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This chapter is from the book

Making Decisions with Conditionals

Control structures are the structures within a language that allow you to control the flow of execution through a program or script. You can group them into conditional (or branching) structures and repetition structures (or loops).

If you want to sensibly respond to your users’ input, your code needs to be able to make decisions. The constructs that tell your program to make decisions are called conditionals.

if Statements

You can use an if statement to make a decision. You should give the if statement a condition to use. If the condition is true, the following block of code will be executed. Conditions in if statements must be surrounded by parentheses ().

For example, if a visitor orders no tires, no bottles of oil, and no spark plugs from Bob, it is probably because she accidentally clicked the Submit Order button before she had finished filling out the form. Rather than telling the visitor “Order processed,” the page could give her a more useful message.

When the visitor orders no items, you might like to say, “You did not order anything on the previous page!” You can do this easily by using the following if statement:

if ($totalqty == 0)
  echo 'You did not order anything on the previous page!<br />';

The condition you are using here is $totalqty == 0. Remember that the equals operator (==) behaves differently from the assignment operator (=).

The condition $totalqty == 0 will be true if $totalqty is equal to zero. If $totalqty is not equal to zero, the condition will be false. When the condition is true, the echo statement will be executed.

Code Blocks

Often you may have more than one statement you want executed according to the actions of a conditional statement such as if. You can group a number of statements together as a block. To declare a block, you enclose it in curly braces:

if ($totalqty == 0) {
  echo '<p style="color:red">';
  echo 'You did not order anything on the previous page!';
  echo '</p>';
}

The three lines enclosed in curly braces are now a block of code. When the condition is true, all three lines are executed. When the condition is false, all three lines are ignored.

else Statements

You may often need to decide not only whether you want an action performed, but also which of a set of possible actions you want performed.

An else statement allows you to define an alternative action to be taken when the condition in an if statement is false. Say you want to warn Bob’s customers when they do not order anything. On the other hand, if they do make an order, instead of a warning, you want to show them what they ordered.

If you rearrange the code and add an else statement, you can display either a warning or a summary:

if ($totalqty == 0) {
    echo "You did not order anything on the previous page!<br />";
} else {
    echo htmlspecialchars($tireqty).' tires<br />';
    echo htmlspecialchars($oilqty).' bottles of oil<br />';
    echo htmlspecialchars($sparkqty).' spark plugs<br />';
  }

You can build more complicated logical processes by nesting if statements within each other. In the following code, the summary will be displayed only if the condition $totalqty == 0 is true, and each line in the summary will be displayed only if its own condition is met:

if ($totalqty == 0) {
    echo "You did not order anything on the previous page!<br />";
} else {
    if ($tireqty > 0)
      echo htmlspecialchars($tireqty).' tires<br />';
    if ($oilqty > 0)
      echo htmlspecialchars($oilqty).' bottles of oil<br />';
    if ($sparkqty > 0)
      echo htmlspecialchars($sparkqty).' spark plugs<br />';
}

elseif Statements

For many of the decisions you make, you have more than two options. You can create a sequence of many options using the elseif statement, which is a combination of an else and an if statement. When you provide a sequence of conditions, the program can check each until it finds one that is true.

Bob provides a discount for large orders of tires. The discount scheme works like this:

  • Fewer than 10 tires purchased—No discount

  • 10–49 tires purchased—5% discount

  • 50–99 tires purchased—10% discount

  • 100 or more tires purchased—15% discount

You can create code to calculate the discount using conditions and if and elseif statements. In this case, you need to use the AND operator (&&) to combine two conditions into one:

if ($tireqty < 10) {
  $discount = 0;
} elseif (($tireqty >= 10) && ($tireqty <= 49)) {
  $discount = 5;
} elseif (($tireqty >= 50) && ($tireqty <= 99)) {
  $discount = 10;
} elseif ($tireqty >= 100) {
  $discount = 15;
}

Note that you are free to type elseif or else if—versions with or without a space are both correct.

If you are going to write a cascading set of elseif statements, you should be aware that only one of the blocks or statements will be executed. It did not matter in this example because all the conditions were mutually exclusive; only one can be true at a time. If you write conditions in a way that more than one could be true at the same time, only the block or statement following the first true condition will be executed.

switch Statements

The switch statement works in a similar way to the if statement, but it allows the condition to take more than two values. In an if statement, the condition can be either true or false. In a switch statement, the condition can take any number of different values, as long as it evaluates to a simple type (integer, string, or float). You need to provide a case statement to handle each value you want to react to and, optionally, a default case to handle any that you do not provide a specific case statement for.

Bob wants to know what forms of advertising are working for him, so you can add a question to the order form. Insert this HTML into the order form, and the form will resemble Figure 1.6:

<tr>
  <td>How did you find Bob's?</td>
  <td><select name="find">
  <option value = "a">I'm a regular customer</option>
  <option value = "b">TV advertising</option>
  <option value = "c">Phone directory</option>
  <option value = "d">Word of mouth</option>
  </select>
  </td>
</tr>
Figure 1.6

Figure 1.6 The order form now asks visitors how they found Bob’s Auto Parts

This HTML code adds a new form variable (called find) whose value will be 'a', 'b', 'c', or 'd'. You could handle this new variable with a series of if and elseif statements like this:

if ($find == "a") {
  echo "<p>Regular customer.</p>";
} elseif ($find == "b") {
  echo "<p>Customer referred by TV advert.</p>";
} elseif ($find == "c") {
  echo "<p>Customer referred by phone directory.</p>";
} elseif ($find == "d") {
  echo "<p>Customer referred by word of mouth.</p>";
} else {
  echo "<p>We do not know how this customer found us.</p>";
}

Alternatively, you could write a switch statement:

switch($find) {
  case "a" :
    echo "<p>Regular customer.</p>";
    break;
  case "b" :
    echo "<p>Customer referred by TV advert.</p>";
    break;
  case "c" :
    echo "<p>Customer referred by phone directory.</p>";
    break;
  case "d" :
    echo "<p>Customer referred by word of mouth.</p>";
    break;
  default :
    echo "<p>We do not know how this customer found us.</p>";
    break;
}

(Note that both of these examples assume you have extracted $find from the $_POST array.)

The switch statement behaves somewhat differently from an if or elseif statement. An if statement affects only one statement unless you deliberately use curly braces to create a block of statements. A switch statement behaves in the opposite way. When a case statement in a switch is activated, PHP executes statements until it reaches a break statement. Without break statements, a switch would execute all the code following the case that was true. When a break statement is reached, the next line of code after the switch statement is executed.

Comparing the Different Conditionals

If you are not familiar with the statements described in the preceding sections, you might be asking, “Which one is the best?”

That is not really a question we can answer. There is nothing that you can do with one or more else, elseif, or switch statements that you cannot do with a set of if statements. You should try to use whichever conditional will be most readable in your situation. You will acquire a feel for which suits different situations as you gain experience.

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