At school most of us are taught that multiplications must be done before additions in a sum.
In E it's different--there is no operator precedence, and the normal order in which the operations are performed is left-to-right, just like the expression is written.
This means that expressions like
1+3*3 do not give the results a mathematician might expect.
1+3*3 represents the number 12 in E.
This is because the addition,
1+3, is done before the multiplication, since it occurs before the multiplication.
If the multiplication were written before the addition it would be done first (like we would normally expect).
3*3+1 represents the number 10 in E and in school mathematics.
To overcome this difference we can use parentheses to group the expression.
If we'd written
1+(3*3) the result would be 10.
This is because we've forced E to do the multiplication first.
Although this may seem troublesome to begin with, it's actually a lot better than learning a lot of rules for deciding which operator is done first (in C this can be a real pain, and you usually end up writing the brackets in just to be sure!).
The logic examples above contained the expression:
(2<1) AND (-1=0)
This expression was false. If we'd left the parentheses out, it would have been:
2<1 AND -1=0
This is actually interpreted the same as:
((2<1) AND -1) = 0
Now the number -1 shouldn't really be used to represent a truth value with
AND, but we do know that
TRUE is the number -1, so E will make sense of this and the E compiler won't complain.
We will soon see how
OR really work (see 10.4.3 Bitwise
OR), but for now we'll just work out what E would calculate for this expression:
2<1can be replaced by
(FALSE AND -1) = 0
TRUEis -1 so we can replace -1 by
(FALSE AND TRUE) = 0
FALSE AND TRUEis
(FALSE) = 0
FALSEis really the number zero, so we can replace it with zero.
0 = 0
So E calculates the expression to be true. But the original expression (with parentheses) was false. Bracketing is therefore very important! It is also very easy to do correctly.
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