Can't double a 0.1*3 value, but 1*0.3 is fine

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Please help me with this problem, don't know why i can't double a value by this 

double a=0.1*3;

But if i use this it will work well

double a=1*0.3;

Even i change the code to this

double a=0.1+0.1+0.1;

it still doesn't work well.


Never have this problem before, don't know why, anyone have the same proble before? 


I print the value of "a", it's the same, both of them is 0.3.

But it just doesn't work in this code  if i double it by 0.1*3  

if(OrderLots()==a)
 

Floating-point has an infinite number of decimals, it's your not understanding floating-point and that some numbers can't be represented exactly. (like 1/10.)
          Double-precision floating-point format - Wikipedia, the free encyclopedia

See also The == operand. - MQL4 programming forum

If you want to see the correct number of digits, convert it to a string with the correct/wanted accuracy.
          question about decima of marketinfo() - MQL4 programming forum 2016.05.18

 

try this:

double a = 0.1*3.0;
 
William Roeder:

Floating-point has an infinite number of decimals, it's your not understanding floating-point and that some numbers can't be represented exactly. (like 1/10.)
          Double-precision floating-point format - Wikipedia, the free encyclopedia

See also The == operand. - MQL4 programming forum

If you want to see the correct number of digits, convert it to a string with the correct/wanted accuracy.
          question about decima of marketinfo() - MQL4 programming forum 2016.05.18

Thanks a lot, looked through all the threads but still a little confused about how to compare double.

Finally i change the code into this and it work

        
        double Lots=0.1;
        int divide=Lots*100;
        double a=3.0/divide;

        if(OrderLots()==a)

Now this can work well.

But another problem about double type value can't be solved whatever i changed the code

      double Lots;
      int m;
      double q=AccountEquity();
      if(q<=500)
      {
         Lots=0.1;
      }
      else
      if(q>500)
      {
         m=(AccountEquity()-500)/20;
         double n=m/100;
         Lots=0.1+n;
      }

The OrderLots is alway 0.1, never add lots when AccountEquity is bigger than 500, please check where can i change the code. I've used NormalizeDouble but still the same result.

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