1. 生查子 ～ 歐陽修
去年元夜時，花市燈如晝， 月上柳梢頭，人約黃昏後；
今年元夜時，月與燈依舊，
不見去年人，淚濕春衫袖。

2. Chapter 3 Decisions and Loops

3. Relational Operators Compare the values of two operands, and return
<
Less than
>
Greater than ==
Equal to
<=
Less than or equal to
>=
Greater than or equal to !=
Not equal to
Compare the values of two operands, and return
true
false

4. Example of Logical Expressions
Suppose two integer variables i = 10, j = -5
The following expressions are all true:
i > j
i != j
j > -8
i <= j + 15
cout << (i < j)
Displays “0” (implicit type conversion, P.63)
cout << (i > j)
Displays “1” (implicit type conversion)

5. True or False
#include <iostream> int main() { int a = 0; int b = 10; if (a = b) std::cout << "True \n"; else std::cout << "False \n"; return 0; }

6. True or False
#include <iostream> int main() { int a = 10; int b = 0; if (a = b) std::cout << "True \n"; else std::cout << "False \n"; return 0; }

7. Flowcharts for three constructs

8. The if Statement The condition to be tested appears in parenthesis
if (letter == 'A') cout << "Apple";
A block of statements between braces could replace the single statement.
if (letter == 'A') { cout << "Apple"; letter = 'a'; }
Nested if Statement (P.94)

9. The if … else … Statement
if (number % 2)
cout << "Odd"
<< endl;
else
cout << "Even"
The condition express
(number % 2)
is equivalent to
(number %2 != 0)
A non-zero value is interpreted as true (implicit cast).
A zero value result casts to false.

10. Logical Operators
if ((letter >= 'A') && (letter <= 'Z')) cout << "This is a capital letter.";
if ( !(i > 5) ) cout << "i is not greater than 5\n";
&&
Logical AND ||
Logical OR !
Logical negation (NOT)

11. The Conditional Operator (P.103)
c = a>b ? a : b ; // set c to the maximum of // a and b
Sometimes called the ternary operator.
condition ? expression1 : expression2
if (a > b)
c = a;
else
c = b;

12. Output Control cout << endl << "We have " << nCakes
<< ((nCakes > 1) ? "s." : ".")
<< endl;
nCakes = 1
We have 1 cake.
nCakes = 2
We have 2 cakes.

13. HSR Ticket Machine

14. Its Code cin >> nTicket;
cout << "The transaction will issue " << nTicket << " ticket(s)." << endl;
Try to modify it using the “conditional operator” of C language.

15. The switch Statement if (option >= 'a' && option <= 'z')
switch (option) {
case 'a':
cout << "Append" << endl;
break;
case 'd':
cout << "Delete" << endl;
case 'q':
cout << "Quit" << endl;
default: cout << "You entered a wrong option."; }

16. Saves the Trouble of Multiple-if
if (option == 'a')
cout << "Append" << endl;
else
if (option == 'd')
cout << "Delete" << endl;
if (option == 'q')
cout << "Quit" << endl;
cout << "You entered a"
<< " wrong option." << endl;

17. Ex3_06.cpp (P.107)
An elegant example to demonstrate the power of C language.
switch (letter * (letter >= 'a' && letter <= 'z')) {
case 'a':
case 'e':
case 'i':
case 'o':
case 'u': cout << "You entered a vowel.";
break;
case 0: cout << "That is not a small letter.";
default: cout << "You entered a consonant."; }

18. Unconditional Branching
myLabel: cout << "myLabel is here"; .
goto myLabel;
Whenever possible, you should avoid using gotos in your program.

19. Loop (Ex3_07 in P.110)

20. Loop (Ex3_07 in P.110) int i = 1, sum = 0; const int max = 5;
KevinLabel:
sum +=i;
if (++i <= max)
goto KevinLabel;
cout << "sum=" << sum << endl
<< "i = " << i << endl;
i = 5, sum = 15
i = 4, sum = 10
i = 2, sum = 3
i = 1, sum = 1
i = 3, sum = 6

21. The for Loop
for (i=1; i<=6; i++)
cout << i << endl;

22. The for Loop (2) for (i=1; i<=6; i+=2)
cout << i << endl; 2

23. Using the for Loop for Summation
int i = 0, sum = 0;
const int max = 5;
for (i=1; i<=max; i++)
sum += i;
i = 4, sum = 10
i = 5, sum = 15
i = 2, sum = 3
i = 1, sum = 1
i = 3, sum = 6

24. Three Expressions in for-loop (P.111)
General form of the for loop:
for (initializing_expression; test_expression; increment_expression) loop_statement;
The first expression sets the initial conditions for the loop. (It only executes once.)
The second expression is a logical expression that determines whether the loop statement (or block of statements) should continue to be executed.
The third expression is evaluated after the loop statement (or block of statements) executes.
After this expression has been evaluated, the second expression is evaluated once more to see whether the loop should continue.

25. Exercises: Fibonacci Sequence Sum of Numbers
Write a program that computes the nth Fibonacci number where n is a value input by the user.
Sum of Numbers
Write a program to find the sum of the first N natural numbers, where the value of N is provided by the user.

26. Nested for Loop const int N = 5; int i, j; for (i=1; i<=N; i++) {
for (j=1; j<=i; j++)
cout << '*';
cout << endl; }
A block of statements between braces could replace the single loop_statement. * **
***
****
*****

27. Increment/Decrement of the Counter
for (i=1; i<=N; i++) {
for (j=1; j<=i; j++)
cout << '*';
cout << endl; }
for (i=N; i>=1; i--) * **
***
****
*****

28. ASCII Table
#include <iostream> using std::cout; using std::endl; int main() { unsigned char c; for (c=32; c<=126; c++) if (c % 8 == 0) cout << endl; cout << c << '\t'; } cout << endl; return 0;

29. ASCII Table (2)
#include <iostream> #include <iomanip> using std::cout; using std::endl; using std::setw; // P.49 int main() { unsigned char c; for (c=32; c<=126; c++) if (c % 8 == 0) cout << endl; cout << setw(3) << static_cast<int>(c) << ' '; cout << c << '\t'; } cout << endl; return 0;

30. Supplementary: Flowchart

31. START
Summing Up Odd Numbers
sum0
i 1
#include <iostream> using std::cout; using std::endl; int main() { int i; int sum=0; for (i=1; i<=9; i+=2) sum += i; cout << sum << endl; return 0; } NO
i9
YES
sumsum+i
ii+2
sum
END

32. break vs. continue
The keyword continue allows you to skip the remainder of the current iteration in a loop and go straight to the next iteration.
The keyword break provides an immediate exit from a loop.
(See P.116 and P.117)

33. Variation on the for Loop
Declare the counter i within the loop scope. The loop statement can be empty.
for (int i = 1; i<=max; sum+= i++) ;
You can omit the initialization expression
int i = 1; for (; i <= max; i++) sum += i;
Use the comma operator (P.60) to specify several expressions:
for (i=0, power=1; i<=max; i++, power *=2)

34. Other Types of Loop The while loop The do-while Loop
while (condition) loop_statement;
Ex3_12.cpp on P.121
The do-while Loop
do { loop_statements; } while (condition);
Always executed at least once.
You may see infinite loops like
while (true) { … }
while (1) { … }
for (;;) { … }

35. 3 n + 1 Input an integer n. Print n. If n is equal to 1, then stop.
START n
Input an integer n.
Print n.
If n is equal to 1, then stop.
If n is odd then n  3n+1;
else n  n/2;
Goto Step 2. n
n == 1 Y N
n%2
== 1 Y N
n3n+1
nn/2
END

36. Greatest Common Divisor (1)
START
Greatest Common Divisor (1)
A, B
Imin(A,B)
#include <iostream> using std::cout; using std::cin; int main() { int a, b, i; cin >> a >> b; for (i=(a>b?b:a); i>=1; i--) if (a % i == 0 && b % i == 0) cout << i << " is the gcd of " << a << " and " << b << std::endl; break; } return 0;
A % I == 0
AND
B % I == 0 NO
II-1
YES
I is the gcd of A and B
END

37. Greatest Common Divisor (2)
START
Greatest Common Divisor (2)
A, B
I2
C 1
#include <iostream> using std::cout; using std::cin; using std::endl; int main() { int a, b; int i =2, c = 1; cin >> a >> b; while (i<=a && i<=b) if (a % i == 0 && b % i == 0) a = a / i; b = b / i; c = c * i; } else i = i + 1; cout << "GCD is " << c << endl; return 0;
I > A OR
I > B
YES NO
A % I == 0
AND
B % I == 0 NO
YES
AA/I
B B/I
CC*I
II+1
C is the gcd of A and B
END

38. Greatest Common Divisor (3)
#include <iostream>    using std::cin; using std::cout; using std::endl; int main() {     int a, b, temp;     cout << "a=? ";     cin >> a;     cout << "b=? ";     cin >> b;     if (a==0 && b==0)     {         cout << "I don't know how to calculate their gcd.\n";         return 1;     }     cout << "The greatest common divisor of " << a << " and " << b << " is ";     while (b != 0)     {         a %= b;         temp = b; b = a; a = temp;    // swap a,b     }     cout << a << endl;     return 0; }

39. Homework Prime number <= N
Extend the “Prime Number Test” program to list all prime numbers less than or equal to N, where N is input from the user.

40. Exercise: Factorization (1)
Input N, and factorize N.
For example, 12 = 2 * 2 * 3

41. Homework (bonus) Perfect Number For example,
In number theory, a perfect number is a positive integer that is equal to the sum of its proper positive divisors; that is, the sum of its positive divisors excluding the number itself.
For example,
6 = 1 + 2 + 3
28 = 1 + 2 + 4 + 7 + 14
Write a program to list all perfect numbers less than or equal to N, where N is input from the user.