Python Basics

This is the first module (Python Basics) for Python for Data Science, a course by IBM's Cognitive Class Labs.

OUTLINE:

- Your First Program

- Types

- Expressions and Variables

- String Operations

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Your first program

A statement or expression is an instruction the computer will run or execute

Print statement

Simplest program to write

Value (argument) displayed in parentheses

Comment code

Tells others what the code does

# symbol followed by comment

Syntactic error

Python does not understand the code

Semantic error

Logic is wrong

Don’t get an error message

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Types

Type is how Python represents different types of data

Chart: expression | data type

Type command

See the actual data type in Python

int

Integer

Can be negative or positive

float

“float”

real number

Include integers, but also decimals

str

“string”

sequence of characters

bool

Boolean

T - true

F - false

Type-casting

Change the type of expression

Cast int into float:

Cast int into str:

BEWARE: can lose info - Cast float into int:

ERROR: Cast str into int when it does not have an integer value:

Cast bool into int:

Cast int into bool:

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Expressions and Variables

Expression

A type of operation

Basic arithmetic operations

Operand - numbers

Operator - math symbols

//

Double slash

Integer division, the result is rounded

Remember PEMDAS? Ahh, memories...

Variables

Store values

Can change variable

Can store results of expressions

Can perform operations on x and save to a new variable y

Can use the type command on variables

Use meaningful variable names

Can use underscore or capital letter to start a new word

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String Operations

Sequence of characters contained within two quotes

A string is an ordered sequence

Each element of the sequence can be accessed using an index represented by the array of numbers

Negative indexing

Bind a string to another variable

Think of a string as a list or tuple

Collection which is ordered or unchangeable

Treat the string as a sequence and perform sequence operations

Stride

Specifies how many characters to move forward after the first character is retrieved from the string

The third number in the slice command specifies the stride

len

Specify the length of the string

cat

Concatenate

Combine strings using the addition symbols

Replicate values of a string

Multiply string

Cannot change value of the string, but can create new string by setting it to the original variable and concatenated with a new string

Strings are immutable - they can’t be changed once they have been created

\

Beginning of an escape sequence - represents strings that may be difficult to input

\n

New line

\t

Inserts a ‘tab’

\\

Inserts a ‘\’

Can also insert an ‘r’ in front of the string

String Methods

Strings are sequences

Have apply methods that work on lists and tuples

Have a second set of methods that just work on strings

upper

Converts lowercase characters to uppercase

replace

Exchange second argument with the segment

find

Finds sub-strings

Argument is the sub-string you would like to find

Output is -1 if the sub-string is not in the string

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