Problems with coding local vs global variables on mql4 | Forex Factory

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Problems with coding local vs global variables on mql4

  • #1
  • First Post: Feb 5, 2014 5:55pm Feb 5, 2014 5:55pm
Hi,
I am pretty new to mql4. I've been doing ok but recently I have been completely stumped by a problem I have run into. I have coded a simple ea which works fine when I declare a local variable for "buyorder"
int buyorder=OrderSend(Symbol(),OP_BUY,LotSize,Ask,3,0,0,NULL,MagicNumber,0, Green);

if (buyorder>0)
OrderModify(buyorder,OrderOpenPrice(),bsl,btp,0,CLR_NONE);

but if I declare buyorder as a global variable (and take away the int) it does not work. It just keeps modifying orders infinitum. For the life of me, I cannot understand why this should happen as a global variable should be able to be used anywhere in the program and it's value should be able to change. I could stick with the local variable but I want to expand the ea to make multiple orders and to use trailing stops etc and I will need to use global variables. I've included the ea below. The first version(local variable) works, the second(global) doesn't. If anyone could help me, that would be so appreciated as I've literally spent days trying to work it out. PS This also applies to the variable "sellorder".
Thanks
Attached File(s)
File Type: mq4 Lesson14x.mq4   3 KB | 597 downloads
File Type: mq4 Lesson14xx.mq4   3 KB | 586 downloads
  • #2
  • Edited 7:03pm Feb 5, 2014 6:47pm | Edited 7:03pm
Quoting Kathrynshah
Hi, I am pretty new to mql4. I've been doing ok but recently I have been completely stumped by a problem I have run into. For the life of me, I cannot understand why this...
Start by performing correct indentation of the code. The formatting is really poor.
https://en.wikipedia.org/wiki/Indent_style

Then enclose all if statements/blocks in curly braces {}.

Then you will see you are using an undefined/uninitialized variable in one of the programs. The compiler should not really compile it successfully.
Or that the variable is not updated in the other version.

I would also suggest defining all local variables at the top of each function and using curly braces to enclose all blocks. That would probably have provided feedback via the compiler that something was wrong.

TLDR:
Poor code formatting; Nesting of if statements without curly braces; Inline variable declaration -> Tricking compiler into approving bad code; Code not running as expected.
  • #3
  • Feb 5, 2014 6:51pm Feb 5, 2014 6:51pm
When you declare a local variable it is only available during the one instance that start{} is called.
Each time start{} runs it re-declares the variable.
Looking at your code the buyorder variable is only called if there are no open BUY orders in place, and a new order is opened.
If an order exists, then the buyorder variable is bypassed by the if statements and so doesn't get declared and so the order modify routine does not get invoked.
ie if buyorder doesnt get called, then buyorder remains undeclared, or has a zero or null value, as buyorder > 0 is false.
Therefore no order modification.

On the other hand, if you declare a global variable then buyorder will retain the values and so the code as written will attempt to modify each time, as buyorder > 0 is true.

Hope this helps
Fred
" check out The Traders Outpost "
  • #4
  • Feb 5, 2014 7:39pm Feb 5, 2014 7:39pm
Thanks Fred,
Thanks Xaphod,

So easy to make a mistake with this coding!!
Will definitely try and make the coding more legible.
Thanks again for your input!
  • #5
  • Feb 5, 2014 7:48pm Feb 5, 2014 7:48pm
Quoting Kathrynshah
Thanks Fred, Thanks Xaphod, So easy to make a mistake with this coding!! Will definitely try and make the coding more legible. Thanks again for your input!
You should only call OrderModify once after the OrderSend. Then it matters not whether buyorder is local or global. Local is preferable.

Inserted Code
int buyorder=0;
 
if (PreviousFast<PreviousSlow && CurrentFast>CurrentSlow) {
    if (OpenOrdersThisPair(Symbol())==0) {
        buyorder=OrderSend(Symbol(),OP_BUY,LotSize,Ask,3,0,0,NULL,MagicNumber,0, Green);
        if (buyorder>0) {
            OrderModify(buyorder,OrderOpenPrice(),bsl,btp,0,CLR_NONE);
        }
    }
}
  • #6
  • Feb 6, 2014 1:27am Feb 6, 2014 1:27am
Quoting Kathrynshah
I cannot understand why this should happen as a global variable should be able to be used anywhere
That's the exact problem. It's a scoping issue.

In both examples the "if(<buyorder/sellorder> > 0)" statements are in the wrong scope; with the difference being that the scope of "buyorder/sellorder" changes with the examples, causing the "ill-effect" in the second example. See Xaphod's sample code for proper scoping.

If I may ask, what language are you used to code in?
Return To Disorder
  • #7
  • Feb 6, 2014 2:59am Feb 6, 2014 2:59am
Quoting rockit
{quote} That's the exact problem. It's a scoping issue. In both examples the "if(<buyorder/sellorder> > 0)" statements are in the wrong scope; with the difference being that the scope of "buyorder/sellorder" changes with the examples, causing the "ill-effect" in the second example. See Xaphod's sample code for proper scoping. If I may ask, what language are you used to code in?
Hi rockit! Actually I don't know how to program, I have been reading a lot and went through the mql4 book. I understand from the examples above where I went wrong but I'm not sure what you mean by "scoping".
Thanks!
  • #8
  • Feb 6, 2014 3:45am Feb 6, 2014 3:45am
Quoting Kathrynshah
{quote} Hi rockit! Actually I don't know how to program, I have been reading a lot and went through the mql4 book. I understand from the examples above where I went wrong but I'm not sure what you mean by "scoping". Thanks!
I mean scope/context: https://en.wikipedia.org/wiki/Scope_(computer_science)

The if-statements that checks for buyorder/sellorder were placed in the wrong context, as they belonged in the context (scope) of the other (outer) if-statements.
Return To Disorder
  • #9
  • Jul 8, 2015 11:05am Jul 8, 2015 11:05am
Hello as can see the differences between a global variable and local is that the global variable can be used by more than EA at once to transfer data between EA's and Indicators, the local variable is known just by the EA declare on it.

I hope you find this useful
  • #10
  • Last Post: Jul 8, 2015 4:20pm Jul 8, 2015 4:20pm
Quoting youssef1985
Hello as can see the differences between a global variable and local is that the global variable can be used by more than EA at once to transfer data between EA's and Indicators, the local variable is known just by the EA declare on it. I hope you find this useful
The terminology is unfortunate and can be confusing to some.
You are referring to a "Global Variable of the client terminal".
The earlier posts are referring to a globalscope variable which can only be used in the EA etc where it is declared.
Please Do Not PM Me With Coding Enquiries
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