GlobalVariables for Client Terminal
My question is based on this lesson in the MQL4 book. https://book.mql4.com/variables/globals
The code used in the example is reproduced for convenience
My question is that (at least the first time this EA executes) it seems after init(),
will be casting an int variable type to a string value since Quantity is a global variable string?
Then Experts is incremented but does it not need to be explicitly cast back to an int in order to do this math operation?
Not a good article or example IMO.
The line you refer to will get the double value of the global variable Quantity (Quantity is a string that holds the name of the GV)
It will then store the value in the integer variable called Experts.
It will automatically cast from double to integer but you would lose data in the process because Experts is not a double.
Experts should therefore be a double.
You can get away with this if you are sure that you are storing only an integer value in the GV in the first place...
Not a good article or example IMO.
The line you refer to will get the double value of the global variable Quantity (Quantity is a string that holds the name of the GV)
It will automatically cast from double to interger but you would lose data in the process because Experts.
Experts should therefore be a double.
You can get away with this if you are sure that you are storing only an integer value in the GV in the first place...
Thanks Paul.
Note that this comes directly from the official MetaQuotes docs (book). Do you know a better example?
>>"The line you refer to will get the double value of the global variable Quantity (Quantity is a string that holds the name of the GV)"
So if it is initialized without a double value (as in the example) the double at initialization is equal to zero?
>> It will then store the value in the integer variable called Experts.
So is the value stored in 'int Experts' the value zero or the string with the GV name?
>>Experts should therefore be a double.
That is my thinking too.
>> You can get away with this if you are sure that you are storing only an integer value in the GV in the first place...
It seems clear in this example that is the case, but can you clarify how we 'get away with this'? Are the values being typecast as I thought or is it that the int Experts is somehow only taking the numeric value part of the GV?
Thanks Paul.
Note that this comes directly from the official MetaQuotes docs (book). Do you know a better example?
>>"The line you refer to will get the double value of the global variable Quantity (Quantity is a string that holds the name of the GV)"
So if it is initialized without a double value (as in the example) the double at initialization is equal to zero?
>> It will then store the value in the integer variable called Experts.
So is the value stored in 'int Experts' the value zero or the string with the GV name?
>>Experts should therefore be a double.
That is my thinking too.
>> You can get away with this if you are sure that you are storing only an integer value in the GV in the first place...
It seems clear in this example that is the case, but can you clarify how we 'get away with this'? Are the values being typecast as I thought or is it that the int Experts is somehow only taking the numeric value part of the GV?
When Experts is used in the example it is receiving the value from the GV. Only the integer part will be stored in Experts and decimal places will be lost
SO you can get away with it if you only store integer values in the GV
examples:
store 55 in GV stores as 55.0 will store 55 in Experts when recalled from the GV.
store 55.123 in GV stores as 55.123 will store 55 in Experts when recalled from the GV.
store 0.5 in GV stores as 0.5 will store 0 in Experts when recalled from the GV.
try it for yourself
When Experts is used in the example it is receiving the value from the GV. Only the integer part will be stored in Experts and decimal places will be lost
SO you can get away with it if you only store integer values in the GV
examples:
store 55 in GV stores as 55.0 will store 55 in Experts when recalled from the GV.
store 55.123 in GV stores as 55.123 will store 55 in Experts when recalled from the GV.
store 0.5 in GV stores as 0.5 will store 0 in Experts when recalled from the GV.
try it for yourself
>> Only the integer part will be stored in Experts
Ok thanks. That's the part that seems mysterious to me because
Experts = GlobalVariableGet(Quantity); seems like it should be fetching the string as
string Quantity = "GV_Quantity"; contains only string data.
>> Only the integer part will be stored in Experts
Ok thanks. That's the part that seems mysterious to me because
Experts = GlobalVariableGet(Quantity); seems like it should be fetching the string as
string Quantity = "GV_Quantity"; contains only string data.
Also, just as deinit decrements the number of experts in one step while setting the GV couldn't I combine these two lines
Experts=Experts+1; // Amount of EAs GlobalVariableSet(Quantity, Experts); // New value
into one line?
GlobalVariableSet(Quantity, Experts+1);
My last concern is that for each of the special functions (init, start, deinit) I get a warning. I assume this would be an error if strict compilation was enforced.
"function must return a value"
I have tried changing return operator to return init(); return start(); return deinit(); which compiles but causes the EA to freeze.
Also, just as deinit decrements the number of experts in one step while setting the GV couldn't I combine these two lines
into one line?
To answer my own question, no you cannot do this. It creates a bug where the previous EA will be detached when a new one is added. I will have to ponder why this is so.
My last concern is that for each of the special functions (init, start, deinit) I get a warning. I assume this would be an error if strict compilation was enforced.
"function must return a value"
I have tried changing return operator to return init(); return start(); return deinit(); which compiles but causes the EA to freeze.
The solution is to use return(0);
completed and corrected code for globalvar.mq4 follows.
//-------------------------------------------------------------------- // globalvar.mq4 //-------------------------------------------------------------------- double Experts; // holds the # of running EAs double Depo=10000.0, // initial deposit size Percent=30, // Percentage to be divided among the EAs Money; // cash value to be divided among the EAs string Quantity="GV_Quantity"; // global var name initial string value //-------------------------------------------------------------------- #property strict; //+------------------------------------------------------------------+ //| | //+------------------------------------------------------------------+ int init() // special function init start { Experts=GlobalVariableGet(Quantity); // Get the value of the global var - Experts=Experts+1; // increment the number of running EAs by this one GlobalVariableSet(Quantity, Experts); // Update the global var so that it can be accessed by other EAs Money=Depo*Percent/100/Experts; // Formula for money sharing. Alert("For EA in window ", Symbol()," allocated ",Money); return(0); // exit special function init() } //-------------------------------------------------------------------- int start() // begin special func start() { double New_Experts= GlobalVariableGet(Quantity);// every tick update the status of the number of running EAs if(Experts!=New_Experts) // check if the number of running EAs has changed { Experts=New_Experts; // if it has update the number of running EAs Money=Depo*Percent/100/Experts; // and update the money sharing scheme Alert("New value for EA ",Symbol(),": ",Money); } /* ... The main EA code would go here. The value of the variable Money is used for the max trade volume. ... */ //Sleep(100000); return(0); // exit special func start() } //-------------------------------------------------------------------- int deinit() // begin specil func deinit { Experts=GlobalVariableGet(Quantity); //update the running EA status //this is necessary to avoid a bug where an incoming tick hasn't yet updated the status via start() if(Experts ==1) // if this is the sole remaining EA running GlobalVariableDel(Quantity); //delete the global varibable Quantity as it's no longer needed else // otherwise GlobalVariableSet(Quantity, Experts-1); //decrement the running EA count by 1 Alert("EA detached from window ",Symbol()); // Notify of detached EA return(0); // exit special func deinit() } //-------------------------------------------------------------------- //+------------------------------------------------------------------+
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My question is based on this lesson in the MQL4 book. https://book.mql4.com/variables/globals
The code used in the example is reproduced for convenience
My question is that (at least the first time this EA executes) it seems after init(),
Experts=GlobalVariableGet(Quantity);will be casting an int variable type to a string value since Quantity is a global variable string?
Then Experts is incremented but does it not need to be explicitly cast back to an int in order to do this math operation?