IF-THEN/ELSE vs SELECT

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When do IF-THEN/ELSE statements in a SAS DATA step always use fewer CPU resources than a SELECT statement?

(A) when there is a small selection of uniformly distributed values

(B) when there is a small selection of randomly distributed values

(C) when there is a large selection of uniformly distributed values

(D) when there is a large selection of randomly distributed values

The answer is (A). But why? Any thoughts? Thanks a lot.

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关键词：Select Elect Then else ect Select else

太大的选择需要输入很多条件，浪费时间也浪费运行时间

原作者年代有点久远，我在备考时候也遇到了这个问题，然后就google了一下。以后还有人需要问答，大家看一下吧，可靠答案。
貌似意思就是如果是那种0/1逻辑条件的情况下，SELECT 比IF更有效。
When you have a long series of mutually exclusive conditions, using a SELECT group is more efficient than using a series of IF-THEN statements because CPU time is reduced. Large numbers of conditions make a SELECT group more efficient than IF-THEN/ELSE statements because CPU time is reduced. SELECT groups also make the program easier to read and debug.