Week 2, Introduction to Functions

7/30/2019 Week 2, Introduction to Functions

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Introduction to FunctionsCS-2301, B-Term 2009 1

Introduction to Functions

CS-2301, System Programming

for Non-Majors

(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)


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DefinitionFunction

A fragment of code that accepts zero or

more argument values, produces a result

value, and has zero or more side effects.

A method ofencapsulating a subset of a

program or a system

To hide details

To be invoked from multiple places

To share with others


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Functionsa big Topic

Examples

Function definition

Function prototypes & Header files Pre- and post-conditions

Scope and storage class

Implementation of functions Recursive functions


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Common Functions

#include

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

printf()

fprintf()

scanf()

sscanf()

...

#include

strcpy()

strcat() strcmp()

strlen()

...


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Common Functions (continued)

In Kernighan & Ritchie// for diagnostics, loop invariants, etc.

// for parsing arguments

// time of day and elapsed time// implementation dependent numbers

// implementation dependent numbers.

// beyond scope of this course// beyond scope of this course


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Common Functions (continued)

See also theman pages of your system forthings like

// concurrent execution

// 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!


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Functions in C

resultType functionName(type1param1, type2

param2, ) {

body

}

If no result, resultType should be void Warning if not!

If no parameters, use voidbetween ()


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Functions in C

resultType functionName(type1param1, type2

param2, ) {

body

} // functionName

If no result, resultType should be void Warning if not!

If no parameters, use voidbetween ()


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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 anyexpression where a value of type int can be

usede.g.,

N = f(pi*pow(r,2), b+c) + d;


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usede.g.,

N = f(pi*pow(r,2), b+c) + d;

Using Functions (continued)This is aparameter

This is an argumentThis is also an

argument


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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


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usede.g.,

N = f(pi*pow(r,2), b+c) + d;

Using Functions (continued)


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usede.g.,

N = f(pi*pow(r,2), b+c) + d;

Using Functions (continued)

Function f is executed and

returns a value of type intResult off is added to dSum is assigned toN


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Questions?


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Function Definition

Every function definition has the formreturn-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


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Note

Functions in Cdo notallow other functionsto be declared within them

Like C++, Java

Unlike Algol, Pascal

All functions defined at top level ofCprograms

(Usually) visible to linker Can be linked by any other program that knows the

function prototype


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Examples

double sin(double radians) {

} // sin

unsigned int strlen (char *s) {

} // strlen


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Note onprintf, etc.

int printf(char *s, ...) {

body

} // printf

In this function header, is nota professorsplace-holder

(as often used in these slides)

but an actual sequence of three dots (no spacesbetween)

Meaning:the number and types of arguments is indeterminate

Useto extract the arguments


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Questions?


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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 Cprogram

In one or more completely separate Cprograms

This is actually the normal case!

Therefore, we need some way to declare afunction separate from defining its body.

Called a Function Prototype


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Function Prototypes (continued)

Definition:a Function Prototype in Cis a

language construct of 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


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Purposes of Function Prototype

So compiler knows how to compile calls to thatfunction, i.e., number and types of arguments

type of result

As part of a contract between developer andprogrammer who uses the function

As part of hiding details ofhow it works and

exposing whatit does.

A function serves as a black box.


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Header files

In applications with multiple Cprograms,

function prototypes are typically provided

in header files

I.e., the .h files that programmers include in theircode

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


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#include

#include

Search the systems directories in orderfor 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 isfound first, before-I and system directories


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Typical CProgramming Style

A lot of small Cprograms, rather than a fewlarge ones

Each .c file contains closely related functions

Usually a small number of functions

Header files to tie them together

Makefiles to build or rebuild them in anorganized way

Later in the term


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DefinitionStub

A stub is a dummy implementation of a

function with an empty body

A placeholder while building (other parts

of) a program

So that it compiles correctly

Fill in one-stub at a time Compile and test if possible


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Questions?


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Contract between Developer and User of a

Function

1. Function Prototype

2. Thepre- andpost-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 functiondoes


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Definitions

Pre-condition:

a characterization or logicalstatement about

the values of the parameters, and

values of relevant variables outside the function

prior to calling the function

Post-condition:a logical statement orcharacterization about

the resultof the function in relation to the values of theparameters and pre-conditions, and

changes to values of variables outside the function

after the function returns


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Example 1

double sin (double angle);

Pre:angle is expressed in radians

Post:result is the familiar sine ofangle

Note: this function does not use or change anyother variables


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Example 2

int printf (string, arg1, arg2, )

Pre:string terminated with '\0' and containing

conversion specifiers

Pre:a buffermaintained by the file system contains

zero or more unprinted characters from previous calls.

Post:args are substituted for conversion codes in

copy ofstring; 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:resultis number of characters added to buffer byprintf


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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


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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 allprevious inputs, or zero if none

Pre:count is number of

previous inputs, or zero if none

Post:if valid input is received

total = totalprev + input,count = countprev + 1


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Important

Pre- andpost-conditions are analogous to

loop invariants

I.e., they describe something about the data before

and after a function is called and the relationshipthat the function preserves

Often are used together with loop invariants to show that loop invariant is preserved from one

iteration to the next


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Questions?

Documents

Week 2, Introduction to Functions