Creating Functions in Programming

Writing Functions It is possible (and common) to create functions when writing a program Why? Reusability Sharing Reduces errors Write and debug the function once Avoid redundancy Professor John Carelli, Kutztown University

Function Definition Creating a function involves 4 basic parts def keyword This tells Python that what follows is a function definition The function name All functions have a name parameters This is the list of variables that accept information passed to the function from the argument list body The statements that do the work in the function Source: Pearson Education, Inc Professor John Carelli, Kutztown University

Elements of a Function def fname (par1, par2, ): statements [return statement] Functions include: def followed by the function name (required) A list of parameters (optional) As needed, internal statements (indented) If output is desired, a return statement (optional) Professor John Carelli, Kutztown University

Function Parameters def fname (par1, par2, ): statements [return statement] Parameters is a list of variables that accept information from the calling routine (via the arguments) When the function is called, the values of the arguments are passed (assigned) to the parameters in the function in the order given The parameters are then used in the function statements to perform computations Professor John Carelli, Kutztown University

Function Parameters def fname (par1, par2, ): statements [return statement] The return statement of a function serves as its output It can be a variable or an expression The resulting value is passed back to the calling routine Ex: input() function returns what the user typed (as a string) Note: print() function has no return Professor John Carelli, Kutztown University

Function Examples def add(x, y): sum= x + y return sum def hello(name): print( Hello ,name); def getName(): name= input( Enter name ) return name Professor John Carelli, Kutztown University

Function Invocation Functions must be defined before they are called Python issues an error if an undefined function is called Functions are called by: Invoking the function name Providing a input list of arguments The arguments are assigned in the order provided to the defined function parameters z= add(a,b) After this statement is executed, z will contain the sum of a and b (using add() as declared previously) Professor John Carelli, Kutztown University

Local Variables and Scope All variables created in a function are local to the function They don t exist outside the function Including both parameters (in the input list) And variables declared in internal function statements They have local scope Any modifications to a local variable do not affect variables in the calling routine Even if they have the same name! All variables created in the main function (the top level) can be seen and used in functions They have global scope They are global variables Professor John Carelli, Kutztown University

Procedural Programming A programming approach wherein a problem is broken down into sub tasks. Functions are written to address each subtask The overall problem is compartmentalized Each function handles a smaller and simpler task Functions are then called in the main program to solve the overall problem Usually involves writing a top-level main function Professor John Carelli, Kutztown University

main() A top-level function that serves as the entry point for the program Program execution begins in the main() function Functions addressing sub-tasks are called, as needed, from main() General Approach: Define the main() function first in the file Followed by other functions, defined as needed, that are called in main After all functions are defined, call main() Professor John Carelli, Kutztown University

Using __name__ __name__ is a global Python variable __name__ is set to the name of the currently running function If a file contains the following if statement and the file is invoked in Python, __name__ is set to the value __main__ result is that main() gets called if __name__ == '__main__ : main() Professor John Carelli, Kutztown University