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Bug in simple code, please help to pinpoint

  • #1
  • First Post: Aug 7, 2013 10:33pm Aug 7, 2013 10:33pm
Hi,

I found a problem in below coding but don't know what causes it. Can someone please point out for me? Below are the coding:-

//Coding start here ------------------------------------------------------------
#property copyright "Copyright 2013, MetaQuotes Software Corp."
#property link "http://www.metaquotes.net"

int start()
{
double douleftprice, dourightprice;
dourightprice = 1.32184;
douleftprice = 1.32187 - 0.00003;
if (douleftprice == dourightprice)
Comment ("Match, douleftprice = ", DoubleToStr(douleftprice,8), ", dourightprice = ", DoubleToStr(dourightprice,8));
else
Comment ("Not Match, douleftprice = ", DoubleToStr(douleftprice,8), ", dourightprice = ", DoubleToStr(dourightprice,8));
return(0);
}
//Coding end here --------------------------------------------------


The result are "Not Match, douleftprice = 1.32184000, dourightprice = 1.32184000"

I got no idea how come it doesn't match. But if I change douleftprice from "1.32187 - 0.00003" to "1.32181 + 0.00003", it match. How come? What did I do wrong? Did I forgot to include anything?
  • #2
  • Aug 7, 2013 11:47pm Aug 7, 2013 11:47pm
Hours and hours I have wasted on exactly this sort of thing.
It doesn't make sense does it?
There's quite a thread about it on the MQL4 forum.
Hopefully Raptor will see this as he knows the answers.
I get over it by
if(MathAbs(double1-double2)<=2*Point)
A suggestion from WHRoeder. if I remember correctly.
Please Do Not PM Me With Coding Enquiries
  • #3
  • Edited 1:02am Aug 8, 2013 12:47am | Edited 1:02am
Quoting anyremote
Hi, I found a problem in below coding but don't know what causes it. Can someone please point out for me? Below are the coding:- //Coding start here ------------------------------------------------------------ #property copyright "Copyright 2013, MetaQuotes Software Corp." #property link "http://www.metaquotes.net" int start() { double douleftprice, dourightprice; dourightprice = 1.32184; douleftprice = 1.32187 - 0.00003; if (douleftprice == dourightprice) Comment ("Match, douleftprice = ", DoubleToStr(douleftprice,8), ", dourightprice =...

comparing doubles directly is not a great idea, you should be able to resolve the problem using the NormalizeDouble() function on both numbers and if you don't really need the 5 digit then compare at the 4th. Otherwise have a look at this article that explains the issue and some other solutions.

http://forum.mql4.com/45053
  • #4
  • Aug 8, 2013 12:31pm Aug 8, 2013 12:31pm
Using your code, I got around it by doing:

dourightprice = NormalizeDouble((1.32187 - 0.00003), 5);

Whole code (I just added a for loop):

//+------------------------------------------------------------------+
//| Test.mq4 |
//| Copyright 2012, MetaQuotes Software Corp. |
//| http://www.metaquotes.net |
//+------------------------------------------------------------------+
#property copyright "Copyright 2012, MetaQuotes Software Corp."
#property link "http://www.metaquotes.net"

#property indicator_chart_window
//--- input parameters
//extern double leftprice = 1.33;
//extern double rightprice = 1.33;
//+------------------------------------------------------------------+
//| Custom indicator initialization function |
//+------------------------------------------------------------------+
int init()
{
//---- indicators
//----
return(0);
}
//+------------------------------------------------------------------+
//| Custom indicator deinitialization function |
//+------------------------------------------------------------------+
int deinit()
{
//----

//----
return(0);
}
//+------------------------------------------------------------------+
//| Custom indicator iteration function |
//+------------------------------------------------------------------+
int start()
{
int i, counted_bars=IndicatorCounted();

for (i = 0; i < counted_bars; i++)
{

double leftprice, rightprice;
leftprice = 1.32184;
rightprice = NormalizeDouble((1.32187 - 0.00003), 5);
if (leftprice == rightprice)
Comment ("Match, douleftprice = ", DoubleToStr(leftprice,8), ", dourightprice = ", DoubleToStr(rightprice,8));
else
Comment ("Not Match, douleftprice = ", DoubleToStr(leftprice,8), ", dourightprice = ", DoubleToStr(rightprice,8));
}


//----

//----
return(0);
}
//+------------------------------------------------------------------+


Screen shot:
Attached Image (click to enlarge)
Click to Enlarge Name: test.png Size: 33 KB
  • #5
  • Aug 13, 2013 12:50am Aug 13, 2013 12:50am
Quoting Gumrai
Hours and hours I have wasted on exactly this sort of thing. It doesn't make sense does it? There's quite a thread about it on the MQL4 forum. Hopefully Raptor will see this as he knows the answers. I get over it by if(MathAbs(double1-double2)<=2*Point) A suggestion from WHRoeder. if I remember correctly.
I also used around 6-8 hours just to find out that this is the bug that causes my indicator to malfunction. I won't use if(MathAbs(double1-double2)<=2*Point) though as I couldn't understand the logic of the algorithm for comparing both double. NormalizeDouble() work well for me. Thanks.

Quoting MaxDoom
{quote} comparing doubles directly is not a great idea, you should be able to resolve the problem using the NormalizeDouble() function on both numbers and if you don't really need the 5 digit then compare at the 4th. Otherwise have a look at this article that explains the issue and some other solutions. http://forum.mql4.com/45053


Thanks for the NormalizeDouble() suggestion. It work well for me and will save me from hours on figuring how to overcome this bug. Thanks a lot, man.


Quoting Bilstein
Using your code, I got around it by doing: dourightprice = NormalizeDouble((1.32187 - 0.00003), 5); Whole code (I just added a for loop): //+------------------------------------------------------------------+ //| Test.mq4 | //| Copyright 2012, MetaQuotes Software Corp. | //| http://www.metaquotes.net | //+------------------------------------------------------------------+ #property copyright "Copyright 2012, MetaQuotes Software Corp." #property link "http://www.metaquotes.net" #property indicator_chart_window //--- input parameters //extern...


Yup, NormalizeDouble() doing just fine for me as well. Thanks for the reply.

  • #6
  • Aug 13, 2013 5:57am Aug 13, 2013 5:57am
Quoting anyremote
{quote} I also used around 6-8 hours just to find out that this is the bug that causes my indicator to malfunction.
It's not a bug, it's just the way that floating point values work . . .

http://en.wikipedia.org/wiki/Floating_point

http://www.cprogramming.com/tutorial...sentation.html
20 pips a day isn't too much to ask . . .
  • #7
  • Aug 13, 2013 7:05am Aug 13, 2013 7:05am
Quoting Gumrai
Hours and hours I have wasted on exactly this sort of thing. It doesn't make sense does it? There's quite a thread about it on the MQL4 forum. Hopefully Raptor will see this as he knows the answers. I get over it by if(MathAbs(double1-double2)<=2*Point) A suggestion from WHRoeder. if I remember correctly.
WHRoeder suggestion is : MathAbs(price1-price2)<=Point/2. The reasoning is that Point is the smallest increment in price, so as long as the difference is smaller than 0.5 Point, it is safe to assume that both prices are numerically equal
  • #8
  • Aug 13, 2013 8:23am Aug 13, 2013 8:23am
Quoting sangmane
{quote} WHRoeder suggestion is : MathAbs(price1-price2)<=Point/2. The reasoning is that Point is the smallest increment in price, so as long as the difference is smaller than 0.5 Point, it is safe to assume that both prices are numerically equal
Yes, you are right.
I was thinking of when comparing doubles to check that they weren't the same.
It was to do with modifying stoplosses.
My code checked that the amended stoploss was not the same as the OrderStopLoss, before trying to modify, but the print was telling me that i was trying to modify the SL with the same figure.
The 2 Point thing was a belt and braces way of making sure that it didn't happen.
Sorry about my misunderstanding.
Please Do Not PM Me With Coding Enquiries
  • #9
  • Last Post: Sep 5, 2013 11:31am Sep 5, 2013 11:31am
Hi SANGMANE.

I have created a strategy in Forex strategy builder and i want to try it on MT4 as an automatic EA. I donīt want to use Forex strategy trader. Could you help me please solve this issue. The strategy is werry simple. Of course you can profit from the strategy. If you will help me contact me on email [email protected] please and i will give you all info you need to code the EA. Have a nice trading day.
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