If statements

The ability to control the flow of your program, letting it make decisions on what code to execute, is valuable to the programmer. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user entered password, your program can decide whether a user is allowed access to the program.

Without a conditional statement such as the if statement, programs would run almost the exact same way every time. If statements allow the flow of the program to be changed, and so they allow algorithms and more interesting code.

Before discussing the actual structure of the if statement, let us examine the meaning of TRUE and FALSE in computer terminology. A true statement is one that evaluates to a nonzero number. A false statement evaluates to zero. When you perform comparison with the relational operators, the operator will return 1 if the comparison is true, or 0 if the comparison is false. For example, the check 0 == 2 evaluates to 0. The check 2 == 2 evaluates to a 1. If this confuses you, try to use a cout statement to output the result of those various comparisons (for example cout<< ( 2 == 1 );) When programming, the aim of the program will often require the checking of one value stored by a variable against another value to determine whether one is larger, smaller, or equal to the other. There are a number of operators that allow these checks. Here are the relational operators, as they are known, along with examples:

>     greater than              5 > 4 is TRUE
<>=    greater than or equal     4 >= 4 is TRUE
<=    less than or equal        3 <= 4 is TRUE ==    equal to                  5 == 5 is TRUE !=    not equal to  5 != 4 is TRUE

It is highly probable that you have seen these before, probably with
slightly different symbols. They should not present any hindrance to
understanding. Now that you understand TRUE and FALSE in computer
terminology as well as the comparison operators, let us look at the
actual structure of if statements.



The structure of an if statement is as follows:

if ( TRUE )
 Execute the next statement

To have more than one statement execute after an if statement that
evaluates to true, use braces, like we did with the body of a function.
Anything inside braces is called a compound statement, or a block.

For example:

if ( TRUE ) {
 Execute all statements inside the braces
}

There is also the else statement. The code after it (whether a single
line or code between brackets) is executed if the if statement is
FALSE.

It can look like this:

if ( TRUE ) {
 // Execute these statements if TRUE
}
else {
 // Execute these statements if FALSE
}

One use for else is if there are two conditional statements that may
both evaluate to true, yet you wish only one of the two to have the
code block following it to be executed. You can use an else if after
the if statement; that way, if the first statement is true, the else if
will be ignored, but if the if statement is false, it will then check
the condition for the else if statement. If the if statement was true
the else statement will not be checked. It is possible to use numerous
else if statements.

Let's look at a simple program for you to try out on your own.


#include  

using namespace std;
  
int main()                            // Most important part of the program!
{
 int age;                            // Need a variable...

 cout<<"Please input your age: ";    // Asks for age
 cin>> age;                          // The input is put in age
 cin.ignore();                       // Throw away enter
 if ( age < 100 ) {                  // If the age is less than 100
    cout<<"You are pretty young!\n"; // Just to show you it works...
 }
 else if ( age == 100 ) {            // I use else just to show an example
    cout<<"You are old\n";           // Just to show you it works...
 }
 else {
   cout<<"You are really old\n";     // Executed if no other statement is
 }
 cin.get();
}


Boolean operators allow you to create more complex conditional
statements. For example, if you wish to check if a variable is both
greater than five and less than ten, you could use the boolean AND to
ensure both var > 5 and var < 10 are true. In the following
discussion of boolean operators, I will capitalize the boolean
operators in order to distinguish them from normal english. The actual
C++ operators of equivalent function will be described further into the
tutorial - the C++ symbols are not: OR, AND, NOT, although they are of
equivalent function.


When using if statements, you will often wish to check
multiple different conditions. You must understand the Boolean
operators OR, NOT, and AND. The boolean operators function in a similar
way to the comparison operators: each returns 0 if evaluates to FALSE
or 1 if it evaluates to TRUE.


NOT: The NOT operator accepts one input. If that input is
TRUE, it returns FALSE, and if that input is FALSE, it returns TRUE.
For example, NOT (1) evalutes to 0, and NOT (0) evalutes to 1. NOT (any
number but zero) evaluates to 0. In C and C++ NOT is written as !. NOT
is evaluated prior to both AND and OR.

AND: This is another important command. AND returns TRUE if
both inputs are TRUE (if 'this' AND 'that' are true). (1) AND (0) would
evaluate to zero because one of the inputs is false (both must be TRUE
for it to evaluate to TRUE). (1) AND (1) evaluates to 1. (any number
but 0) AND (0) evaluates to 0. The AND operator is written &&
in C++. Do not be confused by thinking it checks equality between
numbers: it does not. Keep in mind that the AND operator is evaluated
before the OR operator.


OR: Very useful is the OR statement! If either (or both) of
the two values it checks are TRUE then it returns TRUE. For example,
(1) OR (0) evaluates to 1. (0) OR (0) evaluates to 0. The OR is written
as || in C++. Those are the pipe characters. On your keyboard, they may
look like a stretched colon. On my computer the pipe shares its key
with \. Keep in mind that OR will be evaluated after AND.


It is possible to combine several boolean operators in a
single statement; often you will find doing so to be of great value
when creating complex expressions for if statements. What is !(1
&& 0)? Of course, it would be TRUE. It is true is because 1
&& 0 evaluates to 0 and !0 evaluates to TRUE (ie, 1).


Try some of these - they're not too hard. If you have questions about them, feel free to stop by our forums.

A. !( 1 || 0 )         ANSWER: 0 
B. !( 1 || 1 && 0 )    ANSWER: 0 (AND is evaluated before OR)
C. !( ( 1 || 0 ) && 0 )  ANSWER: 1 (Parenthesis are useful)

If you find you enjoyed this section, then you might want to look more at Boolean Algebra.