Section 1.14 Worked Example: Compound Operators
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 = 5, delta = 7, eta, gamma = 5; double omega = 2.5, theta = -1.3, kappa = 3.0, lambda, rho; gamma += delta / alpha + beta % alpha;
Subsection 1.14.1 SG1 : Determine resultant data type of expression
The expression is usually the right-hand-side (RHS) of the statement, but compound assignment operators are a special shorthand than includes the compounded operation with the left-hand-side (LHS). A much simpler example could be
gamma += 1;, which would be a shorthand for
gamma = gamma + 1;
Thus, the entire expression for this example is
gamma + delta / alpha + beta % alpha;, which will be assigned back into gamma.
At the beginning of the snippet, all of these variables were declared as
inttype, so all of the operations will also result in
Subsection 1.14.2 SG2: Update variable for pre-increment or pre-decrement operators (side effect)
NOT USED IN THIS EXAMPLE
Subsection 1.14.3 SG3: Evaluate arithmetic expression according to operator precedence
The expression may be easiest to conceptualize algebraically, by replacing the variables right away with their initialized values from the declarations above:
5 + 7 / 2 + 5 % 2.
Division and modulus both have higher precedence than addition, so we evaluate them first, resulting in a new expression of
5 + 3 + 1.
In the division, both variables are type
int, so the result is also an
intvalue, with the remainder truncated (i.e. abandoned, i.e. thrown away).
Also remember, the mod % operation returns the remainder after integer division.
Finally, we evaluate the additions left-to-right, 5+3 is 8, and 8+1 is 9.
Subsection 1.14.4 SG4: If an assignment statement (=), is Left Hand Side (LHS) a variable? Check data type of value against data type of variable.
The LHS of the assignment is a variable of type
int, and the expression result is type
int. This is valid.
Subsection 1.14.5 SG5: Update variable for post-increment or post-decrement operators (side effect)
NOT USED IN THIS EXAMPLE
Subsection 1.14.6 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, gamma is declared as an int
Q2-Answer) On the RHS, all of the values are int, so the result is also int
Q3-Answer) Yes, an int can be assigned to an int