Lesson 3.12. Operator precedence

Would you know, categorically, what the variable x will contain?

x =  3 * 2 < 4;

The question raised here is which operator will be executed first: multiplication or comparison? There is a table of operator precedence shown below. You can see that multiplication (precedence 3) has priority over comparison (prcedence 6):

Operator precedence table

Precedence	Operators	Description
1	`++` `--`	Pre-increment and pre-decrement
2	`!` `~` `+` `-`	Unary operators
3	`*` `/` `%`	Multiplication, division and modulo
4	`+` `-`	Addition and subtraction
5	`<<` `>>`	Bit shifting
6	`<` `>` `<=` `>=`	Comparison operators
7	`==` `!=`	Equality operators
8	`&`	Logical bitwise AND
9	`^`	Exclusive bitwise OR
10	`\|`	Inclusive bitwise OR
11	`&&`	Logical ANBD
12	`\|\|`	Logical OR
13	`=` `!=` `+=` `*=` `&=` etc	Assignment operators
14	`++` `--`	Post-increment and post-decrement

Associativity

Would you be able, now, to give with certainty the value of x after these lines of code?

int a=5, x;
x = a = 3;

In this example, the = operator appears twice in the same instruction. Of course, both operators have the same precedence. But fortunately, for each operator is defined an associativity. It is this associativity that determines the order of operators of the same precedence. In the last column of the precedence table we see that the associativity is defined from the right to the left for the = operator. The equivalent instructions are therefore :

int a=5, x;
a = 3;
x = a;

Obviously, no one knows the priority table by heart. The easiest way, if you are doubt, is to add parentheses to impose the order of execution of the of the different operators. This also makes it easier for other developers to u nderstand your code if they have to analyze it. Just remember that the multiplication and division operators have priority over over addition and subtraction.

Exercise

Write a program that calculates the value of x (eliminate unnecessary brackets):

$$ x = \dfrac{125,5+12,5}{5468,12} - 12,5*2,54 $$

The result of the calculation should be -31.724763.

Do the exercise

Solution

Quiz

Will a precedence 1 operator be executed after a precedence 2 operator?

Always

No, it is always the opposite

It depends on the associativity

Yes, unless it is on the left

Check

When does associativity apply?

When there are parentheses

When the same operator appears twice

When there are two operators with the same precedence

When you combine two operators together (for example +=)

Check

How to write this instruction to make it easier to understand.

x = c > a * b;

It's as clear as day.

x = (a * b) < c;

x = a * (b < c);

x = (c > a) * b;

Check

How to write this instruction to make it easier to understand.

x = 3 * 2 && 0;

It's as clear as day.

x = 0;

x = (3 * 2) && 0;

x = 3 * (2 && 0);

Check Bravo! Here, x will always be equal to 0 because the last operation is always && 0.

What will be the value of the variable x after these lines?

int a=3, x;
x = a += 5;

Check Bravo! Operator += is executed first because associativity is from right to left.

What will be the value of the variable x after these lines?

int a=3, x;
x = a == 5;

Check Bravo! Operator == is executed first and comparison will always return 0.

What will be the value of the variable x after these lines?

int a=-3;
x = 0<a<20;

Check

What will be the value of the variable x after these lines?

int a=-3;
x = 0<a && a<20;

Check

Lesson 3.11 Summary Lesson 4.1

Lesson 3.12. Operator precedence

Operator precedence table

Associativity

Exercise

Quiz

See also