if statement can conditionally execute code, and is a frequently used statement in programming. When the value of num is less than 5, the statement will be executed and print "The number is less than 5."

int num  = 2;
// some operations may change num's value
if (num < 5)
    cout << "The number is less than 5." << endl;

If it is a compound statement to be executed, the compound statement should be brace-enclosed. Even it is an one-line statement, it can also be brace-enclosed.

if (num == 5 )
{
    cout << "The number is 5." << endl;
    //some other lines
}

else statement will be executed when the condition follows if is not true.

if (num == 5 )
{
    cout << "The number is 5." << endl;
}
else
{
    cout << "The number is not 5." << endl;
}

If there are multiple conditions to be evaluated, you can use else if as follows. The first condition num < 5 will be evaluated first, and then the second condition num > 10. If no matched conditions, the else statement will be executed.

if (num < 5)
    cout << "The number is less than 5." << endl;
else if (num > 10)
    cout << "The number is greater than 10." << endl;
else
    cout << "The number is in range [5, 10]." << endl;

else will be associated with the closest if. In the following code, else is associated with if(num < 5), not if(num < 10). Even no error is in the following code, some compilers will also give some warning messages since it may confuse readers.

if(num < 10)
if(num < 5)
cout << "The number is less than 5" << endl;
else
cout << "Where I'm?" << endl;

To make the source code easier to understand, I suggest to use braces and indents to format the source code as follows.

if(num < 20)
{
    if(num < 5)
        cout << "The number is less than 5" << endl;
    else
        cout << "Where I'm?" << endl;
}

The ternary conditional operator ? : is also widely used to replace some if else statements.

bool isPositive = true;
int factor = 0;
//some operations may change isPositive's value
if(isPositive)
    factor = 1;
else
    factor = -1;

The if else statement can be replaced by one line code as follows.

factor = isPositive ? 1 : -1;

If the condition before ? is true, the value will be the one between ? and :. Otherwise, the value will be the one after :.

For the previous example, it can even be simplified to an expression without jump. In the instructions compiled from if, ? :, for, while and some others, there are jump statements. Different with them, the following code does not contain jumps. It should be more efficient most of the time. But not all ? : statements can be converted to the following style statement.

factor = (isPositive) * 2 - 1;

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