1. Miscellaneous Flow Control

2. Part I: basics of conditional statements
If (cond) A;
else B;

3. Part II: Alternative ways of writing choice statements --- mostly a syntax problem
If (cond) { A; }
else { B;
If statement
Nested if
If-else-if statement
Operator ?
Selection statement

4. The if Statement Syntax if(Expression)
Action
If the Expression is true then execute Action
Action is either a single statement or a group of statements within braces
Example: absolute value
if(value < 0)
value = -value;
Expression
true
false
Action
If (cond) A;
If (cond) A; else {};

5. Example: Absolute Value (3rd )
// program to read number & print its absolute value
#include <iostream>
using namespace std;
int main(){
int value;
cout << "Enter integer: ";
cin >> value;
if(value < 0)
value = -value;
cout << "The absolute value is " << value << endl;
return 0; }

6. Example: Sorting Two Numbers
int value1;
int value2;
int temp;
cout << "Enter two integers: ";
cin >> value1 >> value2;
if(value1 > value2){
temp = value1;
value1 = value2;
value2 = temp; }
cout << "The input in sorted order: "
<< value1 << " " << value2 << endl;

7. Nested if Statements
Nested means that one complete statement is inside another
if <condition 1 exists>{
<do A>
if <condition 2 exists>{
<do B>
if <condition 3 exists>{
<do C> }
<do D>
<do E>
<do F: sleep>

8. Nested if Statements Example: if <it's Tuesday>{
if <it's 9:00 am>{
if <it's raining>{
<bring umbrella> }
<go to COMP 104>
<call your friends>

9. if-else-if Statements
if <condition 1 exists>{
<do Q> }
else if <condition 2 exists>{
<do R>
else if <condition 3 exists>{
<do S>
else{
<do T> Q R S T

10. if <Tu, or Thur AM>{
<goto COMP 104> }
else if <Mon, Wed, or Fri AM>{
<goto MATH 113>
else if <1PM or 7PM>{
<eat>
else{
<sleep>

11. “Dangling Else” Problem
Problem: Nested if statements can seem ambiguous in their meaning.
What is the value of c after the following is executed?
int a=-1, b=1, c=1;
if(a>0)
if(b>0)
c = 2;
else
c = 3;

12. C++ groups a dangling else with the most recent if.
The following indentation shows how C++ would group this example (answer: c=1).
int a=-1, b=1, c=1;
if(a>0)
if(b>0)
c = 2;
else // dangling else grouped to nearest if
c = 3;

13. Use extra braces { } to clarify the intended meaning, even if not necessary.
int a=-1, b=1, c=1;
if(a>0){
if(b>0)
c = 2;
else // parenthesis avoid dangling else
c = 3; }

14. Summary on conditional statement
The fundamental If-else statement
 if, nested if, if-else-if
Bool type and boolean expression for a test
true/false
relational operators (==, <, …)
logical operators (&&, ||, !)
Full table of operator precedence (c++ technicality)
‘==‘ vs ‘=‘, ‘&&’ vs ‘&’: different!

15. Compact if-else operator: ?
if (condition) { A }
else { B
(condition) ? A : B;
In addition, the expression has a value:
larger = (value1>value2) ? Value1 : value2;

16. (low-level) Bitwise operators (be careful!)
& and
61&122-> & > >56
| or
61|122-> > >127
~ not
~61-> ->
^ xor
<< left shift
>> right shift

17. Selection
Often we want to perform a particular action depending on the value of an expression
Two ways to do this
if-else-if statement
if-else statements “glued” together
switch statement
An advanced construct

18. switch Statement
An alternative way of writing nested if-else-if statements for multiple choice.
if Exp==constant_1 {
action1; }
else if Exp==constant_2 {
action2;
else if … { …
else{
actionDefault
switch (Expression) {
case constant_1: action1; break;
case constant_2: action2; break; …
case constant_n: actionN; break;
default: actionDefault } =

19. Example using if-else-if:
if(score >= 90)
cout << "Grade = A" << endl;
else if(score >= 80)
cout << "Grade = B" << endl;
else if(score >= 70)
cout << "Grade = C" << endl;
else if(score >= 60)
cout << "Grade = D" << endl;
else
cout << "Grade = F" << endl;

20. Example using switch: This version is much easier to understand!
switch(int(score)/10){
case 10:
case 9: cout << "Grade = A" << endl;
break;
case 8: cout << "Grade = B" << endl;
case 7: cout << "Grade = C" << endl;
case 6: cout << "Grade = D" << endl;
default: cout << "Grade = F" << endl; }
Logical OR
This version is much easier to understand!

21. Another example: int left; int right; char oper;
cout << "Enter simple expression: ";
cin >> left >> oper >> right;
cout << left << " " << oper << " " << right
<< " = ";
switch (oper) {
case '+' : cout << left + right << endl; break;
case '-' : cout << left - right << endl; break;
case '*' : cout << left * right << endl; break;
case '/' : cout << left / right << endl; break;
default: cout << "Illegal operation" << endl; }

22. Loops

23. Part I: basics of loops
Initialization
While (cond) { A; }

24. Part II: loops with ‘break’ and ‘continue’

25. break break is not necessary in well designed programs!!!
break leaves the current loop immediately. It is recommended that break be used for situations where the loop needs to be terminated immediately (e.g., due to user intervention or if a fatal error occurs)
break is not necessary in well designed programs!!!

26. while(cond1){ A;
if (cond2) continue; B; }
while(cond1){ A;
if (cond2) break; B; }
If cond2 is true, then the loop is finished.
If cond2 is true, then skip only B, and the loop continues.
!!! break and continue are not necessary in well designed programs, and continue is even less used thant break !!!

27. Example: stop with negative score
int score,sum;
number=100;
n = 1;
sum = 0;
cin >> score;
while((n <= number) && (score>=0)){
sum=sum+score
n=n+1; }
cout << sum/number << endl;
int score,sum;
number=100;
n = 1;
sum = 0;
while(n <= number){
cin >> score;
if (score<0) break;
sum=sum+score
n=n+1; }
cout << sum/number << endl;

28. Example: ignore negative score
int score,sum;
number=100;
n = 1;
sum = 0;
while(n <= number){
cin >> score;
if (score<0) continue;
sum=sum+score
n=n+1; }
cout << sum/number << endl;
int score,sum;
number=100;
n = 1;
sum = 0;
while(n <= number){
cin >> score;
if (score>=0) {
sum=sum+score;
n=n+1; }
cout << sum/number << endl;

29. Tracing a program … int j=0; while(j<3){
cout << j << endl;
if (j==1) break;
j=j+1;
} }
int j=0;
while(j<3){
cout << j << endl;
if (j==1) continue;
j=j+1;
} }

30. Shorthand forms
Very peculiar features of C and C++ 

31. Shortcut Assignment C++ has a set of operators for applying an
operation to a variable and then storing the
result back into the variable
Shortcut assignments: *=, /=, +=, -=, %=
Examples
int i = 3;
i += 4; // i = i + 4
cout << i << endl; // i is now 7
double a = 3.2;
a *= 2.0; // a = a * 2.0
cout << a << endl; // a is now 6.4
int change = 1265;
change %= 100; // change = change % 100
cout << change << endl; // change is now 65

32. Count += 2;
Total -= discount;
Bonus *= 2;
Time /= rushfactor;
Change %= 100;
Amount *= c1 + c2;
Count = Count + 2;
Total = Total - discount;
Bonus = Bonus*2;
Time = Time*rushfactor;
Change = Change % 100;
Amount = Amount*(c1 + c2);

33. Cascading assignments
It’s possible, but may not be clear, to do:
int w,x,y,z;
y = z = 5;
y = (z = 5);
w = x = y + z;
w = (x = (y+z));
Not recommended at all !!!

34. Increment and Decrement
C++ has special operators for incrementing or decrementing an object by one
Examples
int k = 4;
++k; // k=k+1 : k is 5
k++; // k=k+1 : k is 6
cout << k << endl;
int i = k++; // i is 6, k is 7
cout << i << " " << k << endl;
int j = ++k; // j is 8, k is 8
cout << j << " " << k << endl;

35. What is the difference between k++ and ++k?
++k increments first, and the incremented value is
used in the expression (1+k in place value)
k++ uses the initial value of k in the expression,
and increments afterwards (k in place value)
Examples
int a, b, c, d, k;
k = 3;
a = ++k; // k=4, a=4
b = --a; // a=3, b=3
c = b++; // c=3, b=4
d = c--; // d=3, c=2

36. a = k++;
a = ++k;
a = k;
k = k+1; =
k = k+1 ;
a = k; =

37. N! (while, shorthand) int number, factorial, n;
cout << "Enter number: ";
cin >> number;
factorial = 1;
n = 1;
while(n <= number){
factorial *= n;
n++; }
cout << "The factorial of " << number
<< " is " << factorial << endl;

38. 2N (while, shorthand) int number, result, n;
cout << "Enter number: ";
cin >> number;
result = 1;
n = 1;
while(n <= number){
result *= 2;
n++; }
cout << "Two raised to the " << number
<< " power is " << result << endl;

39. Part II: Alternative ways of writing loops
For-statement
Do-while statement
(Break )

40. The for Statement Init; while (Expression) { Action; update; } Syntax
for (Init; Expression; Update) {
Action; }
How it works:
Execute Init statement
While Expression is true
Execute Action
Execute Update
Example
int i;
for(i=1; i<=20; i=i+1)
cout << "i is " << i << endl;
Init;
while (Expression) {
Action;
update; }

41. Iteration Using the for Statement
ForInit
ForExpression
true
false
Action
PostExpression (update)

42. = for loop summary: a compact form of while loop!
for (init(x); expression(x); update(x))
action(x,V)
Init(x);
while (expression(x)) {
action(x,V);
update(x); } =

43. N! (for) int number, factorial, n; cout << "Enter number: ";
cin >> number;
factorial = 1;
for(n=1; n<=number; n=n+1)
factorial = factorial*n;
cout << "The factorial of " << number
<< " is " << factorial << endl;

44. 2N (for) int number, result, n; cout << "Enter number: ";
cin >> number;
result = 1;
for(n=1; n<=number; n=n+1)
result = result*2;
cout << "Two raised to the " << number
<< " power is " << result << endl;

45. The Do-While Statement
Syntax
do Action
while (Expression)
How it works:
Execute Action
if Expression is true then execute Action again
Repeat this process until Expression evaluates to false
Action is either a single statement or a group of statements within braces
Action
true
Expression
false

46. = do-while loop: init(V); do action(V); while (expression(V));
Difference is here!!!
init(V); do
action(V);
while (expression(V));
Init(V);
action(V);
while (expression(V)) { } =

47. Example: N! (do-while) int number, factorial, n;
cout << "Enter number: ";
cin >> number;
factorial = 1;
n = 1;
do{
factorial = factorial*n;
n=n+1;
}while(n <= number);
cout << "The factorial of " << number
<< " is " << factorial << endl;
(when input number is 0, the behavior is different from the while loop!
‘while’ loop is better than ‘do-while’ for this example)

48. Example: 2N (do-while) int number, result, n;
cout << "Enter number: ";
cin >> number;
result = 1;
n = 1;
do{
if(number != 0) result = result*2;
n=n+1;
}while (n <= number);
cout << "Two raised to the " << number
<< " power is " << result << endl;
(‘while’ loop is better than ‘do-while’ for this example!)

49. Waiting for a Reply (‘do-while’ is better than ‘while’ here!)
char reply;
cout << "Continue(y/n): ";
cin >> reply;
while(reply!='n') {
//do something }
char reply;
do{
//do something
cout << "Continue(y/n): ";
cin >> reply;
}while(reply!='n');
(‘do-while’ is better than ‘while’ here!)

50. Summary with Maximum example
Three loop statements: while, for, do-while
One more example: Maximum
The user is to input a sequence of positive integer values from the keyboard.
The end of the sequence is marked as the integer value –1.
Write a programme which computes the maximum of the given sequence of integer values.

51. Maximum (while) MAX int value=0; //input value
cin >> value;
Max = value;
must initialise ‘value’
int value=0; //input value
int max=0; //maximum value
while (value != -1) {
cout << "Enter a value (-1 to stop): ";
cin >> value;
if(value > max) max = value; }
cout << "The maximum value found"
<< " is " << max << endl;
MAX

52. Maximum (do-while) int value; //input value int max=0; //maximum value
No need to initialize ‘value’
int value; //input value
int max=0; //maximum value
do{
cout << "Enter a value (-1 to stop): ";
cin >> value;
if(value > max) max = value;
} while (value!=-1);
cout << "The maximum value found is "
<< " is " << max << endl;
‘do-while’ is slightly better than ‘while’ for this example!

53. Maximum (for)
‘for’ loop is not suitable for this example!

54. Which Loop to Use?
While loop --- the fundamental one, always think it first!
You want to repeat an action without knowing exactly how many times it will be repeated.
You are working with user input
There are situations when the action should not be executed.
For loop
Usually best for sums, products, and counting loops.
Do-while loop
The action should always be executed at least once.
Otherwise, the do-while loops and while loops are used in similar situations.

55. Print out the following triangular pattern *
* * *
* * * * *
* * * * * * *
* * * * * * * * *

56. print triangular pattern: row=1; while(row<=5) { print-spaces;
print-stars;
row++; }
print spaces:
space=1;
while(space<=5-row) {
cout << “ “;
space++; }
print stars:
star=1;
while(star<=2*row-1) {
cout << “*”;
star++;

57. row=1;
while(row<=5) {
// print spaces
space=1;
while(space<=5-row) {
cout << “ “;
space++; }
// print stars
star=1;
while(star<=2*row-1) {
cout << “*”;
star++;
cout << endl;
row++;
for(row=1; row<=5; row++){
for(space=1; space<=5-row; space++) cout << " ";
for(star=1; star<=2*row-1; star++) cout << "*";
cout << endl ; }

58. Example:
Write a function void staircase(int n) to print a "staircase" on the screen with n steps.
We assume that n is always greater than 0.
For example,
if n is 1, the output should be:
+-+
| |
If n is 4, it should be:
| +-+
| |
| +-+
| | |
| |

59. void print-first-row(int i){
if (i == 0) {
print +-+ }
else {
print 2*i-1 spaces
void print-second-row(int i)
print |
print 2*i+1 spaces
void StairCase(int n) {
int i = 0;
while (i < n)
print-first-row(i);
print-second-row(i);
i++; }
print-last-row;

60. void StairCase(int n) {
char SPACE = ' ';
char STROKE = '|';
char PLUS = '+';
char HYPEN = '-';
int i = 0;
while (i < n)
if (i != 0)
print_one_row(STROKE, SPACE, SPACE, 2*i-2);
cout << "+-+" << endl;
print_one_row(STROKE, SPACE, STROKE, 2*i+1);
cout << endl;
i++; }
print_one_row(PLUS, HYPEN, PLUS, 2*n-1);
void print_one_row(char start_symbol, char mid_symbol,
char end_symbol, int length) {
cout << start_symbol;
int j = 0;
while (j < length)
cout << mid_symbol;
++j; }
cout << end_symbol;