Programming Forums > Application Development > C++

jamie7823 · Jun 1st, 2008, 11:23 PM

hello everyone, im new to programming and trying to visualize using pointers. when would i ever use them?

Ancient Dragon · Jun 1st, 2008, 11:31 PM

you can NOT write a C or C++ program of any significance without using pointers. And there are thousands of reasons why you might want to use them.

One example is when you need an array whose size is not known when you write the program. Lets say you need to read a text file into memory. When you write the program you will not know how big the text file is so it is not possible to create an array of the correct size. The solution is to use a pointer and allocate the memory once the program knows the size of the file.

That's only one of a trillion possible examples.

jamie7823 · Jun 2nd, 2008, 12:00 AM

Wow,

Im trying to grasp that question and several more at the same time. programming is fun but mentally exhausting. Just when you think you understand a concept, It dosnt compile!

Im messing around trying to write a script and something I am trying to understand is how to get a variable inside func_2 and use it as a variable in main().

 c Syntax (Toggle Plain Text) 
int func_two(x);
 
int main()
{
     cout<<func_two(x);
     return 0;
}
 
func_two(x)
{
     x=2
}
int func_two(x);

int main()
{
     cout<<func_two(x);
     return 0;
}

func_two(x)
{
     x=2
}

mrynit · Jun 2nd, 2008, 12:20 AM

stop what you are and read about the language.

http://www.cplusplus.com/doc/tutorial/

BstrucT · Jun 2nd, 2008, 3:33 AM

Quote:

Originally Posted by mrynit

stop what you are and read about the language.

http://www.cplusplus.com/doc/tutorial/

This is good link for you as I have personally worked through it and have done almost all the examples.

Where might you find pointers, and why use them?
Another typical use to add to the millions of uses for pointers, is that they are used to open and close files for writing and reading purposes. Example from Sams teach yourself c in 21 days:

(Toggle Plain Text)

/* Demonstrates the fprintf() function. */
2:  #include <stdlib.h>
3:  #include <stdio.h>
4:
5:  void clear_kb(void);
6:
7:  main()
8:  {
9:      FILE *fp;
10:     float data[5];
11:     int count;
12:     char filename[20];
13:
14:     puts("Enter 5 floating-point numerical values.");
15:
16:     for (count = 0; count < 5; count++)
17:         scanf("%f", &data[count]);
18:
19:     /* Get the filename and open the file. First clear stdin */
20:     /* of any extra characters. */
21:
22:     clear_kb();
23:
24:     puts("Enter a name for the file.");
25:     gets(filename);
26:
27:     if ( (fp = fopen(filename, "w")) == NULL)
28:     {
29:         fprintf(stderr, "Error opening file %s.", filename);
30:         exit(1);
31:     }
32:
33:     /* Write the numerical data to the file and to stdout. */
34:
35:     for (count = 0; count < 5; count++)
36:     {
37:         fprintf(fp, "\ndata[%d] = %f", count, data[count]);
38:         fprintf(stdout, "\ndata[%d] = %f", count, data[count]);
39:     }
40:     fclose(fp);
41:     printf("\n");
42:     return(0);
43: }
44:
45: void clear_kb(void)
46: /* Clears stdin of any waiting characters. */
47: {
48:     char junk[80];
49:     gets(junk);
50: }

This is just one pointer example, but should show the importance and usefullness of using them correctly.

Please take note that pointers can and will most probably be hard to grasp at first, but don't be put off by it.

>BstrucT

Ancient Dragon · Jun 2nd, 2008, 10:08 AM

Quote:

Originally Posted by jamie7823

Wow,

Im trying to grasp that question and several more at the same time. programming is fun but mentally exhausting. Just when you think you understand a concept, It dosnt compile!

Im messing around trying to write a script and something I am trying to understand is how to get a variable inside func_2 and use it as a variable in main().

 c Syntax (Toggle Plain Text) 
int func_two(x);
 
int main()
{
     cout<<func_two(x);
     return 0;
}
 
func_two(x)
{
     x=2
}
int func_two(x);

int main()
{
     cout<<func_two(x);
     return 0;
}

func_two(x)
{
     x=2
}

The problem with the above code is that you have to declare vairable x in main() then pass a pointer to it to func_two(), like this:

(Toggle Plain Text)

int func_two(int* x);

int main()
{
   int x = 0;
     cout<<func_two(&x);
     return 0;
}

int func_two(int *x)
{
     *x=2
    return *x;
}

grumpy · Jun 2nd, 2008, 4:37 PM

Quote:

Originally Posted by Ancient Dragon

you can NOT write a C or C++ program of any significance without using pointers. And there are thousands of reasons why you might want to use them.

I don't really agree with this.

My first programming language was Fortran 77, and some of the most significant applications I ever wrote were in that language. Fortran 77 does not support any notion equivalent to pointers.

In C, pointers make some substantial things much easier than other techniques, hence the many thousands of reasons you might want to use them.

That is also true in C++ but, thanks to the standard library, it is quite possible to write substantial programs without knowing anything about pointers (the standard library often uses pointers behind the scenes, but that's implementation detail), and without running into the various mistakes that are possible with pointers. Of course, the trade-off is that it also takes time and effort to learn how to use the standard library effectively.

Jun 1st, 2008, 11:23 PM	#1
jamie7823 Newbie Join Date: Jun 2008 Posts: 2 Rep Power: 0	When will i ever use pointers? hello everyone, im new to programming and trying to visualize using pointers. when would i ever use them?

Jun 1st, 2008, 11:31 PM	#2
Ancient Dragon PFO God In Training Join Date: Jun 2005 Location: near St Louis, MO. (USA) Posts: 499 Rep Power: 4	Re: When will i ever use pointers? you can NOT write a C or C++ program of any significance without using pointers. And there are thousands of reasons why you might want to use them. One example is when you need an array whose size is not known when you write the program. Lets say you need to read a text file into memory. When you write the program you will not know how big the text file is so it is not possible to create an array of the correct size. The solution is to use a pointer and allocate the memory once the program knows the size of the file. That's only one of a trillion possible examples. __________________ I Like Ike. Vote for Dwight Eisenhower this November. --This message brought to you by the the Procrastinators Club Of America.

Jun 2nd, 2008, 12:00 AM	#3
jamie7823 Newbie Join Date: Jun 2008 Posts: 2 Rep Power: 0	Re: When will i ever use pointers? Wow, Im trying to grasp that question and several more at the same time. programming is fun but mentally exhausting. Just when you think you understand a concept, It dosnt compile! Im messing around trying to write a script and something I am trying to understand is how to get a variable inside func_2 and use it as a variable in main(). c Syntax (Toggle Plain Text) int func_two(x); int main() { cout<<func_two(x); return 0; } func_two(x) { x=2 } int func_two(x); int main() { cout<<func_two(x); return 0; } func_two(x) { x=2 } Last edited by Ancient Dragon; Jun 2nd, 2008 at 10:01 AM. Reason: add code tags

Jun 2nd, 2008, 12:20 AM	#4
mrynit Hobbyist Programmer Join Date: Mar 2006 Location: olympia,WA Posts: 332 Rep Power: 3	Re: When will i ever use pointers? stop what you are and read about the language. http://www.cplusplus.com/doc/tutorial/ __________________ i dont know much about programming but i try to help

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Programming Forums > Application Development > C++

When will i ever use pointers?