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programming: the fundamentals

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Python as a Calculator

Arithmetic Operators

Operator Operation Expression English description Result
+ addition 11 + 56 11 plus 56 67
- subtraction 23 - 52 23 minus 52 -29
* multiplication 4 * 5 4 multiplied by 5 20
** exponentiation 2 ** 5 2 to the power of 5 32
/ division 9 / 2 9 divided by 2 4.5
// integer division 9 // 2 9 divided by 2 4
% modulo (remainder) 9 % 2 9 mod 2 1

Types int and float

A type is a set of values and operations that can be performed on those values.

Two of Python's numeric types:

int: integer
For example: 3, 4, 894, 0, -3, -18

float: floating point number (an approximation to a real number)
For example: 5.6, 7.342, 53452.0, 0.0, -89.34, -9.5

Arithmetic Operator Precedence

When multiple operators are combined in a single expression, the operations are evaluated in order of precedence.

Operator Precedence
** highest
- (negation)
*, /, /, %
+ (addition), - (subtraction) lowest
Syntax and Semantics

Syntax: the rules that describe valid combinations of Python symbols

Semantics: the meaning of a combination of Python symbols is the meaning of an instruction — what a particular combination of symbols does when you execute it.

Errors

A syntax error occurs when we an instruction with invalid syntax is executed. For example:

>>> 3) + 2 * 4
SyntaxError: invalid syntax
A semantic error occurs when an instruction with invalid semantics is executed. For example:

>>> 89.4 / 0
Traceback (most recent call last):
File "", line 1, in
89.4 / 0
ZeroDivisionError: float division by zero

Python and Computer Memory

Computer Memory

For the purpose of this course, you may think of computer memory as a long list of storage locations where each location is identified with a unique number and each location houses a value. This unique number is called a memory address. Typically, we will write memory addresses as a number with an "id" as a prefix to distinguish them from other numbers (for example, id201 is memory address 201).

Variables are a way to keep track of values stored in computer memory. A variable is a named location in computer memory. Python keeps variables in a separate list from values. A variable will contain a memory address, and that memory address contains the value. The variable then refers to the value. Python will pick the memory addresses for you.

Terminology

A value has a memory address.
A variable contains a memory address.
A variable refers to a value.
A variable points to a value.
Example: Value 8.5 has memory address id34.
Variable shoe_size contains memory address id34.
The value of shoe_size is 8.5.
shoe_size refers to value 8.5.
shoe_size points to value 8.5.

@темы: coursera

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