Introduction to Computer (C Programming)

Introduction to Computer (C Programming) Chapter 6 User-defined Functions

Roadmap • 6.1 Introduction • 6.2 Definition of functions • 6.3 Function calls • …

6.1 Introduction • C functions are easy to define and use. • We have used functions in every program that we have discussed so far, such as main, printf and scanf. • A function, such as printf, represents a set of programming steps used to perform a useful operation.

6.1.1 Need for Functions • A function is similar to a complete program. • Indeed, the programs you have seen up to now have been written as a function, which happens to have the name main. • The difference between a function and a program lies primarily in who or what makes use of it: • A user run a program. • A function is entirely internal to the program.

6.1.1 Need for Functions cont. • Problem: if we code any program utilizing only main function, the program may become too large and complex. • Solution: if a program is divided into functional parts, then each part may be independently coded and later combined into a single unit.

6.1.1 Need for Functions cont. • Advantages of this “division” approach: • It facilitates top-down modular programming • The length of a source program can be reduced by using functions at appropriate places. • It is easy to locate and isolate a faulty function for further investigations. • A function may be used by many other programs.

6.1.2 Multi-function Program • A function is a self-contained block of code that performs a particular task. • A function can be treated as a ‘black box’ that takes some data from the main program and return value. • The inner details of operation are invisible to the rest of the program. • All the program knows about a function is: What goes in and what comes out.

6.1.2 Multi-function Program cont. • Every C program can be designed using a collection of these black boxes known as functions. • Any function can call any other function. In fact, it can call itself. • A ‘called function’ can also call other functions. • A function can be called more than once. In fact, this is one of the main features of using functions.

6.1.3 Modular Programming • Modular programming is a strategy applied to the design and development of software system. • It is defined as organizing a large program into small, independent program segments called modules. • These modules are separately named and individually callable program units. • These modules are carefully integrated to become a entire software system. In C, each module refers to a function that is responsible for a single task.

6.1.3 Modular Programming cont. • Characteristics of modular programming: • Each module should do only one thing. • Communications between modules is allowed only by calling module. • A module can be called by one and only one higher module. • No communication can take place directly between modules that do not have calling-called relationship. • All modules are designed as single-entry, single-exit systems using control structures.

6.2 Definition of functions • A function definition, also known as function implementation shall include the following elements: • Function name • Function type • List of parameters • Local variable declarations • Function statements • A return statement Function Header Function Body

6.2.1 Function Header • Function header has the following form: return-type name (argument-specifiers) • return-type is the type of value the function returns. • name is the function name. • argument-specifiers is a list, separated by commas, of individual specifications for the argument types. • Example: int getWeekday(int year, int month, int date)

6.2.1.1 Return type • Return type of a function is the type of the function’s return value. • It is assume to be int if not specified. • It is void if return nothing. • It is a good programming practice to code explicitly the return type, even when it is an integer.

6.2.1.2 Function names • Similarities between functions and variables: • Both function names and variable names are considered identifiers. • Like variables, function names and their types must be declared and defined before they are used in a program. • We can define functions and use them like any other variables in C programs.

6.2.1.3 Argument-specifiers • Argument-specifiers forms a parameter List: • It contains the variables that will receive the data sent by the calling program. • It can also be used to send values to the calling programs. • void or empty – no value need to be sent.

6.2.2 Function Body • Function body has the following form: { Local variables declarations Function statements Return statement } Example: int max(int a, int b, int c){ int maxone; if (a>b){ maxone = a>c?a:c; }else{ maxone = b>c?b:c; } return (maxone); }

6.2.2.1 The return statement • If a function returns a result, its body must include at least one return statement, which specifies the value to be return. • The paradigmatic form for the return statement is: • return (expression); Where expression is the value to be returned. • return; If the function has no result.

6.2.2.2 Some notes • When a function reaches its return statement, the control is transferred back to the calling program. • In the absence of a return statement, the closing brace acts as a void return. • A local variable is a variable that is defined and used inside a function.

6.3 Function calls • A function can be called by simply using the function name followed by a list of actual parameters (or arguments). • Example: maxnum = max(3, 4, 5); int max(int a, int b, int c){ int maxone; if (a>b){ maxone = a>c?a:c; }else{ maxone = b>c?b:c; } return (maxone); }

6.3 Function calls cont. • A function which returns a value can be used in expressions like any other variable. • Example: value = max(3, 4, 5)*3; printf(“The max value is %d.”, max(3,4,5)); if(max(a,b,c)==a){…} • A function can not be used on the right side of an assignment statement. • A function that does not return any value can be called in to perform certain tasks specified in the function.

Introduction to Computer (C Programming)