Question

Guys can you teach me how to use malloc feature in my code..........

Asked by: Sambho

Guys .....greetings...

My programme to find the prime number below 10000 is ready to go. But I want to use malloc function in my programme to create memory for 10,000 elements of the array.....So guys can you help me how can I do that........
Tons of thanks......

/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/
 
#include<stdio.h>
#include <stdlib.h>     /* for the malloc() function and exit() function */
#include <assert.h>     /* for assert macro */
 
int main(int argc, char* argv[])
{
    FILE *fp;
    int num[10000], i, j; 
    
    if (argc != 2) {
 	  printf("Usage: SeiveErat.exe output.txt \n");
 	  exit(1);
    }
               
    fp = fopen(argv[1], "w+");
       
    for(i=1;i<=9999;i++)     
          num[i]=i+1;
     
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)
          {
               for(j=(i+1);j<=9999;j++)
               {
                    if(num[j]!=0)
                    {
                         if((num[j]%num[i])==0)     /*check if num[j]*/ 
                         num[j]=0;                    /*is a multiple of num[i]*/
                                                       /*if it is a multiple then set it to 0*/
                    }
               }
          }
     }
 
     for(i=0;i<=9999;i++)
     {
          if(num[i]!=0)                   /*Print all prime numbers*/
               {
               //printf("\n%d",num[i]);
               fprintf(fp, "%d\r\n", num[i]);
               }
     }
      fclose(fp);
}

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Asked On: 2009-10-23 at 10:01:01ID24838601
Topics: C Programming Language
,; C++ Programming Language
Participating Experts: 3
Points: 500
Comments: 23

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Answers

by: jkrPosted on 2009-10-23 at 10:50:50ID: 25646690

Using 'malloc()' here is quite simple, just change that to

/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/
 
#include<stdio.h>
#include <stdlib.h>     /* for the malloc() function and exit() function */
#include <assert.h>     /* for assert macro */
 
int main(int argc, char* argv[])
{
    FILE *fp;
    int i, j; 
    int* num; // instead of the array
    
    if (argc != 2) {
          printf("Usage: SeiveErat.exe output.txt \n");
          exit(1);
    }
               
    fp = fopen(argv[1], "w+");
 
    num = (int*) malloc(10000 * sizeof(int)); // allocate the memory
       
    for(i=1;i<=9999;i++)     
          num[i]=i+1;
     
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)
          {
               for(j=(i+1);j<=9999;j++)
               {
                    if(num[j]!=0)
                    {
                         if((num[j]%num[i])==0)     /*check if num[j]*/ 
                         num[j]=0;                    /*is a multiple of num[i]*/
                                                       /*if it is a multiple then set it to 0*/
                    }
               }
          }
     }
 
     for(i=0;i<=9999;i++)
     {
          if(num[i]!=0)                   /*Print all prime numbers*/
               {
               //printf("\n%d",num[i]);
               fprintf(fp, "%d\r\n", num[i]);
               }
     }
 
     free(num); // clean up
 
      fclose(fp);
}

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by: jkrPosted on 2009-10-23 at 10:51:30ID: 25646694

BTW, see also http://www.cplusplus.com/reference/clibrary/cstdlib/malloc/ for more information on that issue.

by: SambhoPosted on 2009-10-23 at 11:43:52ID: 25647154

Yes thank you
It is really simple to use malloc. I also used assert macro just below the malloc function.I think it is correct to use in that way......

In the output I am getting number 4063608 on the top( before prime number 2). I don't know why I am getting this......

There I want to print something like "There are 1229 prime numbers less than or equal to 10000"

Output is attached below.....

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by: jkrPosted on 2009-10-23 at 11:56:02ID: 25647240

In the ourput loop, you start with i = 0 instead of i = 1, thus printing an uninitialized memory area. Use

 
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)                   /*Print all prime numbers*/
               {
               //printf("\n%d",num[i]);
               fprintf(fp, "%d\r\n", num[i]);
               }
     }

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by: jkrPosted on 2009-10-23 at 12:03:13ID: 25647307

Oh, alternatively, you could initialize the memory to zero, e.g.

/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/
 
#include<stdio.h>
#include <stdlib.h>     /* for the malloc() function and exit() function */
#include <assert.h>     /* for assert macro */
#include <memory.h>
 
int main(int argc, char* argv[])
{
    FILE *fp;
    int i, j; 
    int* num; // instead of the array
    
    if (argc != 2) {
          printf("Usage: SeiveErat.exe output.txt \n");
          exit(1);
    }
               
    fp = fopen(argv[1], "w+");
 
    num = (int*) malloc(10000 * sizeof(int)); // allocate the memory
 
    memset(num,0,10000 * sizeof(int));
       
    for(i=1;i<=9999;i++)     
          num[i]=i+1;
     
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)
          {
               for(j=(i+1);j<=9999;j++)
               {
                    if(num[j]!=0)
                    {
                         if((num[j]%num[i])==0)     /*check if num[j]*/ 
                         num[j]=0;                    /*is a multiple of num[i]*/
                                                       /*if it is a multiple then set it to 0*/
                    }
               }
          }
     }
 
     for(i=0;i<=9999;i++)
     {
          if(num[i]!=0)                   /*Print all prime numbers*/
               {
               //printf("\n%d",num[i]);
               fprintf(fp, "%d\r\n", num[i]);
               }
     }
 
     free(num); // clean up
 
      fclose(fp);
}

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by: SambhoPosted on 2009-10-23 at 13:14:28ID: 25648114

Thank you JKR very much,,

I have one last question for you..............I want to print " There are 1229( what ever it is) prime numbers less than or equal to 10000"
So for this I have to use just fprintf... function but I don't know how to count n[i].......Please tell me how to do that....
Thank you very much..........................

by: jkrPosted on 2009-10-23 at 13:28:57ID: 25648250

Sure, no problem:

/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/
 
#include<stdio.h>
#include <stdlib.h>     /* for the malloc() function and exit() function */
#include <assert.h>     /* for assert macro */
#include <memory.h>
 
int main(int argc, char* argv[])
{
    FILE *fp;
    int i, j; 
    int* num; // instead of the array
 
    unsigned int count = 0;
    
    if (argc != 2) {
          printf("Usage: SeiveErat.exe output.txt \n");
          exit(1);
    }
               
    fp = fopen(argv[1], "w+");
 
    num = (int*) malloc(10000 * sizeof(int)); // allocate the memory
 
    memset(num,0,10000 * sizeof(int));
       
    for(i=1;i<=9999;i++)     
          num[i]=i+1;
     
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)
          {
               for(j=(i+1);j<=9999;j++)
               {
                    if(num[j]!=0)
                    {
                         if((num[j]%num[i])==0)     /*check if num[j]*/ 
                         num[j]=0;                    /*is a multiple of num[i]*/
                                                       /*if it is a multiple then set it to 0*/
                    }
               }
          }
     }
 
     for(i=0;i<=9999;i++)
     {
          if(num[i]!=0)                   /*Print all prime numbers*/
               {
               //printf("\n%d",num[i]);
               fprintf(fp, "%d\r\n", num[i]);
               ++count; // count prime number
               }
     }
 
 
     fprintf(fp,"There are %u prime numbers less than or equal to 10000\n",count);
 
     free(num); // clean up
 
      fclose(fp);
}

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by: SambhoPosted on 2009-10-23 at 14:16:05ID: 25648731

Hi jkr,
the out put is wrong it shows very high number...............I have attached the output...Please have a look.

9689
 
9697
 
9719
 
9721
 
9733
 
9739
 
9743
 
9749
 
9767
 
9769
 
9781
 
9787
 
9791
 
9803
 
9811
 
9817
 
9829
 
9833
 
9839
 
9851
 
9857
 
9859
 
9871
 
9883
 
9887
 
9901
 
9907
 
9923
 
9929
 
9931
 
9941
 
9949
 
9967
 
9973
 
There are 2009314170 prime numbers less than or equal to 10000

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by: jkrPosted on 2009-10-23 at 14:19:50ID: 25648769

Have you changed anything? The code from above gives me the following output (starting with 2):

2
 
3
 
5
 
7
 
11
 
13
 
17
 
19
 
23
 
29
 
31
 
37
 
41
 
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47
 
53
 
59
 
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There are 1229 prime numbers less than or equal to 10000

Select all Open in new window

by: Subrat2009Posted on 2009-10-23 at 15:42:20ID: 25649392

You can replace these two lines with a single statement using calloc() as follows.Calloc initializes memory with value 0.
>>num = (int*) malloc(10000 * sizeof(int)); // allocate the memory

>> memset(num,0,10000 * sizeof(int));
num = (int*) calloc(10000, sizeof(int));

>>the out put is wrong it shows very high number...............
This prog given by JKR is correct. I think u r getting garbage due to some changes in the code. Can u provide the exact code that produces this unexpected result!

by: SambhoPosted on 2009-10-23 at 16:25:49ID: 25649739

Thanks guys for your support and help....................

The exact code is given below. In my programme I have used i = 1 instead of i = 0 in the outer loop instead of using memset .....................It produces wrong output in my case.................

/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/
 
#include<stdio.h>
#include <stdlib.h>     /* for the malloc() function and exit() function */
#include <assert.h>     /* for assert macro */
 
int main(int argc, char* argv[])
{
    FILE *fp;
    int i, j, count; 
    int* num; // instead of the array
    
    if (argc != 2) {
          printf("Usage: SeiveErat.exe output.txt \n");
          exit(1);
    }
               
    fp = fopen(argv[1], "w+");
 
    num = (int*) malloc(10000 * sizeof(int)); // allocate the memory
    assert(num != NULL);   
    
    for(i=1;i<=9999;i++)     
          num[i]=i+1;
     
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)
          {
               for(j=(i+1);j<=9999;j++)
               {
                    if(num[j]!=0)
                    {
                         if((num[j]%num[i])==0)     /*check if num[j]*/ 
                         num[j]=0;                    /*is a multiple of num[i]*/
                                                       /*if it is a multiple then set it to 0*/
                    }
               }
          }
     }
 
     for(i=1;i<=9999;i++)
     {
          if(num[i]!=0)                   /*Print all prime numbers*/
               {
               //printf("\n%d",num[i]);
               
               fprintf(fp, "%d\r\n", num[i]);
               ++count; // count prime number
               }
     }
     fprintf(fp,"There are %u prime numbers less than or equal to 10000\n",count);
     
     free(num); // clean up
 
     fclose(fp);
}

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Select all Open in new window

by: jkrPosted on 2009-10-23 at 16:31:38ID: 25649777

You forgot to set 'count' to '0', make that

int i, j, count = 0;

by: SambhoPosted on 2009-10-23 at 17:15:53ID: 25650037

Thank you very much jkr,
Here is probably last help i want from you... I want to make some modification to my code......I want to use command line arguments to pass the filename and the number. Such as
SeiveErat.exe 10000 Output.txt

I think we have to use something like atoi() to convert 10000 to int.
I don't know how to use the function atoi() in our programme.........
Can you tell me how to do this...........what should we add in our programme...........

Thank you very much in advance..................

by: InternalStaticPosted on 2009-10-23 at 17:50:16ID: 25650157

It doesn't make a functional difference, but (though I'm not big with C) I believe allocating memory like this is supposed to be done using "calloc" instead of "malloc". They are functionally identical, but using one over the other shows your intent in a program, which is valuable in any language. calloc's signature is as follows:

void* calloc(size_t numElements, size_t elementSize);

So you would put the following where you currently call malloc:

calloc(10000, sizeof(int))

Also, your algorithm is using much more memory than is needed (unless I am misunderstanding your intent). It looks to me like you want to output every prime number between 2 and 10,000, and you also want to output how many of these numbers there are. Again, I'm not big with C, but this is much more efficient (I'm gunna hate myself for this, 'cuz this totally sounds like a homework assignment, but here goes anyway...)

Hope this helps,
Nate

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <memory.h>
 
int main(int argc, char* argv[])
{
	FILE *fp;	 // Output file handle
    int i;		 // Outer for loop counter
	int j;		 // Inner for loop counter
    int* primes; // Prime numbers
	int upperBound; // The number to stop at
	unsigned int countPrimes; // Number of primes
 
	if (argc != 3)
	{
		printf("Usage: SeiveErat.exe num output.txt\n");
		exit(1); // Bail; improper arguments
	}
 
	upperBound = atoi(argv[1]);
	if(upperBound == 0)
	{
		// Parse failed, or the user entered 0 (in which case, there are no primes)
		// Decide how you want to deal with this.
	}
 
	// Set default values.
	countPrimes = 0U;
 
	// Allocate enough for half of the upper bound because any number even number isn't prime,
	// and half of any consecutive set of numbers are always even
	primes = (int*)(calloc(upperBound / 2, sizeof(int)));
 
 
	// Open the specified file with write access. Ideally, you should check to make sure a legitimate path was specified.
	fp = fopen(argv[2], "w+");
 
	// Pre-store/write as many as you want. I would keep it small like this, as more will clutter your code.
	fprintf(fp, "%d\r\n", primes[countPrimes++] = 1);
	fprintf(fp, "%d\r\n", primes[countPrimes++] = 3);
 
	for(i = primes[countPrimes - 1]; i < upperBound; /*Update in loop*/)
	{
		// Start at 3 because if only even numbers are divisible by 2, which we are skipping (above)
		for(j = 3; j < i; j++)
		{
			// Skip over adding the value to the 'array'
			if((i % j) == 0)
			{ goto breakloop; }
		}
		// You can do this entire "application" without storing all the primes at all, if you want.
		// Just remove all references to "primes", and replace "primes[countPrimes++] = i" (below) with just "i"
		fprintf(fp, "%d\r\n", primes[countPrimes++] = i);
	breakloop:
		// Increment by two because even numbers can't be prime (make sure i is equal to an odd number, though, first)
		i += 2;
	}
 
	fprintf(fp, "\r\n\r\nTotal prime numbers inclusively between 1 and %u: %u", upperBound, countPrimes); // Output results
 
	fclose(fp);   // Always release external resources first (e.g. database connections, file handles, etc)
	free(primes); // Deallocate your primes, if you no longer need them
 
	printf("Output finished\n");
	return 0;
}

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Select all Open in new window

by: InternalStaticPosted on 2009-10-23 at 17:53:04ID: 25650166

Whoops, forgot to mention I included the parsing for you, too. Hope it's in the format you're looking for.

Also, the word you are spelling as "programme" is spelt "program".

by: SambhoPosted on 2009-10-23 at 18:49:20ID: 25650371

Thank you Internalstatic,
I appreciate your help and support. I am not like you, I am just a beginner in this world. SO, I don't care about how much memory my programme is eating. I am learning and using all differnt kind of things. And your programme is different then mine.
So, can you please update my own programme or code..........In that way it will be way to much easier for me to know whats going on in my programme...........
By the way what do you mean by "countPrimes = 0U;" What is 0U..
Thank you very much......

by: Subrat2009Posted on 2009-10-23 at 21:08:57ID: 25650740

Unsigned is repesented by U. Like 0U.

Here const is represented by unsigned bcz, we know count shoul only hold +ve vaue rather nagative and it's expcted here.

So instead of chosing signed int we use unsigned int. Signed holds + & -ve values.

by: InternalStaticPosted on 2009-10-23 at 21:28:08ID: 25650807

As Subrat2009 said, the "U" after the number tells the compiler that the number it follows is unsigned (in other words, that the number within it cannot be negative). As I've expressed in a previous post, the point of the "U" suffix is to express intent, while it offers no real functionality in this context (you'll start to see when these things matter as your knowledge progresses). Its use is only valid after a literal numeric constant.

By the way, is this little project a homework assignment, or are you teaching yourself the C language?

by: Subrat2009Posted on 2009-10-23 at 21:40:26ID: 25650841

// Plz read the comments carefully by which u can be cleared that why something is used!
/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/

#include<stdio.h>
#include <stdlib.h> /* for the malloc() function and exit() function */
#include <assert.h> /* for assert macro */

int main(int argc, char* argv[])
{
FILE *fp = NULL;//Better try to initialize all variables
int i = 0, j = 0, count = 0;
// better use unsigned count = 0U;
int* num = NULL; // instead of the array.
//Better try to initialize all variables

if (argc != 3) {
printf("Usage: SeiveErat.exe upperbound output.txt \n");
exit(1);
}

fp = fopen(argv[2], "w+"); // change this to 2 bcz argv[1] holds upperbound like 10000.So argv[2] will hold filename.argv[0] holds executable name.
int upperBound = atoi(argv[1]);
//num = (int*) malloc(upperBound * sizeof(int)); // allocate the memory
// Use calloc() here . Already told why in the same thread. Else use memset() as described by jkr which initializes the memory
// Please use calloc() bcz it initializes the memory.But malloc dosn't do that , it only allocates memory and allocated memory will hold garbage values. Which is unsafe.As u r in beginning level, u should check urself in these regards to become a very good programmer in future.
// Use num = (int*) calloc(upperBound, sizeof(int));
num = (int*) calloc(upperBound, sizeof(int));
assert(num != NULL);

for(i=1;i<=upperBound-1;i++)
num[i]=i+1;

for(i=1;i<=upperBound-1;i++)
{
if(num[i]!=0)
{
for(j=(i+1);j<=upperBound-1;j++)
{
if(num[j]!=0)
{
if((num[j]%num[i])==0) /*check if num[j]*/
num[j]=0; /*is a multiple of num[i]*/
/*if it is a multiple then set it to 0*/
}
}
}
}

for(i=1;i<=upperBound-1;i++)
{
if(num[i]!=0) /*Print all prime numbers*/
{
//printf("\n%d",num[i]);

fprintf(fp, "%d\r\n", num[i]);
++count; // count prime number
}
}
fprintf(fp,"There are %u prime numbers less than or equal to %d\n", count, upperBound);

free(num); // clean up

fclose(fp);
}

by: SambhoPosted on 2009-10-23 at 21:50:56ID: 25650862

First of all guys thanks for your help,
This is not homework assignment, I am teaching myself C and I ask you guys exact questions and problems from books. And I want to do exactly what is said in books.
This is how I am learning......

I have one question again...................
In the end of my output page it says," There are 1229 prime numbers less than or equal to 10000" ................I want to put that message in the begening of the page i,e before the prime number starts..

Guys give me some solid idea please ................................Code is attached...

/* Finding Prime Numbers Using The Sieve Of Eratosthenes*/
 
#include <stdio.h>       /* standard header file */
#include <stdlib.h>     /* for the malloc() function and exit() function */
#include <assert.h>    /* for assert macro */
 
 
void get_primes( int *, int);
 
int main(int argc, char* argv[])
{
     FILE *fp;
     int i, count = 0; 
     int *num; 
     int upbound = 10000;
    
    
    if (argc != 3) {
           printf("Usage: SeiveErat.exe 10000 output.txt \n");
           exit(1);
     }
     
   	 upbound = atoi(argv[1]);          
     fp = fopen(argv[2], "w+");
 
     num = (int*) malloc(10000 * sizeof(int)); // allocating the memory
     assert(num != NULL);
       
     get_primes(num, upbound);
                   
     for(i=1; i<upbound; i++)
     {
          if(num[i]!=0)                   
               {
               fprintf(fp, "%d\r\n", num[i]);
               ++count; // count prime number
               }
     }
     fprintf(fp,"There are %u prime numbers less than or equal to 10000\n",count);
     
     free(num); // To clean up 
     
     fclose(fp);
}
 
 
   void get_primes( int *num, int upbound){
     
      int i, j;
 
      for(i=1; i<upbound; i++)     
          num[i]=i+1;
     
      for(i=1; i<upbound; i++)
     {
          if(num[i]!=0)
          {
               for(j=(i+1); j<upbound; j++)
               {
                    if(num[j]!=0)
                    {
                         if((num[j]%num[i])==0)     
                         num[j]=0;                    
                                                       
                    }
               }
          }
     }
}

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by: InternalStaticPosted on 2009-10-23 at 22:10:43ID: 25650890

I would love to help, but I think this question is quite exhausted. Us feeding you code like this really isn't getting you anywhere--you need to learn how to think like a programmer. Syntax is nothing--hell, I've never written a program in straight C before today, and it took me all of about 15 minutes to figure it out.

When you find a book to read, do not skip anything. If you think you know whatever it is you're reading about, assume you're wrong, because you probably are. If you've done some skipping around on your current book, I suggest you start over. Open up notepad, and start putting bulleted notes about things that don't make sense to you (maybe your first entry should include the "U" suffix I mentioned? If you're not familiar with the difference between "signed" and "unsigned", it's time you discovered Bing.com)

Lastly, I strongly suggest you put your learning of C on hold and take up another, very similar language (called C++). Their syntax is remarkably similar, however, they are fundamentally different. C is what we call a procedural language, whereas C++ is an object-oriented or object-based language. Object-oriented languages are quickly becoming the standard, so if you don't have a set-in-stone reason to learn C, I recommend you move on over to C++ instead. However, there is one more language I would recommend, also syntactically similar to C and C++: The language C#.

C# is another object-oriented language, but it's a little different. The language relies on another "platform" (known as the .NET Framework--perhaps you've heard of it). The advantage of this fact is that the .NET Framework offers unsurpassed code documentation, naming consistency, and the world's truly best IDE (Integrated Development Environment), known as Microsoft's Visual Studio. The disadvantage in using a .NET language (eg C#) is there is a somewhat large performance penatly, though it is rarely noticed anymore, with 2GB of RAM being as cheap as a candy bar. This is still no excuse for poor coding, however.

I apologize for this post be a rather "personal" and "targeted" one (not very useful for the knowledgebase in most cases, I'm afraid), but I hope it will at least get you thinking. If you'd like to continue this conversation, you may e-mail me at InternalStatic@live.com, as I'd rather not clog this question up any more than I already have.

- Nate

by: Subrat2009Posted on 2009-10-23 at 23:15:38ID: 25651035

You can use fseek() to achieve your goal...

by: SambhoPosted on 2009-10-24 at 19:38:11ID: 25655299

Oh I am sorry guys for being late...............

I want to conclude this discussion right here. Thank you for everything..................

I am hoping for same kind of help in the future..............

Thank you............

20120131-EE-VQP-002

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Guys can you teach me how to use malloc feature in my code..........

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