Introduction to Functions

Introduction to Functions CS-2301 System Programming C-term 2009 (Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel) Introduction to Functions

Definition – Function A fragment of code that accepts zero or more argument values, produces a resultvalue, and has zero or more side effects. A method of encapsulating a subset of a program or a system To hide details To be invoked from multiple places To share with others Introduction to Functions

Functions – a big Topic Examples Function definition Function prototypes & Header files Pre- and post-conditions Scope and storage class Implementation of functions Recursive functions Introduction to Functions

Common Functions #include <math.h> sin(x) // radians cos(x) // radians tan(x) // radians atan(x) atan2(y,x) exp(x) // ex log(x) // logex log10(x) // log10x sqrt(x) // x  0 pow(x, y) // xy ... #include <stdio.h> printf() fprintf() scanf() sscanf() ... #include <string.h> strcpy() strcat() strcmp() strlen() ... Introduction to Functions

Common Functions (continued) In Kernighan & Ritchie <assert.h> // for diagnostics, loop invariants, etc. <stdarg.h> // for parsing arguments <time.h> // time of day and elapsed time <limits.h> // implementation dependent numbers <float.h> // implementation dependent numbers. <setjmp.h>// beyond scope of this course <signal.h>// beyond scope of this course Introduction to Functions

Common Functions (continued) See also the man pages of your system for things like <pthread.h> // concurrent execution <socket.h> // network communications ... // many, many other facilities Fundamental Rule: if there is a chance that someone else had same problem as you, … … there is probably a package of functions to solve it! Introduction to Functions

Functions in C resultType functionName(type1param1, type2param2, …) { … body … } If no result, resultType should be void Warning if not! If no parameters, use void between () Introduction to Functions

Functions in C resultType functionName(type1param1, type2param2, …) { … body … } // functionName If no result, resultType should be void Warning if not! If no parameters, use void between () It is good style to always end a function with a comment showing its name Introduction to Functions

Using Functions • Let int f(double x, int a) be (the beginning of) a declaration of a function. • Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g., N = f(pi*pow(r,2), b+c) + d; Introduction to Functions

This is a parameter This is also an argument This is an argument Using Functions (continued) • Let int f(double x, int a) be (the beginning of) a declaration of a function. • Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g., N = f(pi*pow(r,2), b+c) + d; Introduction to Functions

Definitions • Parameter:–a declaration of an identifier within the '()' of a function declaration • Used within the body of the function as a variable of that function • Initialized to the value of the corresponding argument. • Argument:– an expression passed when a function is called; becomes the initial value of the corresponding parameter Introduction to Functions

The second argument expression is evaluated, converted to int, and assigned to parameter a The first argument expression is evaluated, converted to double, and assigned to parameter x Using Functions (continued) • Let int f(double x, int a) be (the beginning of) a declaration of a function. • Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g., N = f(pi*pow(r,2), b+c) + d; Introduction to Functions

Function f is executed and returns a value of type int Result of f is added to d Sum is assigned to N Using Functions (continued) • Let int f(double x, int a) be (the beginning of) a declaration of a function. • Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g., N = f(pi*pow(r,2), b+c) + d; Introduction to Functions

Questions? Introduction to Functions

Function Definition Every function definition has the form return-type function-name (parameter declarations){ definitions and statements } See top of page 70 in Kernighan & Ritchie For practical purposes, code between {}(inclusive) is a compound statement Introduction to Functions

Note • Functions in C do not allow other functions to be declared within them • Like C++, Java • Unlike Algol, Pascal • All functions defined at “top level” of C programs • (Usually) visible to linker • Can be linked by any other program that knows the function prototype Introduction to Functions

Examples double sin(double radians) { …} // sin unsigned int strlen (char *s) { …} // strlen Introduction to Functions

Note on printf, etc. int printf(char *s, ...) {body} // printf In this function header, “…” is not a professor’s place-holder (as often used in these slides) …but an actual sequence of three dots (no spaces between) Meaning:– the number and types of arguments is indeterminate Use <stdarg.h> to extract the arguments Introduction to Functions

Function Prototypes There are many, many situations in which a function must be used separate from where it is defined – before its definition in the same C program In one or more completely separate C programs This is actually the normal case! Therefore, we need some way to declare a function separate from defining its body. Called a Function Prototype Introduction to Functions

Function Prototypes (continued) Definition:– a Function Prototype is a language construct in C with the form:– return-type function-name (parameter declarations) ; Introduction to Functions

Function Prototypes (continued) Definition:– a Function Prototype is a language construct in C with the form:– return-type function-name (parameter declarations) ; I.e., exactly like a function definition, except with a ';' instead of a body in curly brackets Introduction to Functions

Purposes of Function Prototype So compiler knows how to compile calls to that function, i.e., number and types of arguments type of result As part of a “contract” between developer and programmer who uses the function As part of hiding details of how it works and exposing what it does. A function serves as a “black box.” Introduction to Functions

Header files • In applications with multiple C programs, function prototypes are typically provided in header files • I.e., the ‘.h’ files that programmers include in their code • Grouped by related functions and features • To make it easier for developers to understand • To make it easier for team development • To make a package that can be used by someone else Introduction to Functions

#include • #include <foo.h> • Search the system’s directories in order for a file of the name foo.h • Directories can be added with ‘-I’ switch to gcc command • E.g., gcc –I myProject/include foo.c • Precedes system directories in search order • #include "foo.h" • Search the directory where the source program is found first, before-I and system directories Introduction to Functions

Typical C Programming Style • A lot of small C programs, rather than a few large ones • Header files to tie them together • Makefiles to build or rebuild them in an organized way • Later in the term Introduction to Functions

Definition – Stub • A stub is a dummy implementation of a function with an empty body • A placeholder while building a program • So that it compiles correctly • Fill in one-stub at a time • Compile and test if possible Introduction to Functions

“Contract” between Developer and User of a Function Function Prototype The pre- and post-conditions I.e., assertions about what is true before the function is called and what is true after it returns. A logical way of explaining what the function does Introduction to Functions

Definitions Pre-condition:–a characterization or logical statement about the values of the parameters, and values of relevant variables outside the function prior to calling the function Post-condition:–a logical statement or characterization about the result of the function in relation to the values of the parameters and pre-conditions, and changes to values of variables outside the function after the function returns Introduction to Functions

Example 1 double sin (double angle); Pre:–angle is expressed in radians Post:– result is the familiar sine of angle Note: this function does not use or change any other variables Introduction to Functions

Example 2 int printf (string, arg1, arg2, …) Pre:–string terminated with '\0' and containing conversion specifiers Pre:– a buffer maintained by the file system contains zero or more unprinted characters from previous calls. Post:– args are substituted for conversion codes in copy of string; resulting string is added to buffer Post:– if '\n' is anywhere in buffer, line is “printed” up to '\n'; printed characters are cleared from buffer Post:–result is number of characters added to buffer by printf Introduction to Functions

Example 3 float total = 0; int count = 0; int GetNewItem(void) {float input;int rc;printf("Enter next item:- "); if ((rc = scanf("%f", &input)) != EOF && (rc > 0)) { total += input; count++;}; // if return rc; } // GetNewItem Introduction to Functions

Example 3 float total = 0; int count = 0; int GetItem(void) {float input;int rc;...; if ((rc = scanf(“%f”, &input)) != EOF && (rc > 0)) { total += input; count++;}; // if return rc; } // GetItem Pre:– total is sum of all previous inputs, or zero if none Pre:– count is number of previous inputs, or zero if none Post:– if valid input is receivedtotal = totalprev + input, count = countprev + 1 Introduction to Functions

Important Pre- and post-conditions are analogous to loop invariants I.e., they describe something about the data before and after a function is called and the relationship that the function preserves Often are used together with loop invariants … to show that loop invariant is preserved from one iteration to the next Introduction to Functions

Introduction to Functions