Added material for Section09 thus far.

Author: anthonyclyan

Section09-Controlling_Program_Flow/README.md

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+# Controlling Program Flow
+
+- Sequence
+    - Ordering statements sequentially
+- Selection
+    - Making decisions
+- Iteration
+    - Looping or repeating
+
+We can implement any algorithm.
+
+## Selection - Decision Making
+
+- `if` statement
+- `if-else` statement
+- Nested `if` statements
+- `switch` statement
+- Conditional operator `?:`
+
+## Iteration - Looping
+
+- `for` loop
+- `while` loop
+- `do-while` loop
+- Range-based `for` loop
+- `continue` and `break`
+- Infinite `loops`
+- Nedted `loops`
+
+### `if` statement 
+
+- `if` statement followed by a boolean true/false value 
+- If the value of the expression is true then execute the statment
+- If the expression is false then skip the statement
+- Indentation is helpful
+
+```c++
+if (expr)
+    statement;
+```
+
+#### Block statement
+
+- Block statement is a sequence of statements inside a block which is made up of `{}`
+
+```c++
+if (num > 10) {
+    ++num;
+    cout << "this too";
+}
+```
+
+- Block statement comes in when more than 1 statement is executed if the statement is true
+- Create a block of code by inclding more than one statement in code block `{}`
+- Blocks can also contain variable declarations
+- These variables are visible only within the block - local scope
+
+### `if-else` statement
+
+```c++
+if (expr)
+    statement 1;
+else 
+    statement 2;
+```
+
+- If the expression is **true** then execute **statement 1**
+- If the expression is **false** then execute **statement 2**
+- Block statements still works
+
+### `if-else-if` construct
+
+```c++
+if (score > 90)
+    cout << "A";
+else if (score > 80)
+    cout << "B";
+else if (score > 70)
+    cout << "C";
+else if (score > 60)
+    cout << "D";
+else                    // all others must be F
+    cout << "F";
+
+cout << "Done";
+```
+
+### Nested `if` statement
+
+```c++
+if (expr1)
+    if (expr2)
+        statement1;
+    else
+        statement2;
+```
+
+- `if` statement is nested within another
+- Allows testing of multiple conditions
+- `else` belongs to closest `if`
+- We have a **dangaling else problem** since we have 2 `if` but only 1 `else`
+
+### `Switch` statement
+
+- The control expression must evaluate to an integer type
+- The case expression must be constant expressions that evaluate to integer or intergers literals
+- Once a match occurs all following case sections are executes UNTIL a `break` is reached the switch complete
+- Best practice: provide break statement for each case
+- Best practice: `default` is optional, but should be handled
+
+```c++
+switch (integer_control_expr) {
+    case expression_1: statement_1; break;
+    case expression_2: statement_2; break;
+    case expression_3: statement_3; break;
+    ...
+    case expression_n: statement_n; break;
+    default: statement_default;
+}
+```
+
+- `integer_control_expr` is a control expression must evaluate to an `int` type or an `enumeration` type
+- then the series of `case` statements enclosed in `{}`
+- the value of the control expression, `integer_control_expr`, is compared with the values following the `case` keyword
+- then the code after the `:` under the case will be executed until it hits a `break` statement
+- best practice to include `break` statement after all cases
+- `default` case is optional and is catch-all if none case matches
+- `break` is not needed in the default case
+
+```c++
+// fall-through example
+switch (selection) {
+    case '1': cout << "1 selected";
+              break;
+    case '2': cout << "2 selected";
+              break;
+    case '3':
+    case '4': cout << "3 or 4 selected";
+              break;
+    default:  cout << "1,2,3,4 NOT selected";
+}
+
+// with an enumeration
+switch (screen_color) {
+    case red:   cout << "red"; break;
+    case green: cout << "green"; break;
+    case blue:  cout << "blue"; break;
+    default:    cout << "should never execute";
+}
+```
+
+### Conditional Operator
+
+```c++
+(cond_expr) ? expr1 : expr2
+```
+
+- cond_expr evaluates to a boolean expression
+    - if cond_expr is true then the value of expr1 is returned
+    - if cond_expr is false then the value of expe2 is returned
+- Similar to `if-else` statement
+- Ternary operator
+- Very useful when used inline
+- Very easy to abuse
+
+```c++
+int a{10}, b{20};
+int score{92};
+int result {};
+
+
+result = (a > b) ? a : b;
+// if a is greater than b , then the result = a
+// otherwise, result = b
+
+result = (a < b) ? (b - a) : (a - b);
+// if b is greater than a, then the result = (b - a)
+// otherwise, result = (a - b)
+
+result = (b != 0) ? (a/b) : 0;
+// if b is not equal to zero, then the result = (a/b)
+// otherwise, result = 0
+
+cout << ((score > 90)) ? "Excellent" : "Good");
+// if score is greater than 90, then output "Excellent"
+// otherwise, output "Good"
+```
+
+### Looping 
+
+- The third basic building block of programming 
+    - sequence, selection, iteration
+- Iteration or repetition
+- Allows the execution of a statement or block of statements repeatedly
+- Loops are made up a loop condition and the body which contains the statements to repeat
+
+#### Use-cases
+
+- A specific number of times
+- For each element in a collection
+- While a specific condition remains true
+- Until a specific condition becomes false
+- Until we reach the end of some input stream
+- Forever 
+- Many, many more
+
+#### C++ Looping Constructs
+
+- `for` loop
+    - iterate a specific number of times
+- `range-based for` loop
+    - one iteration for each element in a range or collection
+- `while` loop
+    - iterate while a condition remains true
+    - stop wen the codition becomes false
+    - check the condition at the beginning of every iteration
+- `do-while` loop
+    - iterate while a condition remains true
+    - stop when the condition becomes false
+    - check the condition at the end of every iteration
+
+### `for` Loop
+
+```c++
+for (initialization; condition; increment)
+    statement;
+
+for (initialization; condition; increment) {
+    statement(s);
+}
+```
+
+#### Example
+```c++
+int i {0};
+
+for (i = 1; i <=5; ++i)
+    cout << i << endl;
+```
+
+#### Comma operator
+
+```c++
+for (int i{1}, j{5}; i <= 5; ++i, ++j){
+    cout << i << " * " << j << " : " << (i * j) << endl;
+}
+// 1 * 5 : 5
+// 2 * 6 : 12
+// 3 * 7 : 21
+// 4 * 8 : 32
+// 5 * 9 : 45
+```
+
+- The basic for loop is very clear and concise
+- Since the for loop's expressions are all optional, it is possible to have
+    - no initialization
+    - no test
+    - no increment
+
+```c++
+for (;;)
+    cout << "Endless loop" << endl;
+```