Operators: addition
The addition operator is the+ (plus) sign, which is fully in line with mathematical standards.
Again, take a look at the snippet of the program below:
print(-4 + 4)
print(-4. + 8)
The result should be nothing surprising. Run the code to check it.
0
4.0
The subtraction operator, unary and binary operators
The subtraction operator is obviously the - (minus) sign, although you should note that this operator has another meaning - it can change the sign of a number.
This is a great opportunity to present a very important distinction between unary and binary operators.
In subtracting applications, the minus operator expects two arguments: the left (a minuend in arithmetical terms) and right (a subtrahend).
For this reason, the subtraction operator is considered to be one of the binary operators, just like the addition multiplication and division operators.
But the minus operator may be used in a different (unary) way - take a look at the last line of snippet below:
print(-4 - 4)
print(4. - 8)
print(-1.1)
By the way: there is also a unary + operator, You can use it like this:
print(+2)
The operator preserves the sign of its only argument - the right one.
Although such a construction is syntactically correct, using it doesn't make much sense, and it would be hard to find a good rationale for doing so.
Take a look at the snippet abor - can you guess its output?
Operators and their priorities
So far, we've treated each operator as if it had no connection with the others. Obviously, sich an ideal and simple situation is a rarity in real programming.
Also, you will very often find more than one operator in one expression, and then this presumption is no longer so obvious.
Consider the following expression:
2+3*5
You probably remember from school that Multiplications preed additions.
You surely remember that you should first multiply 3 by 5 and, keeping the 15 in your memory, then add it to 2, thus getting the result of 17.
The phenomenon that causes some operators to act before others is known as the hierarchy of priorities.
Python precisely defines the priorities of all operators, and assumes that operators of a larger (higher) priority perform their operations before the operators of a lower priority.
So, if you know that * has a higher priority that +, the computation of the final result should be obvious.
Operators and their bindings
The binding of the operator determines the order of computations performed by some operators with equal priority, put side by side in one expression.
Most of Python's operators have left-sided binding, which means that the calculation of the expression is conducted from left to right.
This simple example will show you how it works. Take a look:
print(9 % 6 % 2)
There are two possible ways of evaluating this expression:
- from left to right: first 9%6 gives 3, then then 3%2 gives 1
- from right to left: first 6%2 gives 0, then 9%0 causes a fatal error.
Run the example and see what you get.
The result should be 1. This operator has left-sided binding. But there's one interesting exception.
