Thread: Using pointers instead of subscripts for arrays

For this, I was supposed to build a program that read arrays from pointers instead of subscripts. At the highlighted location, I was having an error that said: [Warning] initialization discards qualifiers from pointer target type. How do I get the EOF message to skip for the second array it reads in? Besides that, there is just something wrong with the program that makes it so every array it compares says they are the same, even when they are clearly different. Also, when I print out what the user enters, some of the values are skewed. I included a run at the bottom so you can see what I mean.

Code:

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

#define MAX 10

int main()
{
  int read_eles(int a[], int max, int sent);
  void print_eles(const int a[], int v);
  int table_diff(const int a[], const int b[], int m, int n);
  
  int a[MAX];
  int b[MAX];
  int m;
  int n;
  int index;
  int sent = -999;
  
  m = read_eles(a, MAX, sent);
  n = read_eles(b, MAX, sent);
  print_eles(a, m);
  print_eles(b, n);
  index = table_diff(a, b, m, n);
  if (index = -1)
     printf("The tables are the same.\n");
  else      // index != -1
     printf("The tables are different.\n");
  
  system("PAUSE");	
  return 0;
}

int read_eles(int a[], int max, int sent)
{
  int value;
  int r;
  int *ptr = a;
  int *endptr = ptr + max;
  
  while((r = scanf("%i", &value)) != EOF && value != sent && ptr < endptr)
  {
     if (r == 0)            // invalid character
     {
        printf("Nonnumeic data has been entered.\n");
        while (getchar() != '\n')   // flush invalid character
           ;
     }
     else
        *ptr++ = value;
  }
  if (r == 1 && value != sent)
     printf("Too many elements were entered.");
  else if(r == EOF) 
     printf("End of file reached before encountering the"
     " sentinel value %i.\n", sent);
  
  return ptr - a;
}

void print_eles(const int a[], int v)     // prints the table of values
{
   int *ptr = a;
   int *endptr = a + v;
   
   while (ptr++ < endptr)
      printf("%i\n", *ptr);
}

int table_diff(const int a[], const int b[], int m, int n)
{
   const int *aptr = a;
   const int *bptr = b;
   const int *aendptr = a + m;
   const int *bendptr = b + n;
   
   while (aptr < aendptr && bptr < bendptr && *aptr == *bptr)
   {
      aptr++;
      bptr++;
      
   }
   if (aptr == aendptr && bptr == bendptr)
      return -1;
   else      // if *aptr != *bptr
      return aptr - a;
}

Code:

1 2 3 -999
1 2 4
^Z
End of file reached before encountering the sentinel value -999.
2
3
2009252579
2
4
4006969
The tables are the same.
Press any key to continue . . .

Find the difference in declartion of variables which are highlighted in ur program

Originally Posted by Sir Andus

For this, I was supposed to build a program that read arrays from pointers instead of subscripts. At the highlighted location, I was having an error that said: [Warning] initialization discards qualifiers from pointer target type. How do I get the EOF message to skip for the second array it reads in? Besides that, there is just something wrong with the program that makes it so every array it compares says they are the same, even when they are clearly different. Also, when I print out what the user enters, some of the values are skewed. I included a run at the bottom so you can see what I mean.

Code:

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

#define MAX 10

int main()
{
  int read_eles(int a[], int max, int sent);
  void print_eles(const int a[], int v);
  int table_diff(const int a[], const int b[], int m, int n);
  
  int a[MAX];
  int b[MAX];
  int m;
  int n;
  int index;
  int sent = -999;
  
  m = read_eles(a, MAX, sent);
  n = read_eles(b, MAX, sent);
  print_eles(a, m);
  print_eles(b, n);
  index = table_diff(a, b, m, n);
  if (index = -1)
     printf("The tables are the same.\n");
  else      // index != -1
     printf("The tables are different.\n");
  
  system("PAUSE");	
  return 0;
}

int read_eles(int a[], int max, int sent)
{
  int value;
  int r;
  int *ptr = a;
  int *endptr = ptr + max;
  
  while((r = scanf("%i", &value)) != EOF && value != sent && ptr < endptr)
  {
     if (r == 0)            // invalid character
     {
        printf("Nonnumeic data has been entered.\n");
        while (getchar() != '\n')   // flush invalid character
           ;
     }
     else
        *ptr++ = value;
  }
  if (r == 1 && value != sent)
     printf("Too many elements were entered.");
  else if(r == EOF) 
     printf("End of file reached before encountering the"
     " sentinel value %i.\n", sent);
  
  return ptr - a;
}

void print_eles(const int a[], int v)     // prints the table of values
{
   int *ptr = a;
   int *endptr = a + v;
   
   while (ptr++ < endptr)
      printf("%i\n", *ptr);
}

int table_diff(const int a[], const int b[], int m, int n)
{
   const int *aptr = a;
   const int *bptr = b;
   const int *aendptr = a + m;
   const int *bendptr = b + n;
   
   while (aptr < aendptr && bptr < bendptr && *aptr == *bptr)
   {
      aptr++;
      bptr++;
      
   }
   if (aptr == aendptr && bptr == bendptr)
      return -1;
   else      // if *aptr != *bptr
      return aptr - a;
}

Code:

1 2 3 -999
1 2 4
^Z
End of file reached before encountering the sentinel value -999.
2
3
2009252579
2
4
4006969
The tables are the same.
Press any key to continue . . .

Sorry about previous suggestion i didn't go through ur code clearly. what i observe is u r trying to assign a value to iundex in if statement. Please opbserve the the bolded statement.
So to avoid such silly mistakes try to code in this way

if( -1 == index) By doing in this way we can avoid some mistakes

For ex. if i placed the assign operator instead of equal the compiler will throw an error message.

if( -1 = index) --> leads to an error

if( index = -1) --> no error and the value assigned to index

Originally Posted by Prelude

>I was having an error that said: [Warning] initialization discards qualifiers from pointer target type.
That's not an error, it's a warning. It's telling you that you're using an int pointer to point to a const int pointer. You can fix it by doing this:

Code:

const int *ptr = a;
const int *endptr = a + v;

>How do I get the EOF message to skip for the second array it reads in?
Can you clarify this?

>every array it compares says they are the same, even when they are clearly different

Code:

if (index = -1)

No, every array is clearly the same according to your test.

>when I print out what the user enters, some of the values are skewed.
That's because your print algorithm is off by one:

Code:

while (ptr++ < endptr)
    printf("%i\n", *ptr);

Should be:

Code:

while (ptr < endptr)
    printf("%i\n", *ptr++);

Ok, everything is in working order except for the EOF occuring before the sentinel value message. Since I call the read_eles function twice, the second time it calls it, the sentinel value has already been used up, so it prints the message no matter what. The only viable solution I can come up with by myself is to create a whole new function, but that seems very inefficient and I'm sure that has to be another way.

You are overloading the read_eles function. Rather than writing a whole new function, you just have to tell the exiting function whether you are interested in hitting EOF before sentinel (1st pass) or not (2nd pass). Perhaps an extra argument to the function, a simple boolean flag indicating whether its important that you've hit EOF before reading the sentinel. It would be "true" for the 1st pass and "false" for the second pass.