If statements in C
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, always following the same sequence of
function calls. If statements allow the flow of the program to be changed,
which leads to 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 printf statement to output the result of those
various comparisons (for example printf ( "%d", 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
== 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 well as the comparison operators, let us
look at the actual structure of if statements.
Basic If Syntax
The structure of an if statement is as follows:
if ( statement is TRUE )
Execute this line of code
Here is a simple example that shows the syntax:
if ( 5 < 10 )
printf( "Five is now less than ten, that's a big surprise" );
Here, we're just evaluating the statement, "is five less than ten", to see if
it is true or not; with any luck, it is! If you want, you can write your
own full program including stdio.h and put this in the main function and run
it to test.
To have more than one statement execute after an if statement that evaluates
to true, use braces, like we did with the body of the main function. Anything
inside braces is called a compound statement, or a block. When using if
statements, the code that depends on the if statement is called the "body" of
the if statement.
For example:
if ( TRUE ) {
/* between the braces is the body of the if statement */
Execute all statements inside the body
}
I recommend always putting braces following if statements. If you do this,
you never have to remember to put them in when you want more than one
statement to be executed, and you make the body of the if statement more
visually clear.
Else
Sometimes when the condition in an if statement evaluates to false, it would
be nice to execute some code instead of the code executed when the statement
evaluates to true. The "else" statement effectively says that whatever 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 */
}
Else if
Another use of else is when there are multiple conditional statements that may
all evaluate to true, yet you want only one if statement's body to execute.
You can use an "else if" statement following an if statement and its body;
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 to ensure that only
one block of code is executed.
Let's look at a simple program for you to try out on your own.
#include <stdio.h>
int main() /* Most important part of the program!
*/
{
int age; /* Need a variable... */
printf( "Please enter your age" ); /* Asks for age */
scanf( "%d", &age ); /* The input is put in age */
if ( age < 100 ) { /* If the age is less than 100 */
printf ("You are pretty young!\n" ); /* Just to show you it works... */
}
else if ( age == 100 ) { /* I use else just to show an example */
printf( "You are old\n" );
}
else {
printf( "You are really old\n" ); /* Executed if no other statement is
*/
}
return 0;
}
More interesting conditions using boolean operators
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 along 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)
evaluates to 0, and NOT (0) evaluates to 1. NOT (any number but zero) evaluates
to 0. In 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
(i.e., 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.
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