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Why the equation like this:
(for x = 5)
int result = 2*x+(++x+3)*3
equals
37 ????
not
39 (2*5+(6+3)*3=39)
(for x = 5)
int result = 2*x+(++x+3)*3
equals
37 ????
not
39 (2*5+(6+3)*3=39)
2*5+(6+3)*3 = 10 + 9*3 = 10+27 = 37.
Though I'm sure that the order of evaluation is undefined, so you shouldn't do this. Though this is ugly, so you shouldn't do this anyway..
Though I'm sure that the order of evaluation is undefined, so you shouldn't do this. Though this is ugly, so you shouldn't do this anyway..
++x is executed first, so x becomes x+1.
In this case, x becomes 6.
Then the expression is evaluated:
2*6+(6+3)*3 = 39
In this case, x becomes 6.
Then the expression is evaluated:
2*6+(6+3)*3 = 39
Sorry there is a bug, my fault
2*5+(6+3)*3=37 it should be but it is 39
I have this on the software engineering classes. In java the result is 37. Why is the result different?
2*5+(6+3)*3=37 it should be but it is 39
I have this on the software engineering classes. In java the result is 37. Why is the result different?
Well, ++x does the following: Before the entire statement is executed, add 1 to x. So before 2*x+(x+3)*3 is calculated, x becomes 5+1 (=6). Therefor, the outcome is not 2*5+(6+3)*3, but 2*6+(6+3)*3 = 37.
I don't know about Java, it's probably defined in a different way there.
I don't know about Java, it's probably defined in a different way there.
The expression
What happens is you are incrementing x (on the right side of the +) and reading from x (on the left side of the +) between sequence points. In C++ (and C) order of evaluation of operands is unsequenced. Depending on the situation, the compiler may read from x (on the left side of the +) before, after, or simultaneously with writing to x (on the right side of the +). The left x may evaluate to the value of x before ++x, the value of x after ++x, or some half-written value.
In practice, all compilers I know compile this (some, like GCC, with warnings which you should never ignore!), but the results are completely unpredictable.
Here, I tested a few:
gcc 4.6.2/linux: 39
clang++ 2.9/linux: 37
Intel C++ 11.1/linux: 39
Further reading:
Wikipedia: http://en.wikipedia.org/wiki/Sequence_point
comp.lang.c FAQ: http://c-faq.com/expr/seqpoints.html
StackOverflow FAQ: http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points
In Java, the rules for expressions evaluation are totally different, this expression is well-defined, and always evaluates 37.
2*x+(++x+3)*3 is an error, classified as "undefined behavior". It is as severe as accessing an array out of bounds: the language specification (both C++ and C) says that the compiler is free to crash and burn and not as much as warn you.What happens is you are incrementing x (on the right side of the +) and reading from x (on the left side of the +) between sequence points. In C++ (and C) order of evaluation of operands is unsequenced. Depending on the situation, the compiler may read from x (on the left side of the +) before, after, or simultaneously with writing to x (on the right side of the +). The left x may evaluate to the value of x before ++x, the value of x after ++x, or some half-written value.
In practice, all compilers I know compile this (some, like GCC, with warnings which you should never ignore!), but the results are completely unpredictable.
Here, I tested a few:
gcc 4.6.2/linux: 39
clang++ 2.9/linux: 37
Intel C++ 11.1/linux: 39
Further reading:
Wikipedia: http://en.wikipedia.org/wiki/Sequence_point
comp.lang.c FAQ: http://c-faq.com/expr/seqpoints.html
StackOverflow FAQ: http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points
In Java, the rules for expressions evaluation are totally different, this expression is well-defined, and always evaluates 37.
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