# 程式語言 Chapter 7 - Expressions and Assignment Statements
## Evaluation of Expressions
* Variables
* Values fetched from memory
* Constants
* Sometimes fetched from memory
* Part of the machine language instruction (memory fetch is not required)
* Parenthesized expression
* Operators in the parenthesised expression must be evaluated first before the expression can be used as an operand.
## Side Effects and Evaluation Order
* Functional side effect: function which changes
* its parameters
* or global variables
### Example

a + fun(a)

* No side effect
* The order of evaluation has no effect on the value of the expression.
* With side effect
* If fun() changes a, a side effect happend.
* Different evaluation order causes different value of the expression.
## Operator Overloading (Multiple Use of Notations)
* Acceptable as long as **readability** and/or **reliability** do not suffer.
* The "+" arithmetic operator
* Integer addition
* Floating-point addition
* String catenation
* The ampersand(&) in C
* Bitwise logical AND (Binary 二元)
* Address-of operator (Unary 一元)
* Same symbol(&) for two completely unrelated operations -> detrimental to readability
* Leave out on of the operands -> an error undected by the compiler
## Short-Circuit Evaluation
* Determine result without evaluating all of the operands and/or operators.
* Arithmetic expression
* Not easy to detect during execution -> Never taken

0 * fun(b)

* Boolean expression
* Something like the truth table
* Easier to discover -> Taken
* Potential Problem with Non-Short-Circuit Evaluation
* Indexing error
* Use short-circuit-evaluation to solve

while ((index < listlen) && (list[index] != key))
// IndexOutOfBoundException

* Errors from Short-Circuit-Evaluation
* Part of the expression that contains a side effect is not evaluated.

(a > b) || (b++ / 3)