PHP Decision-Making - Associativity
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All operators process their operators in a certain direction. This direction is called associativity, and it depends on the type of operator. Most operators are processed from left to right, which is called left associativity. For example, in the expression 3 + 5
- 2, 3 and 5 are added together, and then 2 is subtracted from the result, evaluating to 8. While left associativity means that the expression is evaluated from left to right, right associativity means the opposite.
Since the assignment operator has right associativity, it is one of the exceptions because right associativity is less common. The expression $a=$b=$c processes by $b being assigned the value of $c, and then $a being assigned the value of $b. This assigns the same value to all of the variables. If the assignment operator is right associative, the variables might not have the same value.
If you're thinking that this is incredibly complicated, don't worry. These rules are enforced only if you fail to be explicit about your instructions. Keep in mind that you should always use brackets in your expressions to make your actual meaning clearer. This helps both PHP and also other people who may need to read your code.
If you accidentally use & instead of &&, or | instead of ||, you'll end up getting the wrong operator. & and | compare binary data bit by bit. PHP will convert your operands into binary data and apply binary operators.
Relational Operators In Chapter 3 we discussed assignment and math operators. Relational operators provide the ability to compare two operands and return either TRUE or FALSE regarding the comparison. An expression that returns only TRUE or FALSE is called a Boolean expression, which we discussed in the previous chapter. These comparisons include tests for equality and less than or greater than. These comparison operators allow you to tell PHP when to do something based on whether a comparison is true so that decisions can be made in your code.
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter four of the book Learning PHP and MySQL, Second Edition, written by Michele Davis and Jon Phillips (O'Reilly, 2006; ISBN: 0596101104). Check it out today at your favorite bookstore. Buy this book now.
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