Modularity using Functions Chapter 4

Modularity In programming blocks of code often can be "called up" and reused whenever necessary, for example code to calculate GST and PST, or code to validate inputs or code to convert values (eg from miles to kilometers). A block of reusable code placed in a separate unit is called a function. This reusable code can then be called from another function. main is actually a function itself. Programs become more modular and are easier to maintain through the use of functions.

/* Program to input values between 0-20 for number of large, medium and small coffees sold and calculate total cost of coffees. */ main () { int large, medium, small; double cost; printf ("Enter number of large coffee (0-20): "); scanf ("%d", &large); while (large 20) {printf ("0-20 only. Please try again\n"); scanf ("%d", &large);} printf ("Enter number of medium coffee (0-20): "); scanf ("%d", &medium); while (medium 20) {printf ("0-20 only. Please try again\n"); scanf ("%d", &medium);} printf ("Enter number of small coffee (0-20): "); scanf ("%d", &small); while (small 20) {printf ("0-20 only. Please try again\n"); scanf ("%d", &small); } cost = (large * 1.80) + (medium * 1.50) + (small * 1.10); printf ("The cost for coffee is: $%.2lf\n", cost);}

int GetNumber (char size); /* function prototype */ main () {int large, medium, small; double cost; large = GetNumber('L'); /* function call */ medium = GetNumber('M'); /* function call */ small = GetNumber('S'); /* function call */ cost = (large * 1.80) + (medium * 1.50) + (small * 1.10); printf ("The cost for coffee is: $%.2lf\n", cost); } int GetNumber (char size) /* function heading */ {int num; /* local variable */ printf ("Enter number of %c coffee (0-20): ", size); scanf ("%d", &num); while (num 20) {printf ("0-20 only. Try again. # of %c coffee:\n",size); scanf ("%d", &num); } return num; /* return num to main */ } /* Program to input values between 0-20 for number of large, medium and small coffees sold and calculate total cost of coffees. Uses function called GetNumber to validate input is between 0-20 */

Benefits of Modularity Less coding required  fewer syntax and logic errors Easier to test Easier to understand Function can be reused by other programs

Functions There are 4 elements of a function: function heading function body function call function prototype

Function Heading A function heading defines three things: 1)the type of value to be returned from the function 2)the name of the function (the same rules apply here as for naming variables) 3)the parameter list (the type and name of the values to be passed into the function) Example: int GetNumber (char size)

Function Body The function body is placed just after the function heading. Function body is enclosed in brace brackets and contains the code for the function task. A function may declare some local variables. A function may return only a single value of the type defined in the function heading. Example: int num; /* local variable */ printf ("Enter number of %c coffee (0-20): ", size); scanf ("%d", &num); while (num 20) {printf ("0-20 only. Please try again\n"); scanf ("%d", &num); } return num;

Function Call A function can be called from main or from another function. To call a function, use the name of the function and a list of arguments (separated by commas) that are being sent to the function. The arguments in the function call must match the data type and order of the arguments shown in the parameter list of the function heading. If the function returns a value which you want to store, then you would use an assignment statement when calling the function.For example: large = GetNumber('L');

Function Prototype Functions may be written above main but most programs place all functions below main so it is easy to find the first line that will be executed in the program (execution always begins with the first line in main). When functions are placed below main, a function prototype must appear above main. The function prototype allows C to find and report any illegal data type conversions (for example, sending a double to a function where an integer is expected). Think of the function prototype as the function declaration Example: int GetNumber (char size);

Modularity Guidelines Each function should perform a single task (for example: calculate the volume, or ask the user to input a mark, or print an invoice) and should be given a name that reflects that task (for example: CalcVolume or GetMark or PrintInvoice) Each function should be a manageable length (one page or one screen in length as a guide). Note: this also includes the main function. The number of parameters in a function heading should be reasonable (5 to 10 as a guide) If one function is too long, too complex, or has too many parameters, perhaps you need to subdivide it into several functions.