C Pointers 2013 Author Riko H i

Transcription

1 C Pointers 2013 Author Riko H i

2 Copyright 2013 CDorm.net All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, without the prior written permission of the publisher. Disclaimer All the material contained in this book is provided for educational and informational purposes only. No responsibility can be taken for any results or outcomes resulting from the use of this material. While every attempt has been made to provide information that is both accurate and effective, the author does not assume any responsibility for the accuracy or use/misuse of this information. C Pointers 2013 Author Riko H ii

3 Acknowledgements I would like to thank my father who kept pushing and encouraging me to make this book, for believing I have what it takes to make it real and helping to make this project possible, taking it from an idea to a realized e-book. Thank you, for without you this would not be possible. C Pointers 2013 Author Riko H iii

4 Table of Contents Chapter 1: C Pointers Basic Concept Chapter 2: Using Pointers in C Chapter 3: Arrays Chapter 4: Functions and Function Argument C Pointers 2013 Author Riko H iv

5 Chapter 1: C Pointers basic concepts Welcome to the world of pointers! If you are reading this then that means you probably have some trouble with pointers or maybe just want some extra information on the topic. Let s begin! If we are planning to dive into this vast world we first need to talk about a couple things. For starters we must understand what exactly a pointer is and how does it function in principle. You might be tempted to or have already searched the internet high and low for a quick, simple and easy definition of a pointer and came up with something like: a pointer is a programming language data type whose value refers directly to another value stored elsewhere in the computer memory using its address. It might then go into referencing registers and using all kinds of complicated analogies to explain the concept leaving you more confused than when you started reading. Now, time for that simple explanation you were waiting for. The first question anyone may ask is: What is a pointer and how do they work? Well, a pointer in programming is simply a reference to an actual variable. How do they work? I shall explain the concept two ways: First, let s imagine we have a mailman, some mail and a house for the mail to be delivered. We can look at the data as the mail, the variable as the house and the pointer as the mailman. What does a mailman do? You guessed it! Deliver! If you have a package to be delivered (i.e. data to be stored) you give the address of the house to the mailman, then you give the mailman the package, however, you expect that if you access/look into the house you would see the package. C Pointers 2013 Author Riko H Page 1

6 Similarly, when you use pointers to manipulate data you give data to the pointer but when the variable it is referencing is accessed it too shall have the data because the pointer (just like the mailman) is the gateway to the variable. If that concept didn t clear up anything for you let s look at it another way: Let s say we have a funnel, a marble and a bottle. The bottle is our variable, the marble our data and the funnel as our pointer. If we were to insert the funnel to our bottle and put the marble in the funnel what would you expect to happen? The marble would fall in the bottle. What if you look through the funnel or look directly in the bottle, what would you see? The marble. Similarly, when you use pointers to manipulate data you give data to the pointer but when the variable it is referencing is accessed it too shall have the data because the pointer (just like the funnel) is the gateway to the variable. If it sounds trivial or really simple that s because it isn t complicated once you have the right imagination. This brings up a really important point about pointers (no pun intended). As with the second analogy, you ll notice that the data (marble) is actually in the variable (bottle) and not in the pointer (funnel) itself. That s because pointers don t actually store data, they just tell the data where to go. That s really it guys, that s how simple the concept of a pointer is. C Pointers 2013 Author Riko H Page 2

7 Chapter 2: Using pointers in C Now that we understand how a pointer works let s get into using them because it s no use just knowing how being able to visualize how they work we have to be able to use them at our will. Declaring First, how do you declare a pointer? In C, the asterisk denotes a pointer. (type *variable) NB: the type of the pointer must match the type of data you plan to reference. float *p; This is a pointer float x; This is NOT a pointer. It s as simple as that. Using pointers After declaring but before you can start using a pointer, you must reference a variable (give the mailman the address of the house. This is done by using the & operation. p = &x; Notice when assigning address asterisk isn t used Only after assignment can manipulation can begin. C Pointers 2013 Author Riko H Page 3

8 Example #include <stdio.h> int main (void) { int *ptr; /*DECLARING POINTER*/ int x; ptr = &x; /*ASSIGNING POINTER TO X VARIABLE*/ *ptr = 5; printf(" %d ", *ptr); /*PRINTING POINTER*/ x = 3; printf(" %d ", x); /*PRINTING VARIABLE*/ printf(" %d ", *ptr); /*PRINTING POINTER*/ return (0); } What is the output of this program? Continue reading to find out! C Pointers 2013 Author Riko H Page 4

9 The output would be: We only set the pointer value to 5 once, do you understand why 3 occurred when we printed the pointer a second time? Remember the funnel and marble analogy? Lets recall that a pointer is simply a funnel connected to our variable so even though we didn t directly drop a 3 in the pointer we did drop it in the varibale ( x = 3; ) so when we looked in the funnel the second time we simply saw that 3. C Pointers 2013 Author Riko H Page 5

10 Let s try one more: #include <stdio.h> int main (void) { int *ptr1; int *ptr2; int x,y; ptr1 = &x; *ptr1 = 3; ptr2 = &y; y = 2; printf(" %d ", *ptr1 + *ptr2); printf(" %d ", x + y); printf(" %d ", x + *ptr1); ptr1 = &y; x = 10; printf(" %d ", *ptr1 + *ptr2); return (0);} What s the output this time? Take your time and check the next page when you ve got your answers! C Pointers 2013 Author Riko H Page 6

11 Output: Let s look into this one in detail: Let Line 1 be int *ptr1; Lines 1-3 are standard declaration commands. Line 4 connects the first pointer (*ptr1) to x. Line 5 then sends the number 3 to x via ptr1 Line 6 is the same as Line 4 this time assigning ptr2 to y. Instead of going via the pointer (*ptr2), Line 7 directly inserts 2 into y. Now comes the complicated part: printf(" %d ", *ptr1 + *ptr2); *ptr1 is simply x and *ptr2 is simply y therefore this line is printing x + y which is which is 5. Next, printf(" %d ", x + y); x + y is the same as *ptr1 + *ptr2 because ptr1 is referencing x and ptr2 is referencing y and we established from before to be 5. printf(" %d ", x + *ptr1); *ptr1 is pointing to x so x + *ptr1 is simply x + x which is 6. C Pointers 2013 Author Riko H Page 7

12 What do the next two lines do? ptr1 = &y; is nothing new to us by now all this is doing to taking the funnel out of bottle X and placing it in bottle Y. Therefore the line printf(" %d ", *ptr1 + *ptr2); simply prints the value of *ptr1 + *ptr2 which is the same as y + y since both pointers are now pointing towards the same variable y. C Pointers 2013 Author Riko H Page 8

13 Chapter 3: Arrays Now that we had covered pointers and are now ready to tackle problems involving them let s shift our attention and focus to another area in programming known as Arrays. Simple data types use a single cell to store a variable. As we dive into the world of programming we will see it is more efficient to group data items together than to give each variable its own cell. This group is called an array. Declaring and using an Array and its cells Declaring an array is as simple as declaring any other thing in C. We simply use the format: type variable[size] int my_array[3]; This says I want three integer variable for my using. If you are curious like I am you may be wondering where are the 3; they are: my_array[0] Notice that we start from 0 my_array[1] my_array[2]. We now have access to these three variables to do with what we wish simply by declaring the array. These cells can now be treated as normal variables and can be manipulated as such. This comes in handy in cases where you have lots of data to store. For instance is you have 900 numbers to store would rather have to declare 900 variables or would you rather type float numbers[900]; There are also extremely fast and efficient ways to fill and use arrays which we will get into in a later issue. Imagine filling all 900 of those slots with one line of code (hint: loops). C Pointers 2013 Author Riko H Page 9

14 Example: #include <stdio.h> int main (void) { int my_array[3]; my_array[1] = 7; my_array[2] = 4; my_array[0] = my_array[1] - my_array[2]; printf( " %d ", my_array[0] ); printf( " %d ", my_array[1] ); printf( " %d ", my_array[2] ); return (0); } What s the output? C Pointers 2013 Author Riko H Page 10

15 Chapter 4: Functions and Function Arguments Pointers? Check. Arrays? Check. Now that we have a clear understand on those, time to move on to Functions and Function Arguments. A function is nothing more than a program within a program. A dedicated set of code which when called upon carries out the objective of its code, this could be anything from an addition function which when called adds 4 numbers then multiplies it by 2 or a factorial function which when called takes and displays the factorial of a given number. It could even be a function which when called does nothing. The possibilities are endless when it comes to function because just like how they are trillions of programs that can be coded they are trillions of functions. Why? Because they ARE programs and you, the coder, decide what they do and how they do it. With that said, all I can do is show you how to set it up and you and your imagination and expertise learnt from this e-book and elsewhere can fill the inside with whatever you want! Declaring a function Declaring a function is not like declaring a pointer or array. As matter of fact, they aren t even declared in the same place. Declaring a function goes like this: #include <stdio.h> type name(arguments) We shall discuss arguments later. { /*FUNCTION CODE GOES HERE* } int main() {. return(0); } C Pointers 2013 Author Riko H Page 11

16 Types of functions Value functions These functions give back a value when called and therefore need to be stored when called. e.g. int addition (int num1,int num2) { return(num1+num2); } e.g. float multiply (int num1, int num2) { int x=2; return (num1*num2*x); } Void functions These functions return nothing and therefore do not need to be stored when called e.g. void print() { printf ( This function does nothing other than print this message ) } Notice there is no return command. C Pointers 2013 Author Riko H Page 12

17 Using a function After we declare a function along with the code of what it s going to do we now have to use it. Using a function is referred to as calling it and it is done in the main program and it is as simple and easy as calling a taxi. To call a value function we must first have a variable (must be same date type as function and is declared the standard way) to store the output of this function (i.e. the function is giving us something so we must have somewhere to put it). x = addition (4, 2); we have just called our addition function and the result shall be stored in x. For void functions, because it is not giving anything, we do not need to put it anywhere and can just call by typing its name: print(); Function Arguments Arguments is just a fancy word for inputs. So function arguments just means function inputs. As in, what data do you need (if any) from the main program for your function to operate? For instance if we were to make an addition function that adds two numbers, logically we would need the two numbers to add so our function arguments for example would be (int num1, int num2). They are declared in the heading of the function and do not need to be re-declared in the body. C Pointers 2013 Author Riko H Page 13

18 Example #include <stdio.h> int add (int x, int y) { return (x+y); } void print() { printf("our first void function! "); } int main (void) { int result; print(); printf("i wonder what is 2+3? "); result = add(2,3); printf(" %d ",result); return (0); } Can you tell me what the output is? Be careful! Let s see on the next page. C Pointers 2013 Author Riko H Page 14

19 Output Well folks, That s it basically Understanding C Pointers in a nutshell. I hope you like this book and I know it will be a little pocket guide to C Programming and I will be putting out some more short reports like this one to help struggling Student because I know I was there once. Shoot me an riko@cdorm.net or visit the site for more in the future. Until then. :>) C Pointers 2013 Author Riko H Page 15

20 Personal Message from Riko: I hope you find this ebook useful and that it helps you to create your own highly successful C programs! Feel free to check out my website and see the free video and share the link with anyone who might be interested. Free Video: For more free training on C Programming visit my website: click here C Pointers 2013 Author Riko H Page 16