A Tour Of Decision Making Structures In Php

A Tour of Decision Making Structures in PHP by Timothy Boronczyk 2003-10-19

Synopsis A comprehensive overview of the control structures available in PHP.

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A Tour of Decision Making Structures in PHP by Timothy Boronczyk

Introduction Often times, people find it difficult to make a decision--we're often asking ourselves questions like, "What pair of pants should I wear to work?", "Should I really eat that extra donut?" and "Should I tell my fiancee I wear a toupee?" But computers have a much simpler life. Programs use logical comparisons to help make a decision and depending on the value of the statement (whether it evaluates to true or false) the program can process different sections of code. A logical comparison compares the value of one operation or statement with that of another to see if the overall statement resolves to either "true" or "false." For example, if $x is set to 6, and $y is set to 7, then the statement "the value of $x is less than the value of $y" is true. Statements saying $x's value is great than or equal to the value of $y would be false.

Comparison Operators The following chart shows some common operators you may use in your programs. operator comparison -----------------------------------------------x == y checks if the value of x is equal to the value of y x != y

checks if the value of x is not equal to the value of y

x < y

checks if the value of x is less than the value of y

x > y

checks if the value of x is greater than the value of y

x <= y

checks if the value of x is less than or equal to the value of y

x >= y

checks if the value of x is greater than or equal to the value of y

(a) && (b)

checks if the value of statement a and statement b evaluate to "true"

(a) || (b)

checks if the value of statement a or statement b evaluate to "true"

Let's apply that information to some example comparisons: Assuming $x holds the value 5, $y holds 100 and $z holds 500: http://codewalkers.com/tutorials/52/1.html

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A Tour of Decision Making Structures in PHP by Timothy Boronczyk

($x == $y) && ($x <= $z) (5 == 100) && (5 <= 500) false && true > false ($x * $y) == $z (5 * 100) == 500 500 == 500 > true ($x != $y) || ($y == $z) (5 != 100) || (100 == 500) true || false > true Be sure that you don't confuse = with ==. = assigns a value to a variable while == compares for equality. This can be a source of great frustration for many new programmers until they become more accustomed to working with them.

IF Statements Now let's examine a scenario in which a PHP script executes a section of code only if a certain condition is met using an if-statement. An if-statement is made up of the keyword if followed by the comparison surrounded by parentheses and then the lines of code to be executed set apart by braces.

The script first assigns the value of 2 to $x and then prints a greeting message. The logical comparison is made between $x and 2, which returns true. Because this comparison evaluates to true, the script will process the code within the braces. The script concludes by displaying a parting message. If there's only one line of code to be executed under the conditional circumstance, braces do not have to be used. So, the code could be re-written to resemble:
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A Tour of Decision Making Structures in PHP by Timothy Boronczyk

echo "Hello, World.\n"; if ($x == 2) echo "\$x = $x\n"; echo "Goodbye.\n"; ?>

However, it is considered good practice and good form by the PEAR coding standards to still use the braces.

IF-ELSE Statements The if-statement only allows for a piece of code to be executed or skipped depending on the return value of the comparison. But it can be extended with else to allow for an alternate section of code to be processed.

In this example, an if-statement checks to see if the value of $age is less than 13. Because $age was assigned the value of 25, the first section of code is skipped and the second section is executed, displaying the message "You are 13 or over." Again, when only one line of code is used in a section then braces do not need to be used... but it is considered good practice to use them anyway.

Question-Colon Operator A nice, compact way to write a short if-else statement is to use the ?: operator. Because it uses symbols instead of keywords such as if and else, some may find it difficult to start using and avoid it. However with some practice it does become easy to use and can shorten the length of your code considerably. The previous example can be written as follows:

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A Tour of Decision Making Structures in PHP by Timothy Boronczyk



The first section of code (between the question-mark and the colon) will be used if the conditional evaluates true. The second section of code (following the colon) will be used if the evaluation is false. In the code, the statement ($age < 13) is tested and returns false so "You are 13 or over." is displayed. Typically, the ?: is seen when used to assign a value to a variable.

IF-ELSEIF Statements Several conditional statements can be chained together using if-elseif statements.
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A Tour of Decision Making Structures in PHP by Timothy Boronczyk

else { echo "So who knows what to expect?!\n"; } ?>

In the example, PHP will process the chain of if-elseif. Once the first true comparison has been, PHP will execute the desired code block and then break out of the structure. No further conditions of the structure are be tested.

SWITCH-CASE statements As the ?: is an alternative to if-then statements, a switch-case statement provides a cleaner way to code lengthy if-elseif statements where each test checks the same variable for a specified value.
";

case 'rain': echo "Be sure to bring an umbrella!\n"; break; case 'wind': echo "Take the day off and fly a kite!\n"; break; default: echo "So who knows what to expect?!\n"; break; } ?>

The comparison is made between the variable set by the switch and the values designated by the case keyword. The code block of each case is terminated with break. A default case can be used to execute a block of code if no other cases returned a true comparison.

About the Author Timothy Boronczyk lives in Syracuse, NY, where he works as an E-Services Coordinator for a local credit union. He has a background in elementary education, over 5 years experience in web design and has written tutorials on web design, PHP, Ruby, XML and various other topics. His hobbies include photography and composing music. http://codewalkers.com/tutorials/52/1.html

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A Tour of Decision Making Structures in PHP by Timothy Boronczyk

Timothy Boronczyk lives in Syracuse, NY, where he works as an E-Services Coordinator for a local credit union. He has a background in elementary education, over 5 years experience in web design and has written tutorials on web design, PHP, Ruby, XML and various other topics. His hobbies include photography and composing music.

http://codewalkers.com/tutorials/52/1.html

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