problem reading to a dynamic array

noamfrie · #1 02-Jan-2005, 09:46

Hi!
I've allocated a dynamic array of "double" variables using malloc function.
The problem is when I try to input numbers ("double" type) into this dynamic array, I always get a "segmentation fault (core dumped)" error.
I would appriciate your help. It is preety urgent for me....
Here is the code:

CPP / C++ / C Code:

#include <stdio.h>
#include <stdlib.h>
int main ()
{
int n,k;
double *p;
scanf ("%d",&n);
p=(double *)malloc(n*sizeof(double));
if (p==NULL)
printf ("Error: Not enough memory.\n");
for (k=0;k<n;k++)
scanf ("%lf",p+k);
free (p);
return 0;
}

dsmith · #2 02-Jan-2005, 10:05

Hi noamfrie.

What compiler are you using? I downloaded your code and without any changes it seemed to work fine for me.

I am using gcc under Linux.

davekw7x · #3 02-Jan-2005, 10:20

Quote:

Originally Posted by noamfrie

Hi!
I've allocated a dynamic array of "double" variables using malloc function.
The problem is when I try to input numbers ("double" type) into this dynamic array, I always get a "segmentation fault (core dumped)" error.
I would appriciate your help. It is preety urgent for me....
Here is the code:

CPP / C++ / C Code:

#include <stdio.h>
#include <stdlib.h>
int main ()
{
int n,k;
double *p;
scanf ("%d",&n);
p=(double *)malloc(n*sizeof(double));
if (p==NULL)
printf ("Error: Not enough memory.\n");
for (k=0;k<n;k++)
scanf ("%lf",p+k);
free (p);
return 0;
}

Since there is no way for you (or me) to tell what your code is doing, I am going to suggest that you put some printf() statements in strategic places.

For example:

CPP / C++ / C Code:

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

int main ()
{
  int n, k;
  double *p;

  printf("Enter the number of data items: ");
  scanf ("%d",&n);
  printf("You entered %d\n", n);

  p = malloc(n * sizeof(double));
  if (p == NULL) {
    printf ("Error: Not enough memory.\n");
  }
  else {
    for (k = 0; k < n; k++) {
      printf("%d: ", k);
      scanf ("%lf",p+k);
      printf("You entered %f\n", p[k]);
    }

    for (k = 0; k < n; k++) {
      printf("%d: %f:\n", k, p[k]);
    }

    free (p);
  }
  return 0;
}

Then:

1. Maybe you can see how far it gets before it bombs out.
2. Maybe you can see if the program is reading the user input numbers that you think it should see.

Regards,

Dave

aaroncohn · #4 02-Jan-2005, 12:51

Is n a good value coming into the allocation? The code looks fine to me, other than the fact that you are using scanf(). Try using getchar() coupled with the atoi() function if you think the input might be getting goofed up. The code *should* be working, though.

Now that I know what a segfault is, I can tell you that you are accessing memory that doesn't belong to you, and this is usually caused by going out of the bounds of your array.

My theory: Since your memory is coming from the free store, any time you go outside the array, you're trying to access memory that's in the free store. Of course, that memory doesn't belong to you, since you haven't allocated it with malloc(), so you get a segfault.

Dr. Evil · #5 02-Jan-2005, 13:27

I had that problem when trying to allocate memory for an int array. Try using the function LocalAlloc(), for which you should include <windows.h>. It would look something like this, although I don't know if it works for doubles.

CPP / C++ / C Code:

#include <windows.h>

int main()
{
 int *arr;
 
 arr = LocalAlloc(LPTR, sizeof(int)*/*whatever*/);
 if(arr != NULL)
 {
  //do whatever
  LocalFree(arr);
 }
 return 0;
}

noamfrie · #6 02-Jan-2005, 14:39

Hi..
Sorry but the error happens in the following program, not in the partial code I submitted earlier.
This program gets n number of "double" values, puts them in a dynamic array and prints the minimum and maximum value in the array.
Also, I can't use libary functions in "minmax" function, only in "main".
Thanks in advance...

Noam.

Here is the full code:

CPP / C++ / C Code:

#include <stdio.h>
#include <stdlib.h>
void minmax (double *a,int n,double *min,double *max);
int main ()
{
int n,k;
double *p,*min,*max,x=10000,y=1;
min=&x;
max=&y;
scanf ("%d",&n);
p=(double *)malloc(n*sizeof(double));
if (p==NULL)
printf ("Error: Not enough memory.\n");
for (k=0;k<n;k++)
scanf ("%lf",p+k);
minmax (p,n,min,max);
printf ("Double_min=%f\nDouble_max=%f",*min,*max);
free (p);
return 0;
}
void minmax (double *p,int n,double *min,double *max)
{
for (p;p<p+n;p++)
{
if (*p<=*min)
*min=*p;
if (*p>=*max)
*max=*p;
}
}

Quote:

Originally Posted by noamfrie

Hi!
I've allocated a dynamic array of "double" variables using malloc function.
The problem is when I try to input numbers ("double" type) into this dynamic array, I always get a "segmentation fault (core dumped)" error.
I would appriciate your help. It is preety urgent for me....
Here is the code:

CPP / C++ / C Code:

#include <stdio.h>
#include <stdlib.h>
int main ()
{
int n,k;
double *p;
scanf ("%d",&n);
p=(double *)malloc(n*sizeof(double));
if (p==NULL)
printf ("Error: Not enough memory.\n");
for (k=0;k<n;k++)
scanf ("%lf",p+k);
free (p);
return 0;
}

WaltP · #7 02-Jan-2005, 15:17

Quote:

Originally Posted by noamfrie

Hi..
Sorry but the error happens in the following program, not in the partial code I submitted earlier.

Always test your programs before posting. We can't fix what ain't broke.

Quote:

Originally Posted by noamfrie

This program gets n number of "double" values, puts them in a dynamic array and prints the minimum and maximum value in the array.
Also, I can't use libary functions in "minmax" function, only in "main".

Why not? Did your teacher say you can't? Did the compiler complain?

Quote:

Originally Posted by noamfrie

Thanks in advance...

Noam.

Here is the full code:

CPP / C++ / C Code:

#include <stdio.h>
#include <stdlib.h>
void minmax (double *a,int n,double *min,double *max);
int main ()
{
int n,k;
double *p,*min,*max,x=10000,y=1;
min=&x;
max=&y;
scanf ("%d",&n);
p=(double *)malloc(n*sizeof(double));
if (p==NULL)
printf ("Error: Not enough memory.\n");
for (k=0;k<n;k++)
scanf ("%lf",p+k);
minmax (p,n,min,max);
printf ("Double_min=%f\nDouble_max=%f",*min,*max);
free (p);
return 0;
}
void minmax (double *p,int n,double *min,double *max)
{
for (p;p<p+n;p++)
{
if (*p<=*min)
*min=*p;
if (*p>=*max)
*max=*p;
}
}

You did not mention what happened when you did what Dave suggested. Well?

I suspect minmax() is your culprit. See Dave's post.

You need to learn to indent your code so it's readable, as well as read the sticky at the top of the forum. The Preview button will help you help us help you

Hey I like that! I'll have to use it again...

noamfrie · #8 02-Jan-2005, 15:54

The teacher said I can't use libary functions.
And you're right. The "minmax" function is the problem.
Any idea why it gives segmentation fault?

Quote:

Originally Posted by WaltP

Always test your programs before posting. We can't fix what ain't broke.

Why not? Did your teacher say you can't? Did the compiler complain?

You did not mention what happened when you did what Dave suggested. Well?

I suspect minmax() is your culprit. See Dave's post.

You need to learn to indent your code so it's readable, as well as read the sticky at the top of the forum. The Preview button will help you help us help you

Hey I like that! I'll have to use it again...

WaltP · #9 02-Jan-2005, 18:32

As has been suggested:

Quote:

Originally Posted by davekw7x

Since there is no way for you (or me) to tell what your code is doing, I am going to suggest that you put some printf() statements in strategic places.
. . .

Then:

1. Maybe you can see how far it gets before it bombs out.
2. Maybe you can see if the program is reading the user input numbers that you think it should see.

Regards,

Dave

And pointed out:

Quote:

Originally Posted by WaltP

You did not mention what happened when you did what Dave suggested. Well?

I assume you haven't done this because

Quote:

Originally Posted by noamfrie

The teacher said I can't use libary functions.

This does not mean you can't debug your code with them. You just have to remove them before turning in your assignment
.

davekw7x · #10 02-Jan-2005, 19:35

Quote:

Originally Posted by noamfrie

Hi..
Sorry but the error happens in the following program, not in the partial code I submitted earlier.
This program gets n number of "double" values, puts them in a dynamic array and prints the minimum and maximum value in the array.
Also, I can't use libary functions in "minmax" function, only in "main".
Thanks in advance...

Noam.

Look at this:

CPP / C++ / C Code:

for (p;p<p+n;p++)

The first p does nothing (but it doesn't hurt anything either).

The second p < p+n says to continue looping as long as p is less than p+n.

The third p++ increments p each time.

After incrementing, the test p < p + n always fails, so there is an infinite loop. Why does it fail? Because it is seeing if (the new value of p) is less than (the new value of p) + n. And it is

You can print out values of pointers with printf (although they don't usually tell you this at first).

CPP / C++ / C Code:

  for (; p<p+n; p++) {
    printf("pointer = %p\n", p);
    if (*p<=*min) {
      *min=*p;
    }
    if (*p>=*max) {
      *max=*p;
    }
  }

If you run this, you can abort the continuous printout with Ctrl-C.

Regards,

Dave

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problem reading to a dynamic array

CorrectionSorry..here is the correct and full code

Function gives seg. fault

problem reading to a dynamic array

**Correction**Sorry..here is the correct and full code

Function gives seg. fault

CorrectionSorry..here is the correct and full code