Lecture 5: Modular Programming (functions – part 1 BJ Furman 27FEB2012

Learning Objectives Explain the concept of modular program design Explain the concept of a function in C Explain why functions are important in programming Explain the structure of a function  Return data type  Parameters Apply the concept of a function to a practical problem Explain how larger C programs should be structured using.h and.c files

Observations so far Starting to get the hang of it!  Don’t give up if you are still fuzzy on some of the concepts! See Prof. Furman or lab instructor. Don’t slack off on the steps in the process of developing a program:  Define the problem  Determine requirements/specifications  Define inputs and outputs  Develop the ‘logic’ of the program using pseudocode and/or a flowchart  Develop the C code (including comments!!!)  Test the program (consider full range of possible inputs)

Modular Programming Break a large problem into smaller pieces  Smaller pieces sometimes called ‘modules’ or ‘subroutines’ or ‘procedures’ or functions  Why? Helps manage complexity  Smaller blocks of code  Easier to read Encourages re-use of code  Within a particular program or across different programs Allows independent development of code Provides a layer of ‘abstraction’ a = sqrt(9.0);

Functions The ‘building blocks’ of a C program  You’ve used predefined functions already: main() printf(), scanf(), pow()  User-defined functions Your own code In combination with predefined functions

Functions - Mathematical View X Function Returned value

Functions - Definition Structure Function 'header'  Return data type (if any)  Name Descriptive  Arguments (or parameter list) Notice: data type and name Statements  Variable declaration  Operations  Return value (if any) type function_name (type arg1, type arg2 ) { statements; } double product(double x, double y) { double result; result = x * y; return result; } A function that calculates the product of two numbers

Functions - Example Function prototype  Like a variable declaration Tells compiler that the function will be defined later Helps detect program errors Note semicolon!! Function definition  See previous slide  Note, NO semicolon Function return  return statement terminates execution of the current function  Control returns to the calling function  if return expression; then value of expression is returned as the value of the function call Only one value can be returned this way Function call  main() is the 'calling function'  product() is the 'called function'  Control transferred to the function code  Code in function definition is executed #include /* function prototype */ double product(double x, double y); int main() { double var1 = 3.0, var2 = 5.0; double ans; ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans); } /* function definition */ double product(double x, double y) { double result; result = x * y; return result; }

Function - Practice 1 Write a function named 'sum'  sums two integers  returns the sum 2 min. on your own Share with neighbor Steps 1.Function header return data type function name argument list with data types 2.Statements in function definition variable declaration operations return value

Function - sum() int sum_int(int x, int y) { int result; result = x + y; return result; }

Functions that do not return a value Use the return type of void  void my_fun( arg_list,…) Practice  Write two functions, the first prints out first name, and the second prints out last name

Function - Practice 2 Program to print out two happy :) :) or sad faces :( :(  Continuously prompts for user input: ) for happy face ( for sad face Quits if 'q' or 'Q' entered  calls two functions happy_face() sad_face() Work in pairs Pseudocode first!! Divide tasks of writing the two functions Steps 1.Pseudocode for program logic 2.Function header return data type (if any) function name argument list with data types (if any) 3.Statements in function definition variable declaration (if any) operations return value

Program - Faces logic Pseudocode 1. Declare and initialize variables 2. WHILE user input not equal to q AND not equal to Q 1) Switch on user input to 2) Case ')‘: call happy_face(); break; 5) Case '(‘: call sad_face(); break; 6) Case ‘q’: 7) Case ‘Q’: break; 8) Case ‘0’: 9) Default: re-prompt for user input

Program - Faces code

Structuring C Programs Modularization  Breaking a program up into smaller pieces: Instead of:  one_big_program.c  break into groupings of header files (.h) and source code (.c) files:  module_1.h  module_1.c  etc. Rationale  separates the user-interface description (.h) from the nitty- gritty details of implementation (.c) The Application Programming Interface (API), the.h file, is distinct from the implementation, the.c file (which may already be compiled and not readily viewed) Example: math.h from Ch  can construct and test modules independently  promotes re-use of code

Example: math.h used in Ch See C:/ Ch / include / math.h  Declaration of constants #define M_PI  Declaration of macro subsitutions #define isgreater(x, y) ((x)>(y))  Declaration of global variables (caution!)  Function prototypes extern double sin(double x);  Pertinent comments

Review

Structured Programming All programs can be written using these control structures:  Sequence  Decision (three structures) IF IF-ELSE SWITCH  Repetition (three structures) WHILE DO-WHILE FOR

Structure of a C program Ex. free_fall_d_vs_time.c

C Code for D&D 3.15c Programmer’s block Pre-processor directive Declare and initialize variables While loop (repetition structure) Main function (statements go between { } ) return statement

Arithmetic with Integers and Mixed Data Types Arithmetic with integers  Result is an integer > 2 4/2 --> 2 2/4 --> ?BE CAREFUL!!! Arithmetic with mixed data types  Automatic conversion of operand so that data types match Conversion is ‘upward’ in the sizeof() sense Example in Ch char a = 7; sizeof(a); double b=3; sizeof(b); printf("a+b == %lf and needs %d bytes\n ", a+b, sizeof(a+b));

References Modular Programming in C math.h /xsh/math.h.html /xsh/math.h.html