Lecture 5 Modular Programming functions part 1 BJ

Lecture 5: Modular Programming (functions – part 1 BJ Furman 27 FEB 2012

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

Modular Programming n Break a large problem into smaller pieces ¨ ¨ Smaller pieces sometimes called ‘modules’ or ‘subroutines’ or ‘procedures’ or functions Why? n Helps manage complexity Smaller blocks of code ¨ Easier to read ¨ n Encourages re-use of code ¨ Within a particular program or across different programs Allows independent development of code n Provides a layer of ‘abstraction’ a = sqrt(9. 0); n

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

Functions - Mathematical View X Function Returned value

Functions - Definition Structure n Function 'header' ¨ ¨ Return data type (if any) Name n ¨ ¨ ¨ statements; } Notice: data type and A function that calculates the product of two numbers name double product(double x, double y) Statements ¨ { Arguments (or parameter list) n n Descriptive type function_name (type arg 1, type arg 2 ) Variable declaration Operations Return value (if any) { double result; result = x * y; return result; }

Functions - Example n Function prototype ¨ Like a variable declaration n n Function definition ¨ ¨ n 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; n n n Tells compiler that the function will be defined later Helps detect program errors Note semicolon!! 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 <stdio. h> /* function prototype */ double product(double x, double y); int main() { double var 1 = 3. 0, var 2 = 5. 0; double ans; ans = product(var 1, var 2); printf("var 1 = %. 2 fn" "var 2 = %. 2 fn", var 1, var 2); printf("var 1*var 2 = %gn", ans); } /* function definition */ double product(double x, double y) { double result; result = x * y; return result; }

Function - Practice 1 n Write a function named 'sum' ¨ ¨ sums two integers returns the sum 2 min. on your own n Share with neighbor n 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 n Use the return type of void ¨ n 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 n Program to print out two happy : ) or sad faces : ( ¨ Continuously prompts for user input: n n n ¨ calls two functions n n n ) for happy face ( for sad face Quits if 'q' or 'Q' entered 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 n Pseudocode 1. 2. Declare and initialize variables WHILE user input not equal to q AND not equal to Q 1) 2) 5) 6) 7) Switch on user input to Case ')‘: call happy_face(); break; Case '(‘: call sad_face(); break; Case ‘q’: Case ‘Q’: break; 8) 9) Case ‘ 0’: Default: re-prompt for user input

Program - Faces code

Structuring C Programs n Modularization ¨ Breaking a program up into smaller pieces: n Instead of: ¨ ¨ n 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 nittygritty details of implementation (. c) n n ¨ ¨ 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 n See C: / Ch / include / math. h ¨ Declaration of constants n ¨ ¨ #define isgreater(x, y) ((x)>(y)) Declaration of global variables (caution!) Function prototypes n ¨ 3. 14159265358979323846 Declaration of macro subsitutions n ¨ #define M_PI extern double sin(double x); Pertinent comments

Review

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

Structure of a C program n Ex. free_fall_d_vs_time. c

C Code for D&D 3. 15 c Programmer’s block Pre-processor directive Main function (statements go between { } ) Declare and initialize variables While loop (repetition structure) return statement

Arithmetic with Integers and Mixed Data Types n Arithmetic with integers ¨ Result is an integer n n 1+1 --> 2 4/2 --> 2 2/4 --> ? BE CAREFUL!!! Arithmetic with mixed data types ¨ Automatic conversion of operand so that data types match n n 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 bytesn ", a+b, sizeof(a+b));

References Modular Programming in C http: //www. icosaedro. it/c-modules. html n math. h http: //www. opengroup. org/onlinepubs/007 908799/xsh/math. h. html n