Original message from prad <[EMAIL PROTECTED]>: > suppose that you have 2 conditions A and B where B take a lot of effort to > determine (eg looking for a string match in a huge file). > > either A or B needs to be true before you can execute 'this'. > > the 2 if statements below are equivalent i think: > > if A or B: > do this > > if A: > do this > elseif B: > do this > > now, do they work the same way? >
Both of these forms are equivalent only in languages which short-circuit Boolean expressions (not all language implement short-circuiting...). C/C++ both support this feature. What is the difference? In languages which provide short-circuiting, sub-expressions within a complex expression will be evaluated left to right up to the point where the value of the overall expression is determined. ie. in (A or B), only one sub-expression needs to be true in order for the entire expression to be true. So, if A is true, there is no need to evaluate sub-expression B to know the value of the overall expression. However, if A is false, then B must be computed to determine the value of the overall expression. Thus in C/C++, the two forms given above are equivalent. Note that these sub-expressions within languages which support short-circuiting are evaluated from left to right. Thus, the most expensive sub-expressions should be listed last -- placed as far right as possible. OTOH without short-circuiting, both sub-expressions within (A or B) will be evaluated. This does not match your second statement: > if A: > do this > elseif B: > do this ...which executes "do this;" based upon the truth of expression A. You can consider short-circuiting of Boolean evaluation greedy, but it a feature which may also save clock cycles if the right-most sub-expressions are costly to evaluate. j
