FUNCTIONS

• Function เป็นชื่อของชุดคำสั่ง • Syntax ของฟังก์ชั่น Def NAME (PARAMETERS ): STATEMENTS • NAME เป็นชื่อของฟังก์ชัน สามารถใช้ชื่อใดๆ ตามกฎการตั้งชื่อ ยกเว้น keyword • มีคำสั่งได้ไม่จำกัดจำนวน แต่ต้องเยื้องเข้าไป จาก def ( ควรใช้เป็นมาตรฐาน อาทิ 4 spaces)

FUNCTION PATTERN • A header line which begins with a keyword and ends with colon. • A body consisting of one or more statements, each indented, each indented the same amount (4 spaces) from the head line. def NAME(PARAMETERS) : STATEMENTS

The parameters of the function • draw_square has two parameter – One to tell the function which turtle to move around – The other to tell it the size of the square

Run the function • Defining a new function does not make the function run. • To run the function, we need a function call

Function Call

Functions can call other functions • We want function to draw a rectangle. • We need to be able to call the function with different argument for width and height. And, unlike the case of the square.

A new version of draw_square • A square is a special kind of rectangle. • We can use draw_rectangle function to draw a square. • Functions can call other functions.

• A caller of this function might say draw_square(tess, 50) • tx and sz are assigned the values of the tess object and the int 50 Function Call

Flow of execution • Execution always begins as the first statement of the program. • Statements are executed one at a time, in order from top to bottom. • The statements inside the function are not executed until the function is called.

Put the mouse cursor on the line of the program where create the turtle screen and press F4 key

At this point press the F7 key repeatedly to single step through the code. When start to get bored,we can use the key F8 to step over the function

Functions that require arguments • Most functions require arguments • Some function takes more than one arguments

Functions that return values • A function that returns a value is called a fruitful function • The opposite of fruitful function is void function : one that is not executed for its resulting value, but is executed because it does something useful.

Void functions • Python always wants to return something. So if the programmer doesn’t arrange to return a value, Python will automatically return the value None.

Variables and parameters are local • When a local variable is created inside a function, it only exists inside the function, we cannot use it outside • Cannot use the variable a outside the function

Variable lifetime • The variable a only exists while the function is being executed.

Parameters • Parameters are local and act like local variables. • The lifetimes of p, r, n, t – begin when final_amt is called – Ends when the function completes its execution.

Each call of the function • Each call of the function creates new local variables. • And their lifetimes expire when the function returns to the caller.