Decision Making Statements in C++.

The smallest Independent Unit in a C++ program is a statement i.e. You can relate it to sentences of a natural language. Because, Just like sentences are terminated with a period (.), similarly the statements are also terminated with semicolon (;)

The simplest form of a program statement is the empty statement i.e.( ;)
(NOTE - I will not be explaining this topic so much because, It is mostly known by every programmer, doesn't not matter if he/she is a beginner or a professional)

Let's discuss the Decision Making Statements one by one.

1.If Statement - These statements are used when you want something to happen if your condition is true. Consider an example like this - If it's rainy outside I will bring an Umbrella with me.

#include<iostream>
using namespace std;
int main()
{
    bool rain = true;

    if(rain == true)        // It can simply be written as if(rain){}
    {
        cout << "Take an Umbrella with you" << endl;
    }
}

Actually inside if( __here__ ), you can create any situation as you want, You can mix Logical operators to make it work for more than one condition in a single if block, you can use relational operators like this - 

#include<iostream>
using namespace std;
int main()
{
    int a = 10;
    
    if(a <= 21)         
        cout << "A is less" << endl;
}

2. If-else Statement - These are equivalent of saying if some particular condition is true than do something, and do something else for every other scenario. Below code demonstrate the idea - 

#include<iostream>
using namespace std;
int main()
{
    int a = 10;

    if(a < 12)      // True if A is less than 12
        cout << "A is less than 12" << endl;
    else            // True if A is equal to 12 or greater than 12
        cout << "A is not less than 12" << endl;

    return 0;
}

3. If - else if statement - These statement blocks are used when you have multiple conditions and you have perform different task for each of this conditions, For example consider the below program - Where I have to print different messages for each of the condition.

#include<iostream>
using namespace std;
int main()
{
    int a = 10;

    if(a < 12)
        cout << "A is less than 12" << endl;
    else if(a == 12)
        cout << "A is equal to 12" << endl;
    else
        cout << "A is greater than 12" << endl;

    return 0;
}

4. Nested - if statements - These are simply if statements that are inside other if statements, In some scenario, we can replace nested if statement with the help of relational operators (But it is not necessarily true for every case), Consider the given program - 

#include<iostream>
using namespace std;
int main()
{
    int a = 10;

    if(a < 12)
    {
        if(a < 10)
            cout << "A is less than 10" << endl;
        else
            cout << "A is less than 12 but greater than or equal to 10" << endl;
    }
    
    return 0;
}

You can see that, I wrote one if - else statement block inside the outside if statement. You, can also relate these nested - if statement blocks as a ladder of if statements.

5. Switch Statements - The application is same but the syntax is little different, and when you have a mess of if-else statements in your program, you should consider it replacing with switch statement.

The basic structure of switch like this - 

switch(n)       // Here n is a predicate/variable on which we check our conditions
{
    case 1: 
        // Do something
        break;
    case 2:
        // Do something
        break;
    .
    .
    .
    default:
        // Do something
        break;
}

Let's try to create a simple calculator using switch statement

#include<iostream>
using namespace std;
int main()
{
    int a, b;
    cout << "Enter first number : " << endl; cin >> a;
    cout << "Enter second number : " << endl; cin >> b;
    cout << "Enter symbol of operation to perform : " << endl;
    cin.ignore();
    cout << flush;
    char c;
    cin >> c;               // We take character input like +, -, / or *

    switch(c)               // switch statement take c as parameter.
    {   
        case '+':
            cout << "Sum of " << a << " + " << b << " = " << a+b << endl;
            break;
        case '-':
            cout << "difference of " << a << " - " << b << " = " << a-b << endl;
            break;
        case '*':
            cout << "Mulitplication of " << a << " * " << b << " = " << a*b << endl;
            break;
        case '/':
            cout << "division of " << a << " / " << b << " = " << a/b << endl;
            break;
    }
}

See example run here.

The example must be difficult to understand, only two things that you could have not familiar with is cin.ignore(), To take an Idea of this, you have to remember my previous blogs where I have said that the input doesn't directly go to the desired variables it first get stored in some input buffer. and to clear that input buffer I used cin.ignore(), Why I would need to clear the input buffer,

For, this think of general case when someone is giving the input to my program, he might, for first two numbers enter the number and then hit "Enter" key, but enter key is also a character (Escape sequence character), But, while entering the second number if We enter the "Enter" Key, it will get stored in the character which has to be used for taking arithmetic operators, Which will effect the execution of our program and cause undesired behaviour, To avoid that first we delete that input buffer containing that "Enter" key then take a character in character "c".

 Stay tuned for the upcoming content. For any query leave a comment below.

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References -

• Lippman, Stanley B. c++ primer 3rd edition(april 2, 1998)