PHP: Operator precedence

Consider a simple expression:

<?php

print_r(2 + 2 * 2); // => 6

The result is 6, not 8. This is explained by the concept of operator precedence in math and programming. It defines the order in which operations are performed:

• Multiplication and division are performed before addition and subtraction.
• Exponentiation (**) has an even higher precedence.

Operator precedence (from high to low):

  **         exponentiation
   ↓
  * / %      multiplication, division, remainder
   ↓
  + -        addition, subtraction

For example:

<?php

print_r(2 * 2 ** 3); // => 16, because first 2 ** 3 = 8, then 8 * 2 = 16

If operations with the same precedence are next to each other, they are performed from left to right:

<?php

print_r(8 / 2 * 3); // => 12, because first 8 / 2 = 4, then 4 * 3 = 12

Controlling the order of operations

Sometimes you need to change the order of calculations. Parentheses are used for this. They let you specify which operations should be performed first:

<?php

print_r((2 + 2) * 2); // => 8

Parentheses can be placed around any part of an expression and nested into each other:

<?php

print_r(3 ** (4 - 2));                   // => 9
print_r(7 * 3 + (4 / 2) - (8 + (2 - 1))); // => 14

The main rule: always close your parentheses. Unmatched parentheses cause errors: both beginners and experienced programmers sometimes forget about the closing parenthesis.

Write parentheses as a pair right away. For example, type () and then fill in the inner part. Most code editors (including ours) automatically add the closing parenthesis as soon as you write the opening one. This also applies to other paired characters, such as quotation marks.

Improving readability

Sometimes an expression works correctly but looks confusing. In such cases, parentheses can be added simply for clarity: they won't change the result, but they will improve readability.

<?php

// Before
print_r(8 / 2 + 5 - -3 / 2);       // => 10.5

// After
print_r(((8 / 2) + 5) - (-3 / 2)); // => 10.5

Programs are written by people, and they are read by people too. The computer doesn't care how clearly the code is written: it just needs to be syntactically correct. For a human, clear and tidy code is the key to convenience, especially when working in a team or debugging errors.

You and 4 friends (5 people in total) have ordered 2 pizzas for 300 rubles each and 4 drinks for 50 rubles each. You need to split the bill equally.

Write a single-line program with print_r(), placing the parentheses so that the total amount is calculated first, and then divided among everyone:

without parentheses:   2 * 300 + 4 * 50 / 5  =  640  ← wrong
with parentheses:  (2 * 300 + 4 * 50) / 5  =  160  ← correct