable is safer.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which has higher precedence, && or ||?Show answer

   &&.

2. 2.What is the value of 17 / 5 and 17 % 5?Show answer

   3 and 2.

3. 3.What does final do to a variable?Show answer

   Makes it a constant that cannot be reassigned.

4. 4.What does ++a do differently from a++?Show answer

   It increments first and then uses the new value.

5. 5.What is a short-circuit operator?Show answer

   && or ||, which skip the right side when the left side decides the result.

6. 6.Which side of an assignment is evaluated first?Show answer

   The right-hand side value is computed, then stored in the left-hand variable.

Expressions, Operators, Precedence and Assignment in Java: frequently asked questions