How to store the last n values of the propery?

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Hi everyone!
I am reading integer value from PLC(you can think from file which periodically updated ) in every 5 seconds(or by a click a button) and displaying on the screen and use that value to do some processes. I have a problem that, if the user whom can directly control the PLC machine mistakenly reset(zeroing) the PLC value. How can I backup the last read value or How can I make user to select last read value among last read values list. What is the best practice to store last 10 or 20 values of the property. Can yo give me some advices ??

Best Regards

Posted 25-Jan-16 5:54am

Ermany

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PIEBALDconsult 25-Jan-16 12:06pm

In a stack maybe?

Sergey Alexandrovich Kryukov 25-Jan-16 12:06pm

Is that your property or the property you can redefine? Then it would not be a problem. "Property" is a key here.
What is your problem, the storage or capturing the event of setting the value?
—SA

Ermany 25-Jan-16 14:47pm

Hi Sergey !
Actually both of them is my problem but I will try to handle the storage problem by the help of OriginalGriff1 or ppolymorphe solutions I will try both of them. What would you suggest me to setting the value ?

BillWoodruff 25-Jan-16 15:52pm

Interesting question.

"the user whom can directly control the PLC machine mistakenly reset(zeroing) the PLC value"

How do you detect this ? After the user resets it ?

Ermany 26-Jan-16 3:41am

Actually user wanted me to do this. They said some times mistakenly I initialize the PLC value.For easy use at least not on the HMI screen but on your desktop application I want to see last read value(also user can write that value or can select from the last read vales list) + PLC current value. byt the way I am reading length of a stripe for quality control.I am not planning to write values to PLC.

Ralf Meier 26-Jan-16 6:54am

I'm not quiet sure if you have the right approach ...
Nevertheless ... if you select a last_Value from a List (or something like this) you have to write this value to the PLC to make it possible that it could work in future with it.

Ermany 26-Jan-16 7:11am

Yes you are right but my client doesn't want me to do write operation to PLC so I chose that way. Thank you for your interest

Ralf Meier 26-Jan-16 0:53am

What kind of value is it which is resetted or mistakely changed in the PLC ?
Is it a Parameter-Value ? Or a kind of Counter-Value ?
How do you prevent actually a mistakely change of a value by the HMI ?
I think, for that kind of Problem a value-stack is not the right approach ...

Ermany 26-Jan-16 3:47am

I am reading Weight and Length of a stripe from PLC's HMI screen.It is like counter value(It may be wrong answer I couldn't understand I you want to learn from me sorry). I am not planning to prevent , user will realize that he made a mistake and he will add the last read value(rough,approximate) for the Length to continue from there.

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OriginalGriff · Accepted Answer · 2016-01-25T08:11:00

Solution 1

The Queue<T> class would be a natural fit here, if it would only automatically dequeue when it reached it's limit!
But...It's easy to do:

C#

public class FixedQueue<T> : Queue<T>
    {
    private int _fixedSize = int.MinValue;
    public int FixedSize
        {
        get { return _fixedSize; }
        set
            {
            while (value < _fixedSize  && Count > value)
                {
                Dequeue();
                }
            _fixedSize = value;
            }
        }
    public FixedQueue(int fixedSize)
        : base(fixedSize)
        {
        _fixedSize = fixedSize;
        }
    public new void Enqueue(T item)
        {
        if (Count >= FixedSize)
            {
            Dequeue();
            }
        base.Enqueue(item);
        }
    }

Posted 25-Jan-16 8:11am

OriginalGriff

Comments

Ermany 25-Jan-16 14:56pm

Thank you so much for your detailed answer !
Best Regards

BillWoodruff 25-Jan-16 15:54pm

I'm curious: in your code the 'setter of the Public 'FixedSize Property is never called; in what circumstances would the programmer set it ?

If I assume the programmer would set it to down-size the Queue, would it be necessary to call 'TrimExcess after the Queue had been trimmed ?

OriginalGriff 25-Jan-16 15:56pm

If he wanted to change the number of elements it stored.
It's unlikely, but...

BillWoodruff 25-Jan-16 16:39pm

Hi, Just added to my query a question about the possible requirement for a call to 'TrimExcess.

I note the MSDN docs on 'Queue constructed with a 'Size value are a little less than "transparent:"

"If the size of the collection can be estimated, specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Queue<t>."

cheers, your perennial (in fact, antiquated) student, Bill

OriginalGriff 25-Jan-16 17:06pm

Yes, a Queue is implemented in the same way as a list: it's internally an array, and when it expands it's copied to a new array. Setting the initial size defines the original array dimension to avoid the overhead of repeated allocate-copy operations.
There's a description of it as a list here:
http://www.codeproject.com/Articles/870013/List-T-Is-it-really-as-efficient-as-you-probably-t
The Queue class avoids the Insert/Delete problems because it ask works from indexes and you can only Queue and Dequeue from the head and tail, so there's no shuffling of elements involved.

Patrice T · Accepted Answer · 2016-01-25T08:34:00

Solution 2

I would use an array to store the previous values.
In order to reduce The amount of work to manage the array, I would use it like a circular buffer, the advantage is that it avoid moving the data as the oldest value is replaced by a new one.
Circular buffer - Wikipedia, the free encyclopedia[^]

Posted 25-Jan-16 8:34am

Patrice T

Comments

Ermany 25-Jan-16 14:54pm

Thank you mate I will read about it and if I can do I will adapt it to my project

BillWoodruff 25-Jan-16 16:00pm

fyi: a Queue is a data-structure manages an Array.

MSDN: "implements a generic queue as a circular array."

Ermany 26-Jan-16 3:51am

Thanks for your interest.
I am using the code of Rob Blackbourn's "Circular Buffer in C#" post. Here is the link http://geekswithblogs.net/blackrob/archive/2014/09/01/circular-buffer-in-c.aspx
Best regards