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;

Range-based for loop

• Introduced in C++ 11

for (var_type var_name: sequence)
    statement;  // can use var_name

for (var_type var_name: sequence){
    statement;  // can use var_name
}

Example

int scores [] {100, 90, 97};    // array of integers

for (int score : scores)
    cout << score << endl;

// Output:
// 100
// 90
// 97

We actually do not have to declare the type of a collection

int scores [] {100, 90, 97};    // array of integers

for (auto score : scores)
    cout << score << endl;

// Output:
// 100
// 90
// 97

Example: Vector - temperautre average

vector <double> temps {87.2, 77.1, 80.0, 72.5};

double average_temp {};
double running_sum {};

for (auto temp : temps){
    running_sum += temp;
}

average_temp = running_sum / temps.size();
cout << average_temp << endl;

Example: Initializer list - temperautre average

double average_temp {};
double running_sum {};
int size {0};

for (auto temp : {60.2, 80.2, 90.0, 78.2}){
    running_sum += temp;
    ++size;
}

average_temp = running_sum / size;
cout << average_temp << endl;

Example: String

for (auto c : "Frank") {
    cout << c << endl;

// Output:
// F
// r
// a
// n
// k
}

While loop

while (expression)
    statement;

while (expression) {
    statement(s);
}

• Useful for input validation
• If user's input is not valid, you would like to ask them to input again-and-again until input is correct

int number {};

cout << "Enter an integer less than 100: ";
cin >> number;

while (number >= 100) {
    cout << "Enter an integer less than 100: ";
    cin >> number;
}

cout << "Thanks" << endl;

int number {};

cout << "Enter an integer between 1 and 5: ";
cin >> number;

while (number <= 1 || number >= 5) {
    cout << "Enter an integer between 1 and 5: ";
    cin >> number;
}

cout << "Thanks" << endl;

• boolean flag

// initialization
bool done {false};
int number {0};

while (!done) {
    cout << "Enter an integer between 1 and 5: ";
    cin >> number;

    if (number <=1 || number >= 5)
        cout << "Out of range, try again" <<endl;
    else {
        cout << "Thanks" << endl;
        done = true;
    }
}

do-while Loop

do {
    statements;
} while (expression);

• Useful for input validation

int number {};

do {
    cout <<"Enter an integer between 1 and 5: " << endl;
    cin >> number;
} while (number <= 1 || number >= 5);

cout << "Thanks" << endl;

Area calculation with calculate another

char selection {};

do {
    double width {}, height {};
    cout << "Enter width and height seperated by a space: ";
    cin >> width >> height;

    double area {width * height};
    cout << "The area is " << area << endl;

    cout << "Calculate another? (Y/N): ";
    cin >> selectionl
} while (selection == 'Y' || selection == 'y');

cout << "Thanks!" << endl;

continue and break statements

• continue

  • no further statements in the body of the loop are executed
  • control immediately goes directly to the beginning of the loop for the next iteration

• break

  • no further statements in the body of the loop are executed
  • loop is immediately terminate
  • control immediately goes to the statement following the loop construct

Example

vector <int> values {1, 2, -1, 3, -1, -99, 7, 8, 10};

for (auto val : values) {
    if (val == -99) 
        break;
    else if (vale == -1) 
        continue;
    else 
        cout << val << endl;
}

// Output
// 1
// 2
// 3

Infinite loop

• aka endless loop
• Loops whose condition expression always evaluate to true
• Ususally this is unintended and a programmer error
• Sometimes programmers use infinite loop and include and break statements in the body to control them
• Sometimes infinite loops are exactly what we need
  • event loop in an event-driven program
  • operating system

Infinite for loop

for (;;)
    cout << "This will print forever" << endl;

Infinite while loop

while (true)
    cout << "This will print forever" << endl;

Infinite do-while loop

do { 
    cout << "This will print forever" << endl;
} while (true);

Example of useful infinite loop

while (true) {
    char again {};
    cout << "Do you want to loop again? (Y/N): " << endl;
    cin >> again;

    if (again == 'N' || again == 'n')
        break;
}

Nested loop

• Loop nested within another loop
• Can be many levels deep as the program needs
• Very useful with multi-dimensinoal data structures
• Outer loop vs Inner loop

Example

for (outer_val {1}; outer_val <= 2; ++outer_val)
    for (inner_val {1}; inner_val <= 3; ++inner_val)
        cout << outer_val << ", " << inner_val << endl;

// Output
// 1, 1
// 1, 2
// 1, 3
// 2, 1
// 2, 2
// 2, 3

// Note: inner loop looops "faster"

Example: Multiplcation Table

// Display 10 x 10 nultiplication table

for (int num1 {1}; num1 <= 10; ++num1) {
    for (int num2 {1}; num2 <= 10; ++num2) {
        cout << num1 << " * " << num2 << " = " << num1 * num2 << endl;
    }
    cout << "----------" << endl;
}

Example: 2D arrays - set all elements to 1000

int grid[5][3] {};

for (int row {0}; row < 5; ++row) {
    for (int col {0}; col < 3; ++col) {
        grid[row][col] = 1000;
    }
}

// =========

int grid[5][3] {};

for (int row {0}; row < 5; ++row) {
    for (int col {0}; col < 3; ++col) {
        cout << grid[row][col << " ";
    }
    cout << endl;
}

Example: 2D vector - display elements

vector <vector <int>> vector_2d 
{
    {1, 2, 3},
    {10, 20, 30, 40},
    {100, 200, 300, 400, 500}
};

for (auto vec : vector_2d) {
    for (auto val : vec) {
        cout << val << " ";
    }
    cout << endl;
}

// Output
// 1 2 3
// 10 20 30 40 
// 100 200 300 400 500