PHP: Expressions

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Expressions

Aims:

• to learn about expressions in PHP
• to learn about precedence and associativity

Revision: data types

• A data type is defined by giving a set of values, and the operations we can use to create, access and modify the values
• String operators
  • Concatenation (.) and some others
• Numeric operators
  • Addition (+), subtraction (-), multiplication (*), division (/), modulus, i.e. remainder (%), and some others

Arithmetic expressions

• An expression combines operators and data (operands), and can be evaluated to give a result
• E.g.
  • 2 + 3
  • 7 % 2
  • $age + 1
• But there can be more than one operator in an expression, e.g.
  • 1 + 2 + 3
  • 3 * 4 * 2
• And there can be sub-expressions within parentheses, e.g.
  • 3 * (4 + 2)
  • (1 + 5) * (3 * 2)
  • 3 * (4 + (2 * 6))

Arithmetic expressions

• If there are multiple operators in an expression, in what order will they be evaluated?
• Class exercise: Evaluate each of these in two different orders:
  1. 2 + 3 * 4
  2. 8 / 2 - 1
  3. 5 - 2 + 3
  4. 18 / 6 / 3

Arithmetic expressions

• The order PHP evaluates the operators depends on their precedence and associativity
• Precedence:
  • PHP evaluates higher precedence operators ahead of lower precedence ones
  • *, /, % have higher precedence than +, -
• Associativity:
  • what to do in the event of a tie in precedence
  • +, -, *, /, % are left-associative
  • left-associative means evaluate the equal precedence operators from left-to-right
• Class exercise: Hence, evaluate each of these:
  1. 2 + 3 * 4
  2. 8 / 2 - 1
  3. 5 - 2 + 3
  4. 18 / 6 / 3

Arithmetic expressions

• You can force any ordering you want by using parentheses, which raise the precedence of whatever is within them
• Class exercise: Evaluate each of these:
  1. (2 + 3) * 4
  2. 2 + (3 * 4)
  3. (8 / 2) - 1
  4. (5 - 2 + 3)
  5. 18 / (6 / 3)

Class exercise

• The following formula converts a temperature from degree centigrade (Celsius) to degrees Fahrenheit:
• In a PHP program, suppose $temp_in_centigrade contains a temperature in degrees centigrade
• Does this assignment statement need any parentheses to correctly implement the above formula?

  $temp_in_fahrenheit = 32 + $temp_in_centigrade * 9 / 5;

Class exercise

• The following formula calculates the average of two numbers x and y:
• In a PHP program, suppose $age_of_mother and $age_of_father contain the ages of the user's parents
• Complete this assignment statement to calculate the average age of the user's parents

  $avg_age_of_parents = _____________________________________________________

Class exercise

• In a PHP program, $prev_mark contains a student's mark for the previous CS1109 assignments
• And $new_mark contains his/her mark for a new assignment
• Complete this assignment statement to calculate the difference between the new one and the previous one as a percentage of the previous one:

  $percentage_change = __________________________________________________

Class exercise

• In Computer Science, there are many ways to specify colours, e.g. by name, by RGB value, by HSV value
• Even with RGB values, we can specify the amounts of red, green and blue
  • as percentages, e.g. rgb(70%,0%,70%)
  • as decimal numbers between 0 and 255 inc., e.g. rgb(0,255,0)
  • as hexadecimal numbers, e.g. #00FF00
• Suppose rgb_form.html contains this form:

  <form action="rgb_convert.php" method="get">
      <input type="text" name="r_per_cent" value="0" />
      <input type="text" name="g_per_cent" value="0" />
      <input type="text" name="b_per_cent" value="0" />
      <input type="submit" />
  </form>

• Write rgb_convert.php to convert the user's RGB value expressed as percentages to an RGB value expressed as decimal integers between 0 and 255 inclusive