The ability to control the flow of your
program, whether or not your program will run a section of code based on
specific tests, is valuable to the programmar. 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 checks, the operator will return a nonzero number if the
comparison is true, or zero if the comparison is false. For example, the check
0==2 evaluates to 0. The check 2==2 evaluates to a nonzero number, although we
cannot be certain of its value. 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
< less than 4<5 is TRUE
>= greater than or equal 4>=4 is TRUE
<= less than or equal 3<=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)
Do whatever follows on the next line.
To have more than one statement execute
after an if statement that evaluates to true, use brackets.
For example:
if (TRUE)
{ Do everything between the brackets.}
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(FALSE)
{ Not executed if its false}
else
{ do all of this}
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 <iostream.h> 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
if(age<100) //If the age is less than 100
{
cout<<"You are pretty young!"; //Just to show it works
}else if(age==100) //I use else just to show an example
{cout<<"You are old"; //Just to show you it works...
} else {cout<<"You are really old"; //Executed if no other statement is executed
} return 0;}
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 t