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

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

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:





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






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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