Computer Programming Lecture 3 Hung-Yu Wei Department of Electrical Engineering National Taiwan University
10/03/2006
HW#1
Deadline 10/17 12:00 (noon) Assignment: Pick any topic you like Use whatever you have learned
Submission: Email to:
[email protected]
Subject: [HW#1] b95xxxxxx_Name
Attach C++ source codes
Input/output Arithmetic If statement
b95xxxxxx_name.cpp (only 1 file)
1-2 pages of descriptions/results (Word or txt file) Write your 姓名學號 in description file and source codes
Make sure your source codes can be complied in PC room 305
10/18 Lab: you will show your program to TAs
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Review: compiling environment
Visual C++ 6.0 (we used in the lab)
New Project
Win32 console program
Add cpp file to the project
Visual C++ .Net 2003
(an empty project)
專案 Win32 主控台專案 在”應用程式設定”中，勾選”空專案”
Dev C++
New project Console applications 3
Review Question I X=3; Y=7; Z=X==Y What are the values of X, Y, Z? You also need to know what are the memory operations when you do these calculations. 4
Review Question II X=3; Y=7; Z1=Y/X Z2=Y%X What are the values of X, Y, Z1,Z2?
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What will we learn today
Chapter 4
How do you control the “flow” of your program?
Control statement
Selection Repetition
If If … else while More about UML 6
Introduction: solving problem and writing program
Before writing a program
Have a thorough understanding of problem Carefully plan your approach for solving it
While writing a program
Know what “building blocks” are available Use good programming principles
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Algorithms
Computing problems
Algorithm is a procedure determining
Solved by executing a series of actions in a specific order Actions to be executed Order to be executed Example: cooking/recipe
Program control
Specifies the order in which statements are executed 8
Pseudocode (fake code)
Pseudocode
Artificial, informal language used to develop algorithms Similar to plain English
Not executed on computers
Used to think out program before coding
Only executable statements
Easy to convert into C++ program No need to declare variables (optional)
Help you to analyze/design/program 9
Goto statement
It is BAD!
Jumping between the lines is unstructured
Structured programming
Avoid using goto statement Also known as “goto elimination” C is a structured language
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Control Structures
Sequential execution
Transfer of control
Statements executed in order Next statement executed not next one in sequence
3 control structures (Bohm and Jacopini)
Sequence structure Programs executed sequentially by default Selection structures if, if/else, switch Repetition structures while, do/while, for
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Flowchart
Graphical representation of an algorithm BEGIN ACTION1
ACTION2
END
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Types of statement
Sequence statement Selection statement
Single selection statement: If Double selection statement: If … else Multiple selection statement: switch
Repetition statement
Also known as loops or looping statement While Do … while For 13
If statement (revisited)
Pseudocode If student’s grade is greater than or equal to 60 Print “Passed”
C++ if (grade >= 60) cout << “Passed”;
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UML
Unified Modeling Language (UML) An industry standard for modeling software systems Useful for object-oriented programming
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Sequence-structure activity diagram
Initial state Action state Final state
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If single-selection statement activity diagram
Decision symbol Guard condition
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if...else double-selection
Pseudocodes If student’s grade is greater than or equal to 60 Print “passed” else Print “Failed” C++ codes if (grade >= 60) cout << “Passed”; else cout << “Failed”; 18
if...else double-selection statement activity diagram.
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?:
Conditional Operator Example
Cout << (grade >=60 ? “Passed” : “Failed” ); Grade >=60 ? cout << “Passed” : cout << “Failed”;
Usage is similar to if … else
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?: Syntax (condition) ? (if it’s true) : (if it’s false)
Can be used within 1 line
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Nested if … else statement
Pseudocodes
If student’s grade is greater than or equal to 90 Print “A” Else If student’s grade is greater than or equal to 80 Print “B” Else If student’s grade is greater than or equal to 70 Print “C” Else If student’s grade is greater than or equal to 60 Print “D” Else Print “F”
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C++ if (studentGrade >=90) cout <<"A"; else if (studentGrade >=80) cout <<"B"; else if (studentGrade >=70) cout <<"C"; else if (studentGrade >=60) cout <<"D"; else cout <<"F"; 23
Continued
C++ if (studentGrade >=90) cout << “A”; else if (studentGrade >=80) cout << “B”; else if (studentGrade >=70) cout << “C”; else if (studentGrade >=60) cout << “D”; else cout << “F”;
if else if else
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Some tips
A nested if...else statement can perform much faster than a series of single-selection if statements In a nested if... else statement, test the conditions that are more likely to be true at the beginning of the nested if...else statement.
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Dangling-else problem An example If (x>5) if (y>5) cout << “x,y are greater than 5”; else cout << “x is <= 5”;
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Continued The reality (it is confusing) Use {} to make your program clear If (x>5) if (y>5) cout << “x,y are greater than 5”; else cout << “x is <= 5”;
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{} Use {} for clarity and multiple lines of statement in if…else If (x>5) { if (y>5) cout << “x,y are greater than 5”; } else cout << “x is <= 5”;
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Tips
Always putting the braces in an if...else statement (or any control statement) helps prevent their accidental omission, especially when adding statements to an if or else clause at a later time. You could put {} before writing any statements within {} 29
Review: if..else and {} One statement If (condition) one statement; else another statement; Multiple statements if (condition) { multiple statements; } Else { more statements;
} 30
While
A repetition statement Pseudocodes While there are more items on my shopping list Buy the next item and update the list
Syntax while (condition) statement to do;
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An example with while
Example: compute a value which is and is greater than 100
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N
int product =3; While (product <= 100) product=3*product;
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while repetition statement UML activity diagram
UML uses the same icon for merge symbol and decision symbol
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Be careful with while
infinite loop
Repetition statement never terminates
This can make a program appear to “hang” or “freeze” if the loop body does not contain statements that interact with the user. Use while loop carefully
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What have we learned so far?
Basic C++ program Main program Include C++ standard library (e.g. iostream) I/O (cin/cout) Arithmetic Control statement
Create your program from pseudocodes to C++ If, If…else, While 35
Conclusion
Read your textbook
Install/use your C++ compiler HW #1
2.1~2.7 4.1~4.7 1.7~1.10, 1.13
10/17 12:00 (noon)
We will come back to Chapter 3 next week and continue from 4.8 after finishing Chapter 3. 36