为什么Fortran DO循环索引大于循环后的上限？ [英] Why is the Fortran DO loop index larger than the upper bound after the loop?

问题描述

非常简单的例子说明了我的意思：

  PROGRAM CARLA 
 IMPLICIT NONE 
！ 
 INTEGER，PARAMETER :: LM = 24，DEZASSEIS = 16 
 INTEGER :: L，VARIAVEL，SOMA 
！ 
 DO L = 1，LM 
 WRITE（*，*）'L ='，L 
 END DO 
！ 
 WRITE（*，*）'我现在是DO循环的外部'
 WRITE（*，*）'我期望L = LM = 24 ...并且SOMA = 40'
 WRITE（*，*）'L ='，L 
 SOMA = DEZASSEIS + L 
 WRITE（*，*）'SOMA ='，SOMA 
结束程序carla 
  

我预计L = LM = 24 ...而SOMA = 40 ...
但是， get：

  L = 25 
 SOMA = 41 
  

我不明白为什么一旦我们在DO循环之外，L就不会保留假设的最后一个值（因此SOMA将等于40），并保持增加...

有人可以给我一个提示吗？

解决方案

从1到24循环，
所以当它到达25时，循环结束。

  
（伪代码）
 LL = 1 
当LL < 25 
 LL = LL + 1; 
  

正如GummiV所说，不要这样做。循环是编译器优化的主要目标，不能保证循环执行后的内容。可能同样容易为0，因为在某些机器上检测LL = 0比LL> 24快，所以一次优化可以反转计数。速度并不快，但编译器人员会遇到实际问题。

I am looking inside the code of an air quality model written in fortran, and have some questions regarding the way fortran passes variables out from do-loops.

This very simple example illustrates what I mean:

   PROGRAM carla
   IMPLICIT NONE
   !
   INTEGER, PARAMETER :: LM = 24, DEZASSEIS = 16
   INTEGER            :: L, VARIAVEL, SOMA
   !
   DO L=1,LM
   WRITE(*,*) 'L = ', L
   END DO
   !
   WRITE(*,*) 'I am now ouside of the DO loop.'
   WRITE(*,*) 'I would expect L=LM=24... And SOMA=40'
   WRITE(*,*) 'L = ', L
   SOMA = DEZASSEIS + L
   WRITE(*,*) 'SOMA = ', SOMA
   END PROGRAM carla

I would expect L=LM=24... And SOMA=40... But instead I get:

   L =           25
   SOMA =           41

I don't understand why once we are outside of the DO loop, L does not keep the last value assumed (SOMA would be thus equal to 40), and keep increasing...

Can somebody give me a hint?

解决方案

Loop from 1 to 24, So when it gets to 25, loop has finished.

Think of it as 
(pseudocode)
LL = 1
While LL < 25
 LL = LL + 1;

As GummiV stated, don't do this. Loops are prime targets for compiler optimisations, no guarantee what's in there after the loop has executed. Could have just as easily been 0, one optimisation reverses the count because detecting LL = 0 is quicker than LL > 24 on some machines. Not much quicker, but compiler guys have a real problem with it'll do.

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