can some one explain the logic

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can some one explain the logic 

dir=((OP==OP_BUY)?1.0:-1.0);

 
Perhaps you should read the manual. 
dir=((OP==OP_BUY)?1.0:-1.0);
Equivalent to: 
if(OP==OP_BUY) dir= 1.0;
else           dir=-1.0;
Ternary Operator ?: - Operators - Language Basics - MQL4 Reference Works on either side of the equals
This works in the debugger: 
(b ? x : y) = z;
Equivalent to: 
if(b) x = z;
else  y = z;
 
thanks dear for your valuable time and fruitful comments,
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