Worked Example: Basic Knowledge

Section 1.4 Worked Example: Basic Knowledge

Subgoals for evaluating an assignment statement.

Determine resultant data type of expression
Update variable for pre-increment or pre-decrement operators (side effect)
Evaluate arithmetic expression according to operator precedence
If an assignment statement (=), is Left Hand Side (LHS) a variable? Check data type of value against data type of variable.
Update variable for post-increment or post-decrement operators (side effect)

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Given the following code snippet, evaluate the final statement (the last line). If invalid, give the reason. If valid, what value is assigned to the variable? Note any possible side effects.

int alpha = 2,
   beta = 1,
   delta = 3,
   eta,
   gamma;
double omega = 2.5,
   theta = -1.3,
   kappa = 3.0,
   lambda,
   rho;

lambda=alpha + delta;

Subsection 1.4.1 SG1 : Determine resultant data type of expression

The expression is the right-hand-side (RHS) of the statement: alpha + delta. In the first two lines, these variables were declared to be int type. Their sum will also be an int value.

Subsection 1.4.2 SG2: Update variable for pre-increment or pre-decrement operators (side effect)

NOT USED IN THIS EXAMPLE

Subsection 1.4.3 SG3: Evaluate arithmetic expression according to operator precedence

In the declarations, alpha was initialized with a value of 2, and delta was initialized with a value of 3.

The RHS of the statement is alpha + delta, so we replace those variable names with their actual values 2 + 3, which results in a sum of 5.

Subsection 1.4.4 SG4: If an assignment statement (=), is Left Hand Side (LHS) a variable? Check data type of value against data type of variable.

The LHS is a variable of type double, and the RHS is type int. This is valid, as Java automatically promotes an int value when we assign it to a double.

Subsection 1.4.5 SG5: Update variable for post-increment or post-decrement operators (side effect)

NOT USED IN THIS EXAMPLE

Subsection 1.4.6 Questions to check understanding

Q1) Is the left-hand-side (LHS) of the statement a variable? What type?

Q2) What is the resulting type after evaluating the right-hand-side (RHS)?

Q3) Can the type of the RHS result be assigned to the LHS variable?

Answer.

Q1-Answer) Yes, lambda is declared as a double

Q2-Answer) On the RHS, alpha + delta is evaluated as int + int which is int

Q3-Answer) Yes, an int can be assigned to a double (automatic promotion by adding .0)

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