ch03 controal statement i - 國立臺灣大學ccf.ee.ntu.edu.tw/~ypchiou/cpp_programming/ch03...

Photonic Modeling and Design Lab.Graduate Institute of Photonics and Optoelectronics &Department of Electrical EngineeringNational Taiwan University

C++ ProgrammingChapter 3 Control Statements: Part 1

Yih-Peng ChiouRoom 617, BL Building

(02) [email protected]

YPC - NTU GIPO & EE Introduction to C++ ProgrammingNTU BA2

Objectives


Outline

YPC - NTU GIPO & EE Introduction to C++ ProgrammingNTU BA

Before writing a program to solve a problem, we must have a thorough understanding of the problem and a carefully planned approach to solving it.

When writing a program, we must also understand the types of building blocks that are available and employ proven program construction techniques.

In this chapter and in Chapter 4, Control Statements: Part 2, we discuss these issues as we present the theory and principles of structured programming

4

3.1 Introduction


Any solvable computing problem can be solved by the execution of a series of actions in a specific order.

Algorithm: procedure for solving a problem in terms of the actions to execute the order in which the actions execute

Program Control: specifying the order in which statements (actions) execute in a computer program

This chapter investigates program control using C++’s control statements.

5

3.2 Algorithms (演算法)


Pseudocode (or “fake” code): an artificial and informal languagehelping you develop algorithms.

Similar to everyday English Convenient and user friendly. Helps you “think out” a program before attempting to write it. Carefully prepared pseudocode can easily be converted to a

corresponding C++ program. Normally describes only

executable statements. Declarations (that do not

have initializers or do not involve constructor calls) arenot executable statements.

6

3.3 Pseudocode


Sequence Execution statements in a program execute one after the other in the order in which

they’re written Transfer of Control

Various C++ statements enable you to specify that the next statement to execute may be other than the next one in sequence

Three control Structures - All programs could be written using them Sequence structureSelection structureRepetition structure

7

3.4 Control Structure

goto elimination


Sequence Structure in C++Unless directed otherwise, the computer executes C++ statements

one after the other in the order in which they’re written Unified Modeling Language (UML) activity diagram

workflow (also called the activity) action state symbols (a rectangle) diamonds and small circles, etc

8


connected by transition arrows

initial-state

final-state

notes


if selection statement single-selection statement selects or ignores a single action (or a group of actions)

if…else statementdouble-selection statement selects between two different actions (or groups of actions)

switch selection statementmultiple-selection statement selects among many different actions (or groups of actions)

Three types of repetition statementsalso called looping statements or loopswhile and for statements: perform the action (or group of

actions) in their bodies zero or more timesdo…while statement: performs it (them) at least once

9




Keywords must not be used as identifiers, such as variable names.


Single-entry/single-exit control statementsconnecting the exit point of one to the entry point of the nextcalled control-statement stackingonly one other way to connect control statements—called

control-statement nesting, in which one control statement is contained inside another.

11





Single-selection statement Selects or ignores a single action (or a group of actions) Ex: whether “student’s grade is greater than or equal to 60” is true or false

Pseudocode: If student’s grade is greater than or equal to 60Print “Passed”

If true, “Passed” is printed. If false, the print statement is ignored C++ code if ( grade >= 60 )

cout


Specifies an action to perform when the condition is true and a different action to perform when the condition is false.

Ex:

15

3.6 if…else Double-Selection Statement

If student’s grade is greater than or equal to 60Print “Passed”

ElsePrint “Failed”

psedocode

if ( grade >= 60 ) cout


Conditional operator (?:)C++’s only ternary operator—it takes three operands.

The first operand is a condition The second operand is the value for the

entire conditional expression if the condition is true The third operand is the value for the entire conditional expression if

the condition is falseThe values in a conditional expression also can be actions to

execute. Ex:

17


cout = 60) ? ”Passed” : “Failed“);if ( grade >= 60 ) cout


C++ code

if ( studentGrade >= 90 ) // 90 and above gets "A"cout = 80 ) // 80-89 gets "B"

cout = 70 ) // 70-79 gets "C"cout = 60 ) // 60-69 gets "D"

cout




Dangling-else problem if ( x > 5 )

if ( y > 5 )cout 5 )

cout 5 )cout




To include several statements in the body of an if or in either part of an if…else, enclose the statements in braces ({ and }).

A set of statements contained within a pair of braces is called a compound statement or a block.

Null statement or an empty statement It’s also possible to have no statement at all.The null statement is represented by placing a semicolon (;)

where a statement would normally be.

24



Repetition statement (also called looping statement or a loop) repeat an action while some condition remains true

While there are more items on my shopping listPurchase next item and cross it off my list

Eventually, the condition will become false, the repetition will terminate, and the first statement after the repetition will execute

Find the first power of 3 larger than 100int product = 3;

while ( product


Often called definite repetition The number of repetition is known before the loop begins executing. Use a variable, counter, to control the number of times a group of

statements will execute (a.k.a. the number of iterations of the loop) The class average of quiz grades

27

3.8 Formulating Algorithms: Counter-Controlled Repetition

counter: 計數器


// gradeCounter =11 after while loop


Variables used to store totals are normally initialized to zero before being used in a program; otherwise, the sum would in-clude the previous value stored in the total’s memory location (garbage value or undefined value).

Counter variables are normally initialized to 0 or 1, depending on their use. The variables grade and average need not be initialized before they’re

used — will be assigned as they’re input or calculated later in the function. Dividing two integers results in integer division—any fractional part of the

calculation is lost (i.e., truncated).29


Q: how to store grades? A: arrays and vectors in ch. 7.






Often called indefinite repetition the number of repetitions is not known in advance use a special value called a sentinel value (also called a signal value, a

dummy value or a flag value) to indicate “end of data entry.” The sentinel value must not be an acceptable input value.

Ex: Develop a class average program that processes grades for an arbitrary number of students each time it’s run

Develop pseudocode algorithm w. Top-down, stepwise refinement Top: a complete representation of a program

Determine the class average for the quiz for an arbitrary number of students

First refinement: divide the top into a series of smaller tasks and list these in the order Initialize variablesInput, sum and count the quiz gradesCalculate and print the total of all student grades and the class

average32

3.9 Formulating Algorithms: Sentinel-Controlled Repetition


Second refinement: commit specific variables

33


Input, sum and count the quiz grades

Calculate and print the total Of all student grades and the class average

Initialize variables

First refinement

(= -1?)


This would cause an infinite loop in the program if the user did not input –1 for the first grade

Test for the possibility of division by zeroNormally a fatal logic error that, if undetected, would cause the

program to fail (often called “crashing”).An averaging calculation is likely to produce a number with a

decimal point—a real number or floating-point number (e.g., 7.33, 0.0975 or 1000.12345).

C++ floating-point numbers in memory: float: single-precision, 4 Bytes, 7 significant digits on most 32-bit

systems.double: double-precision, 8 Bytes, 15 significant digits ……

C++ treats all floating-point numbers in a program’s source code as double values by default.Known as floating-point constants.

Floating-point numbers often arise as a result of division

34



average = total / gradeCounter;

integer division, fractional part of the calculation is lost (truncated)Unary cast operator ( unary vs. binary)

static_cast(total) creates a temporary floating-point copy of its operand in parentheses.

Explicit conversion. The value stored in total is still an integer.A floating-point value divided by the integer gradeCounter.

The compiler knows how to evaluate only expressions in which the operand types of are identical.

Compiler performs promotion (also called implicit conversion) on selected operands.

In an expression containing values of data types int and double, C++ promotes int operands to double values.

Cast operators are available for use with every data type and with class types as well.

37



Parameterized stream manipulator (argument in parentheses). Include the header .Setprecision(2)

Rounded to 2 digits of precision to the right of the decimal point (e.g., 92.37, 12345.78, 0.17)

Precision not specified: normally output with 6 digitsStream manipulator fixed: output in fixed-point format, as

opposed to scientific notation (scientific,e.g. 9.24e+001, 1.23e+004, 1.73e-001)

Also possible to force a decimal point to appear by using stream manipulator showpoint

38



41


Nesting one control statement within another

42

3.10 Formulating Algorithms: Nested Control Statements


43


3.10 Formulating Algorithms: Nested Control Statements


Assignment operators for abbreviating assignment expressionsvariable = variable operator expression;

variable operator= expression;

45

3.11 Assignment Operators

// equivalently


Increment operator, ++: adding 1 from the value of a numeric variable Decrement operator, --: subtracting 1 ……

46

3.12 Increment and Decrement Operators



When you increment (++) or decrement (--) a variable in a statement by itself, the preincrement and postincrement forms have the same effect, and the predecrement and postdecrement forms have the same effect.

It’s only when a variable appears in the context of a larger expression that preincrementing the variable and postincrementingthe variable have different effects (and similarly for predecrementing and post-decrementing).

48


ch03 controal statement i - 國立臺灣大學ccf.ee.ntu.edu.tw/~ypchiou/cpp_programming/ch03...

Documents