Control Statements

C++  Programming 

# C++ operators 

Because you will start to learn about control statements in C++, it's important to understand the C++ operators that exist in the language first, as they play an important role in control statements.

You will work with comparison operators to determine if values are equal, greater, or less than each other.  C++ also allows you to use mathematical operators for incrementing values to help control the number of iterations in a loop.  You can also make use of bitwise operators to speed up some operations in your code. 

Operator	Description
+	addition
-	subtraction
*	multiplication
/	division
%	modulo
+= (y += x)	same as y = y + x
-= (y -= x)	same as y = y - x
*= (y *= x)	same as y = y * x
++	increment by 1
--	decrement by 1
==	equal to
!=	not equal to
>	greater than
<	less than
>=	greater than or equal to
<=	less than or equal to
&&	logical AND
||	logical OR
!	logical NOT

This video explains some of the C++ operators available to you.

c++ operators and operators overloading

# Decision Statements 

Introduction

C++ decision structures provide logic in your application code that allows the execution of different sections of code depending on the state of data in the application. You might ask users whether they wish to save any changes to a file that is open in the application. The decision structure permits you to code behavior to execute based on the answer provided by the user. C++ uses conditional statements to achieve this functionality.

The primary conditional statement in C++ is the if statement. An alternative to the if statement is a switch statement.  As you will see in the section on the switch statement, you might want to use it for more complex decisions.

if Statements 

In C++, if statements are concerned with Boolean logic. If the statement is true, the block of code associated with the if statement is executed.  If the statement is false, control either falls through to the line after the if statement, or after the closing curly brace of an if statement block.

The following code sample demonstrates an if statement to determine if a response contains a value of y or Y.

char response = 'y';
if (response == 'y' || response == 'Y')
{
    cout << "Positive response received" << endl;
}

Note the use of curly braces in the code sample.  You can eliminate the curly braces if your statement to execute is a single line statement.  C++ understands that if no curly braces are used, the line immediately after the if(condition) will be executed if the condition is true.  Otherwise that line of code is not executed and the code resumes after that line.  If you need to have multiple statements execute if the condition is true, then you must use curly braces to surround the body of the if structure as in the code sample.  

TIP: To avoid confusion as to which lines will execute for a true condition, a recommended practice is to always use curly braces for your if statement.

In C++, if statements can also have associated else clauses. The else clause executes when the if statement is false.

The following code example shows how to use an if else statement to execute code when a condition is false.

if else Statements

string response;
if (response == "connection_failed")
{
    // Block of code executes if the value of the response variable is "connection_failed".
}
else
{
    // Block of code executes if the value of the response variable is not "connection_failed".
}

if statements can also have associated else if clauses. The clauses are tested in the order that they appear in the code after the ifstatement. If any of the clauses returns true, the block of code associated with that statement is executed and control leaves the block of code associated with the entire if construct.

The following code example shows how to use an if statement with an else if clause.

else if Statements

string response;
if (response == "connection_failed")
{
    // Block of code executes if the value of the response variable is "connection_failed".
}
else if (response == "connection_error")
{
    // Block of code executes if the value of the response variable is "connection_error".
}
else
{
    // Block of code executes if the value of the response variable is neither above responses.
}

You can create as many else if blocks as necessary for your logic, or until you become completely lost from too many else if clauses.  If you require any more than five else if clauses, you might want to consider the switch statement, presented next.

This video explains if statements completely ..if statements 

switch Statements

If there are too many else if statements, code can become messy and difficult to follow. In this scenario, a better solution is to use a switchstatement. The switch statement simply replaces multiple else if statements. The following sample shows how you can use a switchstatement to replace a collection of else if clauses.

switch Statement

char response = 'y';
switch (response)
{
   case 'y':
      // Block of code executes if the value of response is y.
      break;
   case 'Y':
      // Block of code executes if the value of response is Y.
      break;
   case 'n':
      // Block of code executes if the value of response is n.
      break;
   default:
      // Block executes if none of the above conditions are met.
      break;
}

Notice that there is a block labeled default:. This block of code will execute when none of the other blocks match.  The default block is optional.

In each case statement, notice the break keyword. This causes control to jump to the end of the switch after processing the block of code. If you omit the break keyword, the application may not perform as you anticipate.  In other languages, such as C#, omitting the break; keyword will cause the code to no longer compile. 

Without the break statement, the code will "fall through" to the remaining cases until it encounters a break statement.   Be very careful in using fall through logic in your switch statements.  The most common use for a fall through scenario is when you want to handle multiple cases with a single statement or set of statements.

If you are coming from another programming language, such as C#, that also uses the switch statement, you might notice that in the C# language, you can use string values in your switch statements and don't have to use integers or enumerated types.  C++ switch statements support the following data types as expressions:

• intrinsic data types such as int or char
• enumerations

In this video you will understand Switch statements briefly. switch statements

# The Conditional (Ternary) Operator

The C++ conditional operator is also known as a ternary operator because it takes three operands.  How this operator functions is somewhat similar to an if statement or a switch statement, but in a more compact form and for one single Boolean value with one of two possible outputs.   That is to say, the first operand is evaluated as a Boolean result.  If the result is true, then the second operand will be the one evaluated.  Otherwise, the third operand will be evaluated.   A sample helps amplify this.

#include <iostream> 
using namespace std; 
int main() 
{ 
     int i = 1, j = 2; 
     cout << ( i > j ? i : j ) << " is greater." << endl; 
}

In this example, we have two integer variables, i and j which are initialized to 1 and 2 respectively.  The ternary operator is embedded inside the cout statement and essentially follows this pattern:

1. it checks whether i is greater than j
2. it outputs the proper numeric value along with is greater.

In the code example here, j is greater than i so the condition evaluates to false and the value for j (2), is output to the console along with the text is greater.  In other words, the output is "2 is greater."  If i was 5 and j was 2, the output would be, "5 is greater." 

i > j ? i : j where i is greater than j then the bold value is selected

i > j ? i : j where j is greater than i, then the bold value is selected.

In this video you will understand conditional operators briefly ..

# Control Flow in C++ (continue, break, return)

In this video you will understand the Control Flow briefly ...

# Repetition Statements 

Introduction

C++ provides a number of standard constructs known as loops that you can use to implement iteration logic.  If you are coming from other programming languages, you might recognize for loops, while loops, and do loops.  C++ supports all three of these iteration statements.

The ability to repeat a series of statements is a core requirement of most application code.  You might need to iterate over a series of items in an array or perhaps you need to count the number of characters in a string or the number of words in a document.   Repetition, through the use of loops, is the way in which you achieve these operations.

for Loops

he for loop executes a block of code repeatedly until the specified expression evaluates to false. You can define a for loop as follows.

for ([initializer(s)]; [condition]; [iterator]) 
{
   // code to repeat goes here
}

The [initializer(s)] portion is used to initialize a value, or values, as a counter for the loop. On each iteration, the loop checks that the value of the counter is within the range to execute the for loop, specified in the [condition] portion., and if so, execute the body of the loop.   At then end of each loop iteration, the [iterator] section is responsible for incrementing the loop counter.

The following code example shows how to use a for loop to execute a code block 10 times.

for Loop
for (int i = 0 ; i < 10; i++) 
{
    // Code to execute.
}
In this example, i = 0; is the initializer, i < 10; is the condition, and i++ is the iterator. 

while Loops

A while loop enables you to execute a block of code while a given condition is true. For example, you can use a while loop to process user input until the user indicates that they have no more data to enter.  The loop can continue to prompt the user until they decide to end the interaction by entering a sentinel value.   The sentinel value is responsible for ending the loop.

The following code example shows how to use a while loop.

while Loop

string response;
cout << "Enter menu choice " << endl << "More" << endl << "Quit" << endl;
cin >> response;

    while (response != "Quit")
    {
        // Code to execute if Quit is not entered

        // Prompt user again with menu choices until Quit is entered
        cout << "Enter menu choice " << endl << "More" << endl << "Quit" << endl;
        cin >> response;
    }

It's imperative to include the prompt again, inside the loop braces.  Failure to put this into the loop body will result in an infinite loop because the sentinel value can never be changed.

In this video you will understand flow loop and while loop briefly ...

do Loops

A do loop, sometimes also referred to as a do...while loop, is very similar to a while loop, with the exception that a do loop will always execute the body of the loop at least once.  In a while loop, if the condition is false from the start, the body of the loop will never execute.

You might want to use a do loop if you know that the code will only execute in response to a user prompt for data. In this scenario, you know that the application will need to process at least one piece of data, and can therefore use a do loop.

The following code example shows the use of a do loop.

do Loop

string response;

do
{        
     cout << "Enter menu choice " << endl << "More" << endl << "Quit" << endl;
     cin >> response;

     // Process the data.

} while (response != "Quit");

A couple of aspects to note about this loop.  First of all, the  response variable is declared outside of the loop.  This is important due to scope resolution requirements.  If you declare the variable inside the loop, then the while(response != "Quit") portion will not "see" the response variable.

Second, note that in comparison with the while loop, the prompt only needs to be placed inside the loop body and is not required ahead of the loop.  This is possible because the do loop executes the contents of the loop at least once due to the condition check being at the end of the loop.

Third, notice the semicolon at the end of the loop.  This is required in the do loop and not in the while or for loop.

Nesting Loops

Nesting of loops is possible in C++.  The most common is to nest for loops.  An example of nesting a for loop might be used for games, such as dealing four hands to card players from a deck of 52.  Your outer loop would count from 0 to 51 for the total number of cards in the deck and the inner loop would count from 0 to 3 to represent the 4 hands being dealt.   Of course, if you aren't dealing the entire 52 cards, you can change the outer loop counter to represent that fact.

The following code example shows nesting for loops to output a chess or checkerboard representation using the characters X and O.

bool alternate = true;

for (int x = 0; x < 8; x++)
{
    for (int y = 0; y < 4; y++)
    {
        if (alternate)
        {
            cout << "X ";
            cout << "O ";

        }
        else
        {
            cout << "O ";
            cout << "X ";

        }
    }
    alternate = !alternate;

    cout << endl;
}

In this video you will understand Nested loop briefly.