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

if (expr)
    statement;

Block statement

• Block statement is a sequence of statements inside a block which is made up of {}

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

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

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

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

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

// 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

(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

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

for (initialization; condition; increment)
    statement;

for (initialization; condition; increment) {
    statement(s);
}

Example

int i {0};

for (i = 1; i <=5; ++i)
    cout << i << endl;

Comma operator

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

for (;;)
    cout << "Endless loop" << endl;