1 objectives ❏ to be able to list and describe the six expression categories ❏ to understand the...

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Objectives ❏ To be able to list and describe the six expression categories ❏ To understand the rules of precedence and associativity in

evaluating expressions ❏ To understand the result of side effects in expression evaluation ❏ To be able to predict the results when an expression is evaluated ❏ To understand implicit and explicit type conversion ❏ To understand and use the first four statement types: null,

expression, return, and compound

Chapter 3Chapter 3 Structure of a C ProgramStructure of a C Program

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3-1 Expressions

• An expression An expression (( 運算式運算式 )) is a sequence of operands is a sequence of operands (( 運算元運算元 )) and operators and operators (( 運算子運算子 )) that reduces to a that reduces to a single value. single value.

• Expressions can be simple or complex. Expressions can be simple or complex.

• An operator is a syntactical token that requires an An operator is a syntactical token that requires an action be taken. action be taken.

• An operand is an object on which an operation is An operand is an object on which an operation is performed; it receives an operator’s action.performed; it receives an operator’s action.

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FIGURE 3-1 Expression Categories

An expression always reduces to a single value.

NoteNote

Primary:

Name (variable): a b12 price calc …

Literal Constants ( 文數字常數 ) : 5 123.98 ‘A’ “Welcome”

後序前序三元一元二元主要

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FIGURE 3-2 Postfix Expressions ( 後序運算式 )

運算子運算元

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FIGURE 3-3 Result of Postfix a++

(a++) has the same effect as (a = a + 1)

NoteNote

The operand in a postfix expression must be a variable.

NoteNote

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PROGRAM 3-1 Demonstrate Postfix Increment

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FIGURE 3-4 Prefix Expression ( 前序運算式 )

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FIGURE 3-5 Result of Prefix ++a

The operand of a prefix expression must be a variable.

NoteNote

(++a) has the same effect as (a = a + 1)

NoteNote

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PROGRAM 3-2 Demonstrate Prefix Increment

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If ++ is after the operand, as in a++, the increment takes place after the expression is evaluated.

If ++ is before the operand, as in ++a, the incrementtakes place before the expression is evaluated.

NoteNote

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FIGURE 3-6 Unary Expressions ( 一元運算式 )

Examples of Unary Plus And Minus ExpressionsTable 3-1

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FIGURE 3-7 Binary Expressions ( 二元運算式 )

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Both operands of the modulo operator (%) must be integral types.

NoteNote

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PROGRAM 3-3

Binary Expressions

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The left operand in an assignment expression must be a single variable.

NoteNote

Table 3-2 Expansion of Compound Expressions ( 複合運算式 )

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PROGRAM 3-4

Demonstration of Compound Assignments

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3-2 Precedence and Associativity

Precedence is used to determine the order in which Precedence is used to determine the order in which different operators in a complex expression are different operators in a complex expression are evaluated. Associativity is used to determine the order evaluated. Associativity is used to determine the order in which operators with the same precedence are in which operators with the same precedence are evaluated in a complex expression. evaluated in a complex expression.

Precedence ( 優先順序 )Associativity ( 結合性 )

Topics discussed in this section:Topics discussed in this section:

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PROGRAM 3-5 Precedence

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FIGURE 3-8 Left-to-Right Associativity

FIGURE 3-9 Right-to-Left Associativity

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3-3 Side Effects

A side effect is an action that results from the A side effect is an action that results from the evaluation of an expression. evaluation of an expression.

For example, in an assignment, C first evaluates the For example, in an assignment, C first evaluates the expression on the right of the assignment operator and expression on the right of the assignment operator and then places the value in the left variable.then places the value in the left variable.

Changing the value of the left variable Changing the value of the left variable is a side effect.is a side effect.

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3-4 Evaluating Expressions

Now that we have introduced the concepts of Now that we have introduced the concepts of precedence, associativity, and side effects, let’s work precedence, associativity, and side effects, let’s work through some examples.through some examples.

Expressions without Side EffectsExpressions with Side Effects


Computer Science: A Structured Programming Approach Using C 23

PROGRAM 3-6

Evaluating Expressions

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3-5 Type Conversion

Up to this point, we have assumed that all of our Up to this point, we have assumed that all of our expressions involved data of the same type. But, what expressions involved data of the same type. But, what happens when we write an expression that involves happens when we write an expression that involves two different data types, such as multiplying an integer two different data types, such as multiplying an integer and a floating-point number? To perform these and a floating-point number? To perform these evaluations, one of the types must be converted.evaluations, one of the types must be converted.

Implicit Type ConversionExplicit Type Conversion (Cast)


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FIGURE 3-10 Conversion Rank


PROGRAM 3-7

Implicit Type Conversion

Visual C++ 不支援 bool 型態


Explicit Casts

PROGRAM 3-8

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3-6 Statements

A statement A statement (( 敘述敘述 )) causes an action to be performed causes an action to be performed by the program. It translates directly into one or more by the program. It translates directly into one or more executable computer instructions.executable computer instructions. You may have noticed that we have used a You may have noticed that we have used a semicolon at the end of the statements in our semicolon at the end of the statements in our programs. Most statements need a semicolon at the programs. Most statements need a semicolon at the end; some do not.end; some do not.

Statement TypeThe Role of the SemicolonStatements and Defined Constants


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FIGURE 3-11 Types of Statements

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FIGURE 3-12 Compound Statement

The compound statement does not need a semicolon.

NoteNote

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3-7 Sample Programs

This section contains several programs that you This section contains several programs that you should study for programming technique and style.should study for programming technique and style.


PROGRAM 3-9

Calculate Quotient and Remainder


PROGRAM 3-10

Print Right Digit of Integer


PROGRAM 3-11

Calculate Average of Four Numbers

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PROGRAM 3-11 Calculate Average of Four Numbers


PROGRAM 3-12

Convert Radians to Degrees

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PROGRAM 3-13

Calculate Sales Total

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PROGRAM 3-13 Calculate Sales Total

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PROGRAM 3-14

Calculate Student Score

1 objectives ❏ to be able to list and describe the six expression categories ❏ to understand the...

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