Python Strings: Indexing, Slicing, and Operations

Indices and Slices

• Position or index of a character in a string is identified with numbers starting from 0.
• To access characters, you use square brackets [].
• s[0] gives the first character which is 'a'.
• s[4] gives the fifth character which is 'd'.
• Trying to access an index that is out of range, like s[5] in the example, results in an error.
• s[0:2] yields 'ab', excluding the character at index 2.
• s[2:] yields 'c d', starting from index 2 to the end of the string.
• This is called "string slicing".

String Operations

• len(str1) returns the number of characters in the string.
• str1.upper() uppercases every alphabetical character in the string.
• str1.lower() lowercases every alphabetical character in the string.
• str1.count('th') returns the number of non-overlapping occurrences of the substring "th".
• "coDE".capitalize() capitalizes the first letter of the string and lowercases the rest to give "Code".
• "beN hur".title() capitalizes the first letter of each word and lowercases the rest, resulting in "Ben Hur".
• "ab ".rstrip() removes spaces from the right side of the string, resulting in "ab".

User Defined Functions

• A function is a group of statements within a program designed to perform a specific task.
• Most programs benefit from being divided into subtasks, each handled by a function.
• The return statement indicates the value returned by the function.
• The return statement is optional, and if a function does not return a value, it is often called a procedure.

Benefits of using Functions

• Simpler code is achieved by using smaller functions, which are easier to read.
• Code reuse is possible, the same function can be called multiple times.
• Duplication of code within a program is reduced.
• Better testing is achieved by testing functions separately to isolate and fix errors.
• Focus only on one function at a time while testing.
• Faster development due to reusing common tasks and functions in other programs.
• Examples of common tasks are security functions, displaying current date and time, and arithmetic operations.
• Teamwork is easier to facilitate when each team member is responsible for writing a specific function.
• This allows you to share the workload.

Void Functions

• Executes the statements it contains and then terminates.

Value-returning Functions

• Executes the statements it contains, and then it returns a value back to the statement that is called it.

Defining a Function

• Function name must follow variable naming rules.
• List of parameters being passed should be in parenthesis and comma separated
• Suite of the function follows the colon
• Keyword indicating the function is being defined is def
• Return statement indicates the value returned when the function finishes

Flow of program

• The program flows in order of the code

Scope of a Variable / Local Variables

• Scope describes the part of the program in which a variable can be accessed.
• A local variable is created inside a function and cannot be accessed by statements outside of that function.
• A local variable is destroyed when the function exits.
• If variables in two different functions have the same name, they are independent.

Passing Arguments to Functions

• Sometimes, one or more pieces of data need to be sent to a Function.
• An argument is any piece of data that is passed into a Function when the Function is called.
• A parameter is a variable that receives an argument that is passed into a Function.
• Multiple arguments can be passed sequentially into a parameter list.

Passing Multiple Arguments

• Python allows writing a function that accepts multiple arguments
• Parameter list replaces single parameter
• Parameter list items separated by comma
• Arguments are passed by position to corresponding parameters
• First parameter receives value of first argument, second parameter receives value of second argument, etc.

Global Variable

• Created by assignment statement written outside all the functions
• Can be accessed by any statement in the program file, including from within a function
• If a function needs to assign a value to the global variable, the global variable must be redeclared within the function
• General format: global variable name

Returning a Value

• The function will return a value to the calling function
• To return a number, you simply create a return val
• To return a string, you create a function to parse the data, and return the required string
• To Return a Boolean, the function contains a conditional statement. Will return true or false depending on the statement.
• The function must be defined before the starting point of your program.
• In Python, a function can return multiple values which is specified after the return statement separated by commas
• Format: return expression1, expression2, etc
• When you call such a function in an assignment statement, you need a separate variable on the left side of the = operator to receive each returned value