# Fortran passing parameters to a function of one variable

by Thereso
Tags: fortran, function, parameters, passing, variable
 P: 1 Hi all! I have a question about fortran. So far in my coding, if i wanted to define a function ( say f(x) ) I would manually set a grid for x, and then define f for each x value, so that i would have a vector for f and a vector for x. However, I'd now like to use the 'function' command, so i can call f for ANY x. I'll do my best to explain my question: say I define a function g(x,y) by g(x,y) = x**2 + y and I want to define another function f(x), which is EQUAL to g(x,y) but with y held constant. I want to use a set of subroutines on the function f, but the problem is that all of the subroutines call f as a function of one variable and not two. So basically I want to pass y as a parameter into a function defining f(x). Later I will change y and then repeat. One way I can do it is to write y to a data file, and then read it from inside the function f: program lolz implicit none real, external :: f real y,c,x ! g(x,y) = x^2 + y !calculate g(3.0,2.0) y=2.0 open(26,file='y.dat',status='unknown') write(26,*) (y) close(26) !test x=3.0 write(*,*)"f(x) is",f(x) end function f (x) implicit none real f real x,y open(26,file='y.dat',status='unknown') read(26,*) y close(26) f= x**2.0 +y return end function f this program correctly returns "f(x) is 11.". But I'm sure that this constant opening and closing of data files is inefficient, there must be a better way of doing it? Thanks everyone :D
 P: 821 Well...I don't quite get what you REALLY are trying to achieve...why can't you just pass x AND y to the function f? ...just declare the function f to take two parameters and return one...nothing wrong with that. Other than that, you could use common block (up to fortran 77 style) to set y before you start calling the function with just x; or, you could go the module/use route (fortran 90 forward style). These two methods allow the function and the main program to share values without having to pass them around as arguments/parameters.
P: 5,379
 Quote by gsal Well...I don't quite get what you REALLY are trying to achieve...why can't you just pass x AND y to the function f? ...just declare the function f to take two parameters and return one...nothing wrong with that.
That won't work, because the OP has some existing code (which presumably can't be changed) that called f() as a function with one variable.

The way to do this in fortran 77 is use a common block, something like

function f(x)
common/fcom/y
...
g(x,y)
...
end

program main
common/fcom/y
...
y = 2.0
...
f(x)
...
y = 3.0
...
f(x)
...
end
I expect you can acheive the same effect in Fortran 90 without using COMMON, but with far more typing

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