Logical Error(bug) in the code for page replacement algorithm

Question

In the following code for optimal page replacement algorithm, the scanf under first for loop is not stopping at the value of 'n' if i enter integer inputs. It works perfectly if the input is repetitive.

EX: If the value of n is 7, and page in put is 1 1 1 1 1 1 1 The code works fine But if the page input is 1 2 3 4 3 2 1 it never stops taking input.

I have even tried explicit declaration in the for loop like i<7 but it still doesn't work Here is my code:

#include<stdio.h>
#include<stdlib.h>
static int MAX =999;
//to find out maximum
int maxim(int a[], int size)
    {
        if(size <= 0) return -1;
        int i, max_i = 0;
        int max = a[0];
        for(i = 1; i < size; ++i)
        {
            if(a[i] > max)
            {
                max = a[i];
                max_i = i;
            }
        }
        return max_i;
    }
int main()
{
    int i,j,k,n,temp_i=0,maximum,temp_j,
    count=0,l,pageFault,
    page[100],frame[50],position[50];
    printf("Enter the number of pages\n");
    scanf("%d",&n);
    printf("\nEnter the number of frames\n");
    scanf("%d",&k);
    printf("\nEnter the page sequence\n");
    //The problem is in the following two line
    for(i=0;i<n;i++)
        scanf("%d",&page[i]);
    for(i=0;i<k;i++)
        frame[i]=-1;
    i=0;
    while(i<k)
    {
        frame[i]=page[i];
        i++;
        count+=1;
    }
    for(i=k;i<n;i++)
    {
        for(j=0;j<k;j++)
        {
            if(frame[j]==page[i])
            break;
        }
        if(j==k)
        {
            temp_i=i;temp_j=0;
            for(l=0;l<k;l++)
                position[l]=MAX;
            while(temp_i<n)
            {   
                while(temp_j<k)
                {
                    if(page[temp_i]==frame[temp_j])
                    {
                        position[temp_j]=temp_i;
                        temp_j++;
                    }
                    //temp_i++;
                }
                temp_i++;
            }
            maximum=maxim(position,k);
            frame[maximum]=page[i];
            count+=1;
        }
    }
    printf("\nThe final frames status is:\n");
    for(i=0;i<k;i++)
    printf("%d",frame[i]); 
    pageFault=count;
    printf("The number of page fault is %d\n", pageFault);
    printf("The hit ratio is %lf\n",(float)pageFault/n); 
}

Try put bellow your scanf this command: `fflush(stdin);`. Should look like: `scanf("%d",&page[i]); fflush(stdin);` — Esdras Xavier, Feb 25 '19 at 17:43
Can you create a [mcve]? Are you sure the error is not elsewhere? — Johnny Mopp, Feb 25 '19 at 17:46
Did you mean to reset `temp_j=0;` just before the `while(temp_j — Weather Vane, Feb 25 '19 at 17:53
Add `printf("I/O done\n"); fflush(stdout);` after the input loop. Does that appear on the screen in both the OK and failure cases? My suspicion is that you have a problem elsewhere. If the problem is in the input, you don't need an awful lot of the code you show to demonstrate the problem. (Incidentally, repeating `int` is not harmful — don't spread three lines of variable definitions with a single `int`.) — Jonathan Leffler, Feb 25 '19 at 17:58
_[…time passes…]_ I added the `printf()` I suggested and it worked for me with `n` at 7, `k` at 4, and input `1 2 3 4 3 2 1` on a single line. It said: `The final frames status is:` — `12343The number of page fault is 5` — `The hit ratio is 0.714286`. I worry about the fact that 5 frames are printed where I only allowed it to use 4. With luck, the repeated 3 is a consequence of that — otherwise, it is a bad problem to have. (I think there should be just 4 page faults for the data given — there are other problems for you to solve as yet.) — Jonathan Leffler, Feb 25 '19 at 18:04
@EsdrasXavier: Be cautious about suggesting `fflush(stdin)`. See [Using `fflush(stdin)`](https://stackoverflow.com/questions/2979209/using-fflushstdin) for the details, but it only works reliably on Windows systems and is not a general purpose solution. It isn't even particularly desirable; it would have stopped me entering `1 2 3 4 3 2 1` on a single line — which would be annoying as it would have thrown away the data I carefully typed. — Jonathan Leffler, Feb 25 '19 at 18:06
@JonathanLeffler Please try it with n=12,k=3 or 4, page sequence: 1 2 3 4 1 2 5 1 2 3 4 5 — Satyam Srivastava, Feb 25 '19 at 18:26
Can you guys please check that i f I am failing at the logical level. I want to implement optimal page replacement algorithm. Thank you — Satyam Srivastava, Feb 25 '19 at 18:28

bruno · Answer 1 · 2019-02-25T18:40:16.637

EX: If the value of n is 7, and page in put is 1 1 1 1 1 1 1 The code works fine But if the page input is 1 2 3 4 3 2 1 it never stops taking input.

For me it is not getting input without ending, but loops forever because of :

while(temp_j<k)
{
  if(page[temp_i]==frame[temp_j])
  {
     position[temp_j]=temp_i;
     temp_j++;
  }
  //temp_i++;
}

if (page[temp_i]==frame[temp_j]) is false there is no change allowing (temp_j<k) to become false and the while never ends.

In case of

pi@raspberrypi:/tmp $ ./a.out
Enter the number of pages
7

Enter the number of frames
1

Enter the page sequence
1 1 1 1 1 1 1

The final frames status is:
1The number of page fault is 1
The hit ratio is 0.142857

you do not enter in the while forever case, but

pi@raspberrypi:/tmp $ ./a.out
Enter the number of pages
7

Enter the number of frames
1

Enter the page sequence
1 2 3 4 5 6 7

you enter in it

It seems more logic to always do temp_j++; like that :

        while(temp_i<n)
        {   
            temp_j=0;
            while(temp_j<k)
            {
                if(page[temp_i]==frame[temp_j])
                {
                    position[temp_j]=temp_i;
                }
                temp_j++;
            }
            temp_i++;
        }

also reseting temp_j of course

now the execution is :

Enter the number of pages
12

Enter the number of frames
3

Enter the page sequence
1 2 3 4 1 2 5 1 2 3 4 5

The final frames status is:
523The number of page fault is 9
The hit ratio is 0.750000

If I uncomment the temp_i++ accompanied with an else and remove the temp_i++ below it. The input problem is so;ved but it is going me segementation fault(core dumped) — Satyam Srivastava, Feb 25 '19 at 18:18
@SatyamSrivastava probably because `temp_i++` become too large and you go out of _page_, you also need to check `temp_i` value in the while — bruno, Feb 25 '19 at 18:19
Thats what I did in my original code, by then thats giving me a problem in input as you pointed out. Any suggestions? — Satyam Srivastava, Feb 25 '19 at 18:31

score -1 · Accepted Answer · answered Feb 25 '19 at 18:42

I found answer in two ways. One as suggested by @bruno in the original answer

and the other as following by using for loops instead of while

if(j==k)
    {
        temp_i=i;temp_j=0;
        for(l=0;l<k;l++)
            position[l]=MAX;
        for(temp_i=i;temp_i<n;temp_i++)
        {   
            for(temp_j=0;temp_j<k;temp_j++)
            {
             if(page[temp_i]==frame[temp_j])
                {
                    position[temp_j]=temp_i;
                }
             //temp_i++;
            }
        }
        maximum=maxim(position,k);
        frame[maximum]=page[i];
        count+=1;
    }

Logical Error(bug) in the code for page replacement algorithm

2 Answers2