- くだすれFORTRAN（超初心者用）その6
694 ：デフォルトの名無しさん[sage]：2014/12/20(土) 11:31:57.54 ID:9/Q8xBDD - 『if … else if … end if』と『select case …』はどっちが速いのか，ふと思って計測してみた．
cygwin gcc 4.8.3@core-i5で繰り返し計測してみたけど，時間差はばらつきの範囲内で有意な差はなさそうな感じ．
二つの構文ともCPU時間同じ，機能も同じで，いまいち，使い分け方が分からないんだよねぇ．．．
使ったコードはこれ↓
module mod_util
implicit none
integer(kind=4) :: t1, t2, t_rate, t_max, t_diff
contains
real(8) function elapsedTime()
implicit none
call SYSTEM_CLOCK(t2, t_rate, t_max)
if ( t2 < t1 ) then
t_diff = t_max - t1 + t2
else
t_diff = t2 - t1
endif
elapsedTime = t_diff/real(t_rate)
end function elapsedTime
end module mod_util
| - くだすれFORTRAN（超初心者用）その6
695 ：694[sage]：2014/12/20(土) 11:33:00.40 ID:9/Q8xBDD - program testSelectIf
use mod_util, only : t1, elapsedTime
implicit none
real (kind=8) :: randX
integer(kind=4) :: i, j, randI
#ifdef ifelse
write(*,'("# if ... else if ... ")')
#else
write(*,'("# select case")')
#endif
call init_randomSeed(972384)
call SYSTEM_CLOCK(t1)
do i = 1, 100000000
call random_number(randX)
randI = int( randX * 10, kind=4 )
#ifdef ifelse
if ( randI == 1 ) then; j = j + 1
else if ( randI == 2 ) then; j = j - 2
else if ( randI == 3 ) then; j = j + 3
else if ( randI == 4 ) then; j = j - 4
else if ( randI == 5 ) then; j = j + 5
else if ( randI == 6 ) then; j = j - 6
else if ( randI == 7 ) then; j = j + 7
else if ( randI == 8 ) then; j = j - 8
else if ( randI == 9 ) then; j = j + 9
else if ( randI ==10 ) then; j = j - 10
| - くだすれFORTRAN（超初心者用）その6
696 ：694[sage]：2014/12/20(土) 11:33:45.85 ID:9/Q8xBDD - end if
#else
select case (randI)
case ( 1 ); j = j + 1
case ( 2 ); j = j - 2
case ( 3 ); j = j + 3
case ( 4 ); j = j - 4
case ( 5 ); j = j + 5
case ( 6 ); j = j - 6
case ( 7 ); j = j + 7
case ( 8 ); j = j - 8
case ( 9 ); j = j + 9
case ( 10 ); j = j - 10
end select
#endif
end do
write(*,'("j=",I0)') j
write(*,'("#time:", F10.3)') elapsedTime()
end program testSelectIf
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