Lecture 5: Modular Programming (functions – part 1

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

2. 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

3. 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’ Modular Programming a = sqrt(9.0);

4. 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

5. Returnedvalue Function X Functions - Mathematical View

6. 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) • typefunction_name (type arg1, type arg2){ • statements; • } A function that calculates the product of two numbers double product(double x, double y) { double result; result = x * y; return result; }

7. Functions - Example #include <stdio.h> /* 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 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 returnexpression; • 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

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

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

10. 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

11. Function - Practice 2 • Steps • Pseudocode for program logic • Function header • return data type (if any) • function name • argument list with data types (if any) • Statements in function definition • variable declaration (if any) • operations • return value • 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 • Pseudocodefirst!! • Divide tasks of writing the two functions

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

13. Program - Faces code

14. 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

15. Example: math.h used in Ch • See C:/ Ch / include / math.h • Declaration of constants • #define M_PI 3.14159265358979323846 • Declaration of macro subsitutions • #define isgreater(x, y) ((x)>(y)) • Declaration of global variables (caution!) • Function prototypes • extern double sin(double x); • Pertinent comments

16. Review

17. 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

18. Structure of a C program • Ex. free_fall_d_vs_time.c

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

20. Arithmetic with Integers and Mixed Data Types • Arithmetic with integers • Result is an integer • 1+1 --> 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));

21. References • Modular Programming in C http://www.icosaedro.it/c-modules.html • math.h http://www.opengroup.org/onlinepubs/007908799/xsh/math.h.html