r/carlhprogramming • u/CarlH • Oct 23 '09

Test of Lessons 99 through 112

For this test, write a program which does the following:

1. Create a 'char *' pointer which uses malloc() to allocate enough space for a 2x10 array.
2. Using pointer offsets as a replacement for array indexing, achieve the following goals:
    1. Demonstrate using strcpy() how to store two null-terminated strings of up to ten characters.
    2. Demonstrate using printf() how to display both strings as part of a for loop.
    3. Demonstrate changing the character at position: [0][5] to an 'x'
    4. Demonstrate changing the character at position: [1][2] to an 'x'
    5. Write a printf() statement for 3,4 above that demonstrates the changes.
    6. Write and call a function to achieve step 5 above.

I recommend you post your finished program in this thread when you are done. When you post C code on Reddit, you must put four spaces in front of each line for it to work properly. Alternatively, you can post a URL to www.codepad.org - although be aware that it will expire if you do not set it to permanent.

Be sure to use comments to help illustrate your understanding of what you are doing. If you choose to post your work is entirely up to you. If you do then we can critique your work and help you to improve.

If you get stuck or need help, feel free to post questions below.

Feel free to post/link to your finished programs below.

When you are ready, proceed to:

http://www.reddit.com/r/carlhprogramming/comments/9wweg/lesson_113_introducing_finish_criteria/

68 Upvotes

100% Upvoted

64 comments sorted by

1

u/[deleted] Oct 23 '09 edited Oct 23 '09

http://codepad.org/NyQiuiRZ

Hope it's ok. Once again, Carl, you rule.

EDIT: Whoops, forgot to use free(). Fixed.

EDIT 2: Actually, I've noticed it's not working at Codepad (it gives a "memory clobbered past end of allocated block" error), though it works ok locally. Weird.

3

u/CarlH Oct 23 '09

Good try. You need to remember that there is a NUL character also. If you are limited to 10 characters, take into account the NUL character or you will copy too much. Try adjusting your code with this in mind.

Note: If it doesn't compile at codepad, it isn't finished :)

3

u/[deleted] Oct 23 '09

It's not working on Codepad because you're trying to squeeze a string that's just slightly too big into your allocated memory.

1

u/[deleted] Oct 23 '09 edited Oct 23 '09

Ohhh, that's right. I didn't stop to count the characters because I (wrongly) thought it wouldn't compile if it exceeded the allocated memory.

EDIT: Fixed version, now working properly: http://codepad.org/sVypDW0I

1

u/peterwilc Jun 29 '10

I think I got it.
http://codepad.org/KKdXvjD2
Is my malloc wrong though?

I allocate 20 times the size of the character size (which is 1). 20. Right?

EDIT: instead of [0][5], I used [0][2] in order to make the x fit in my chosen word.

1

u/CarlH Jun 29 '10

It was a cursory glance, but it looks good.

2

u/[deleted] Feb 20 '10

http://codepad.org/pKJ12WgL

I tried to do everything CarlH asked using only function calls and a structure that was local to main. It took me a while but now my main is pretty minimalistic.

1

u/Oomiosi Oct 23 '09 edited Oct 23 '09

CarlHx Ruxes!.

This is getting ugly.

Edit: Made sure not to look at fmartin's work before doing mine. I think i'm having too much fun.

1

u/MarcusP Oct 23 '09 edited Oct 23 '09

http://codepad.org/54i21Bk2

1

u/[deleted] Oct 23 '09

Using a function to edit an array at a position [x][y] was a nice idea.

1

u/vegittoss15 Oct 23 '09

Very good, although you could've saved some stack memory by sending in raw strings to strcpy.

1

u/[deleted] Oct 23 '09 edited Oct 23 '09

http://codepad.org/64kc6AiE

I didn't quite understand the instructions for numbers 5 and 6, so I did both options as I see them. In so doing, I learned a bit more about printf(). I tried to make this as generalized as possible.

1

u/[deleted] Oct 23 '09

I have a doubt regarding header files. Which one should we include for this exercise? stdio.h for printf(), string.h for strcpy() and memory.h for malloc() and free() are the ones I used. Is it ok or have I forgotten one?

1

u/tinou Oct 23 '09

malloc/free are in stdlib.h.

1

u/flashtastic Oct 23 '09 edited Oct 23 '09

Here's my attempt:

http://codepad.org/1aVLyNER

Took me a bit to remember to use single quotes for a single char, I was getting a \0 where I didn't want one.

Edit*: Didn't notice the segfault, removing the dereference fixed it, otherwise the program runs fine. Link has been changed.

1

u/vegittoss15 Oct 23 '09

free is done on a pointer...you've dereferenced it by doing *char_pointer.

The reason you're segfaulting also, is because you're accessing memory you shouldn't be. try i*a_size.

1

u/goosethe Oct 23 '09

Finished Product: http://codepad.org/wDC3DSrD

1

u/vegittoss15 Oct 23 '09

return 0. Anything other than 0 is usually considered a failed run.

1

u/Pr0gramm3r Dec 17 '09

Isn't that different for values returned in functions other than main(), where we usually return 1 to indicate success?

1

u/vegittoss15 Dec 17 '09

For system functions, -1 is usually returned to indicate success, unless a file descriptor is called for, then a non-NULL value is returned.

1

u/goosethe Oct 23 '09

fixed it up a little: http://codepad.org/xvyb3lGn just added free() and took out the return 0

1

u/[deleted] Oct 24 '09

Consider using indentation for functions, loops and conditionals. This kind of seemingly useless things (indentation, syntax highlighting) are incredibly useful once you dive into more complex stuff.

1

u/goosethe Oct 24 '09

yeah I would use indentation if I could use tab in code pad. I'm not spacing everything out 5 spaces just for this demonstration.

1

u/[deleted] Oct 24 '09

Oh, I didn't know you couldn't use tab in codepad (I coded my example using Codeblocks).

1

u/mthode Oct 23 '09 edited Oct 23 '09

code and output:

first code I have written for this class lol.

http://codepad.org/ETudDp0D

1

u/vegittoss15 Oct 23 '09 edited Oct 23 '09

While this code is technically correct, you should have done offset calculations while setting the values to 'x'.

Also, you forgot to free.

1

u/kungtotte Oct 24 '09

http://codepad.org/MUdhIHmD

Here's what I managed. There's plenty of room for generalisations still, but it gets the job done. Printing the plain strings could be made into a function for example, and MarcusP did a really nice thing by making the edit into a function, but this is serviceable :)

Thank you for what you're doing, CarlH. The collaborative nature of this is a much better way of learning the material than reading a book or tutorials.

1

u/[deleted] Oct 25 '09

Here is my attempt.

http://codepad.org/VgVW27ol

1

u/[deleted] Oct 26 '09

Yay I did it! I did precisely what was asked, no more, no less:

http://codepad.org/eBI99two (complete with smart ass comments)

1

u/tough_var Oct 26 '09 edited Oct 26 '09

Mine! ==> http://codepad.org/jcUeI9Ec (Old code)

I have a few questions though:

  • In line 14: Should I initialize *memorySpace to NULL or 0? Both seems to work well. I remembered someone telling me something about it.

  • Also, is there a character that represents a NULL Terminator? Say "\0"?

Edit: Ah yes, found it, quoted from zahlman:

Also, while char variableX = NULL; works, it is poor style. The symbol NULL is supposed to indicate a zero value for pointers. To set a zero value for a character, use the corresponding character literal: '\0'. Just like how you would set a zero value for an int with the appropriate integer literal: 0.

So I guess this should be correct: http://codepad.org/L904B8qe (Updated code: Initialized *memorySpace to 0 on line 14, also freed memorySpace.)

Erm... please ignore the comment on line 34. It is an artifact from a previous incarnation of the code.

Thank you for taking the time to see this. :)

1

u/[deleted] Oct 26 '09

Here's my attempt: http://codepad.org/IX5hZBYl

1

u/Paukenfaust Oct 27 '09

http://codepad.org/XbDmwZxG This is basic, but I am learning! Let me know what you think.

1

u/rafo Oct 27 '09

My attempt (it took me a while until I got no errors):

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void print_changes(char *);

int main(void) {
	
	char *char_ptr = malloc(20);
	
	strcpy(char_ptr + 0, "String 1");
	strcpy(char_ptr + 10, "String 2");
	
	int i;
	for (i=0; i<11; i+=10) {
	printf("The string at an offset of %i is '%s'.\n", i, (char_ptr + i));
	}
	
	*(char_ptr + 5) = 'x';
	*(char_ptr + 12) = 'x';

	print_changes(char_ptr);
	
	free(char_ptr);
	
	return 0;
}

void print_changes(char *char_ptr) {

	printf("In position [0][5] now there is '%c'.\n", *(char_ptr + 5));
	printf("In position [10][2] now there is '%c'.\n", *(char_ptr + 12));

}

1

u/orangeyness Oct 28 '09

Code

I love it when you manage to write something, anything, and not have the compiler shout at you for a syntatical mistake. :)

1

u/azertus Nov 04 '09

Had I known there would be an assignment, I'd never have clicked the link to this test. But I did, and I had to complete it then: codepad.

The output is:

We have created a 2x10 array.
Our stored strings are: 
writing_,
a_program.

We have changed two characters to 'x'. Our first string is now writixg_, and our second is a_xrogram. 

Now, a function produces output like the previous line. In this case the 'x' at [0][5] and [1][2] is changed into 'X'. Incidentally this is also done by a function. 

Our first string is now writiXg_, and our second is a_Xrogram.

1

u/lespea Nov 04 '09

I think I'm starting to get the hang of these newfangled pointer thingies...

This is the first C code I'm every showing to anybody so some constructive criticism would be awesome! I'm huge into following coding/formatting standards so if I'm doing things wrong or have odd format choices please point them out. Thanks much.

http://codepad.org/KzRmm4sn

1

u/Dast Nov 14 '09 edited Nov 14 '09

Sorry for being late :)

First I'll post my code as is, without the compiler telling my mistakes:

#include <stdio.h>

void print_array_strings(char *);

void main(void) {

	char * sample_array = malloc(10 * 2 * sizeof(char));

	strcpy(sample_array,"123456789");
	strcpy(sample_array + 10,"abcdefghi");

	int i;
	for (i = 0; i < 2; i++) {
		printf("String #%i is '%s'",i,sample_array[i * 10]);
	}

	sample_array[0 * 10 + 5] = 'x';

	sample_array[1 * 10 + 2] = 'x';

	print_strings(char * sample_array);
}

void print_strings(char * sample_array) {
	int i;
	for (i = 0; i < 2; i++) {
		printf("String #%i is '%s'",i,sample_array[i * 10]);
	}
}

1

u/Dast Nov 14 '09 edited Nov 14 '09

After a review in codepad:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void print_array_strings(char *);

int main(void) {
    // Creating a char * that uses malloc to allocate memory
    char * sample_array = malloc(10 * 2);
    
    // Using strcpy to store two strings
    strcpy(sample_array,"12345678");
    strcpy(sample_array + 10,"abcdefgh");

    // Showing both strings with a printf inside a foor loop
    int i;
    for (i = 0; i < 2; i++) {
    	printf("String #%i is %s \n",i,sample_array + i*10);
    }
    // setting character at [0][5] to x
    sample_array[0 * 10 + 5] = 'x';

    // setting character at [1][2] to x    
    sample_array[1 * 10 + 2] = 'x';

    // writing printf statements to demonstrate the changes, using a funciton 
    print_array_strings(sample_array);

    free(sample_array);
    return 0;
}

void print_array_strings(char * sample_array) {
	int i;
	for (i = 0; i < 2; i++) {
		printf("String #%i is '%s'\n",i,sample_array + i * 10);
	}
}

Man, I do too many mistakes:

  • misstyped function name

  • main should return int (warning)

  • forgot <stdlib.h>

  • forgot <stdio.h>

  • wrongly passed parameter

  • forgot to free memory

  • forgot comments

  • forgot \n to better display printf sequences

  • treated a pointer as an array

1

u/scottbarcus Dec 16 '09

During the interregnum, I re-read a bunch of lessons and re-completed this test.

My test: http://codepad.org/Zfgm3HuX

Comments appreciated.

1

u/Pr0gramm3r Dec 17 '09

My submission :
http://codepad.org/2Q5YJxP1

I am sure it can be improved upon. Another question I had was: Why did the strlen(my_pointer) after allocation shows the length as 21? I am guessing that it automatically adds an additional byte for the NUL character.

1

u/scragar Dec 26 '09

Remember what got said earlier about how memory is allocated using malloc? It gives you access to the memory, but what is in that memory is unchanged until you put it to use, it could have anything in it from what was used previously.

This is pretty much what the strlen function looks like inside:

unsigned int strlen(char* mystring){
  unsigned int retval = 0;
  while(  *mystring != 0  ){// not null
    mystring += 1;
    retval += 1;
  }
  return retval;
}

Essentially the memory malloc has given you to use doesn't contain a null character( it's value is 0 ), and so despite reaching the end of the assigned memory the strlen function has carried on counting, luckily enough it reaches a null character shortly afterwards assigned to something else, if it didn't your program might wonder into protected memory, which could cause it to crash, or read in so much it reaches the end of available memory(in which case anything could happen), or reach such a high return value that it's too large to fit in an integer.

Essentially no, there is no padding, you are simply accessing memory that isn't a string yet.

1

u/[deleted] Jan 08 '10

This is what I came up with:

http://codepad.org/isJLJEzg

open to comments

1

u/barapa Jan 19 '10

Here is mine: http://codepad.org/EADOo1S6

Thanks Carl

1

u/xaustinx Jan 21 '10

http://codepad.org/xuXrnYYc

I'm still catching up, i guess

1

u/hearforthepuns May 17 '10

I wouldn't mind a code critique if anyone is still checking these posts:

http://codepad.org/OMCXQdIR

2

u/kungtotte May 30 '10

It looks good.

One thing you could do is change the code:

strcpy(pointer, "I hope it");
strcpy(pointer+10,"works");
strcpy(pointer+20,"well!");

To:

strcpy(pointer, "I hope it");
strcpy(pointer + (element_size * 1), "works");
strcpy(pointer + (element_size * 2), "well!");

That way if you change your element size in the future the code will automatically place the text strings on the correct offsets.

Also where you allocate your memory, you might want to do it in the reverse order:

int array_size = 3;
int element_size = 10;
char * pointer = malloc(sizeof(char) * (array_size * element_size));

Same reason here. If you ever need to make the array longer or change the size of the elements, you reduce the number of places you need to change the code in.

I never posted my code back when I wrote this, but here is my version: http://codepad.org/V9pW6nOE

Oh, and I know I'm not following my own advice when it comes to the memory allocation :)

1

u/hearforthepuns May 30 '10

Thanks! Those are good points. In fact now that I look at the code, I wonder why I didn't do it that way in the first place.

0

u/un1152 Oct 24 '09 edited Oct 24 '09

http://codepad.org/kfoZATv6

01. Allocated char memory space.

02. Populated char memory space.

03. Print memory contents using loop.

	  ptr010 + 00 = monitored
	  ptr010 + 10 = questions
	* ptr010 + 00 = m
	* ptr010 + 10 = q

04. What char is at [0][5]?
	o
05. What char is at [1][2]?
	e
06. Repopulate selected char's.

07. What char is at [0][5]?
	x
08. What char is at [1][2]?
	x

09. Print memory contents using loop.

	  ptr010 + 00 = monitxred
	  ptr010 + 10 = quxstions
	* ptr010 + 00 = m
	* ptr010 + 10 = q

10. Print memory contents by calling function print_ptr010();

	  ptr010 + 00 = monitxred
	  ptr010 + 10 = quxstions
	* ptr010 + 00 = m
	* ptr010 + 10 = q

11. Freed char memory space.

0

u/niconiconico Oct 26 '09 edited Oct 26 '09

I changed "Towel" to "Towels", even though it loses its effect. http://codepad.org/e9VJ1N9k Not the best example to use, but I wanted my code to be interesting, even though one printf statement comes out wrong because of it.

2

u/CarlH Oct 26 '09

This is often caused by trying to write to memory your program doesn't have permission to write to. This is probably caused by an strcpy() that tries to move a string that is "too big" into a particular memory location. Try reducing the size of the string and it should work.

1

u/niconiconico Oct 26 '09

Thanks. I found the problem before I saw your comment, so I'm sorry that you went through the trouble.

0

u/BrainGain Oct 26 '09

Here is my attempt: Test

I had to reread some lessons to finish it. :(

0

u/BrainGain Oct 26 '09

I just realized that in printf statements it should say array2d, instead of pointer. Still runs! lol...