If element in array 1 is larger than element in array 2 then

Question

Im new to using fortran and im having some problems with an if statement. Im trying to write something along the lines of if element in array 1 is larger than element in array 2 then let h = some expression, else h = 0. I have coppied my code below

  DO I=1,NPOIN 

  IF ((X(I)*COS(0.0)+0.0*Y(I)*COS(0.0)+0.1)*0.15 .GT. ZF(I)) THEN  
    H%R(I) =0.15*((X(I)*COS(0.0))+(0.0*Y(I)*COS(0.0))+0.1)
  ELSE
    H%R(I) = 1 
  ENDIF

  ENDDO

The error im getting is:

 IF ((X(I)*COS(0.0)+0.0*Y(I)*COS(0.0)+0.1)*0.15 .GT. ZF(I)) THEN  

Error: Syntax error in IF-expression at (1)

Apologies if this is somethign really obvious but ive tried all sorts to fix it with no joy so far. Any help would be appreciated.

There may be a better way to compare the two arrays but yeah essentially im trying have ZF as my main array, but where H is larger than ZF i want the ZF value to be replaced with the H value. SO essentially i have a new array with the maximum value possible between the two arrays.

Thanks in advance

The expression compiles fine... I assumed real :: X(NPOIN), Y(NPOIN), ZF(NPOIN), R(NPOIN) and exchanged 'H%R' -> 'R'. Maybe there is something else going wrong. Could you post something compilable? — Alexander Vogt
– Alexander Vogt, Commented Jul 23, 2014 at 14:44
This expression -- (X(I)*COS(0.0)+0.0*Y(I)*COS(0.0)+0.1) -- occurs twice in your code and can be reduced to X(I)+0.1. I can't see the extra terms as the source of the problem you report but they get in your way, and ours, in trying to understand what is going on. Get rid of them. — High Performance Mark
– High Performance Mark, Commented Jul 23, 2014 at 15:13
Read about the MERGE statement in Fortran 90 and later versions of the language. — Fortranner
– Fortranner, Commented Jul 23, 2014 at 17:07

High Performance Mark · Accepted Answer · 2014-07-23 16:18:09Z

2

There may be a better way to compare the two arrays but yeah essentially im trying have ZF as my main array, but where H is larger than ZF i want the ZF value to be replaced with the H value. SO essentially i have a new array with the maximum value possible between the two arrays.

If h and zf have the same shape you can simply write:

where (h>zf) 
    zf = h
end where

If they don't have the same shape clarify what you are trying to do.

(Modern) Fortran is an array language (up to a first approximation) so think whole-array operations rather than loops. They may be faster, more likely they're not, but they're quicker to write and easier to understand. If performance isn't good enough then think about a do loop replacement.

answered Jul 23, 2014 at 16:18

High Performance Mark

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2 Comments

francescalus Over a year ago

Or even zf=MAX(zf,h)? Performance may well be even worse than the where construct but it could be clearer in intention.

High Performance Mark Over a year ago

@francescalus: yes, even better your way.

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