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, 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.
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's not! 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.
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 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; } |
While
Loops
Loops are used to repeat a block of code. Being able to have your program repeatedly execute a block of code is one of the most basic but useful tasks in programming -- many programs or websites that produce extremely complex output (such as a message board) are really only executing a single task many times. (They may be executing a small number of tasks, but in principle, to produce a list of messages only requires repeating the operation of reading in some data and displaying it.) Now, think about what this means: a loop lets you write a very simple statement to produce a significantly greater result simply by repetition.
FOR - for loops are the most useful type. The syntax for a for loop is
for ( variable initialization; condition; variable update ) {
Code to execute while the condition is true
}
The variable initialization allows you to either declare a variable and give it a value or give a value to an already existing variable. Second, the condition tells the program that while the conditional expression is true the loop should continue to repeat itself. The variable update section is the easiest way for a for loop to handle changing of the variable. It is possible to do things like x++, x = x + 10, or even x = random ( 5 ), and if you really wanted to, you could call other functions that do nothing to the variable but still have a useful effect on the code. Notice that a semicolon separates each of these sections, that is important. Also note that every single one of the sections may be empty, though the semicolons still have to be there. If the condition is empty, it is evaluated as true and the loop will repeat until something else stops it.
Example:
#include
int main()
{
int x;
/* The loop goes while x < 10, and x increases by one every loop*/
for ( x = 0; x < 10; x++ ) {
/* Keep in mind that the loop condition checks
the conditional statement before it loops again.
consequently, when x equals 10 the loop breaks.
x is updated before the condition is checked. */
printf( "%d\n", x );
}
getchar();
}
WHILE - WHILE loops are very simple. The basic structure is
while ( condition ) { Code to execute while the condition is true } The true represents a boolean expression which could be x == 1 or while ( x != 7 ) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x ==5 || v == 7) which says execute the code while x equals five or while v equals 7. Notice that a while loop is like a stripped-down version of a for loop-- it has no initialization or update section. However, an empty condition is not legal for a while loop as it is with a for loop.
Example:
#include
int main()
{
int x = 0; /* Don't forget to declare variables */
while ( x < 10 ) { /* While x is less than 10 */
printf( "%d\n", x );
x++; /* Update x so the condition can be met eventually */
}
getchar();
}
DO..WHILE - DO..WHILE loops are useful for things that want to loop at least once. The structure is
do {
} while ( condition );
Notice that the condition is tested at the end of the block instead of the beginning, so the block will be executed at least once. If the condition is true, we jump back to the beginning of the block and execute it again. A do..while loop is almost the same as a while loop except that the loop body is guaranteed to execute at least once. A while loop says "Loop while the condition is true, and execute this block of code", a do..while loop says "Execute this block of code, and then continue to loop while the condition is true".
Example:
#include
int main()
{
int x;
x = 0;
do {
/* "Hello, world!" is printed at least one time
even though the condition is false */
printf( "Hello, world!\n" );
} while ( x != 0 );
getchar();
}
Break and Continue
Two keywords that are very important to looping are break and continue. The break command will exit the most immediately surrounding loop regardless of what the conditions of the loop are. Break is useful if we want to exit a loop under special circumstances. For example, let's say the program we're working on is a two-person checkers game. The basic structure of the program might look like this:
while (true)
{
take_turn(player1);
take_turn(player2);
}
This will make the game alternate between having player 1 and player 2 take turns. The only problem with this logic is that there's no way to exit the game; the loop will run forever! Let's try something like this instead.
Example:
while(true)
{
if (someone_has_won() || someone_wants_to_quit() == TRUE)
{break;}
take_turn(player1);
if (someone_has_won() || someone_wants_to_quit() == TRUE)
{break;}
take_turn(player2);
}
FOR LOOP
This code accomplishes what we want--the primary loop of the game will continue under normal circumstances, but under a special condition (winning or exiting) the flow will stop and our program will do something else.
Continue is another keyword that controls the flow of loops. If you are executing a loop and hit a continue statement, the loop will stop its current iteration, update itself (in the case of for loops) and begin to execute again from the top. Essentially, the continue statement is saying "this iteration of the loop is done, let's continue with the loop without executing whatever code comes after me." Let's say we're implementing a game of Monopoly. Like above, we want to use a loop to control whose turn it is, but controlling turns is a bit more complicated in Monopoly than in checkers. The basic structure of our code might then look something like this:
Example:
for (player = 1; someone_has_won == FALSE; player++)
{
if (player > total_number_of_players)
{player = 1;}
if (is_bankrupt(player))
{continue;}
take_turn(player);
}
