Why Does My Fortran 95 Code for Calculating a Piecewise Function Fail?

[NicolasPan]

Homework Statement

. [/B]Hello all ! I am currently trying to make my fortran95 code run in vain.I would really appreciate it if someone could lend my a hand since I am a starter in programming.So the problem goes like that:Create a program capable of calculating the following function: f(x)=1/x when x<3 , x+2(x)^2 when 3≤x≤8 and finally sqrt(x-10) when x>8
The problem is that my code will pop up a window 'Error' and it is indicated that the problem is with the square root .Thanks in advance!

The Attempt at a Solution

.[/B]
Mod note: Added code tags.

program function
implicit none
integer::st=0
real::x,y1,y2,y3
  print*,'Welcome to the function  y=1/x , x<3 &
& 2.*x**2.+x   3<=x<=8               and sqrt(x-10),x>8 '
  print*,'input to x'
  read (*,*,iostat=st) x
        pr:if (st /=0) then
          print*,'possible error'
     stop
          else pr
         ex:if (abs(x-3)<1e-6 .or. abs(x)<1e-6) then
      
      y1=1./x
        print*,y1

        else ex

      in: if (abs(x-3)>=1e-6 .and. abs(x-8)<=1e-6) then
        y2=(2*x)**2+x
        print*,y2

    else in
    
           out:if (abs(x-8)>1e-6 .and. abs(x-10)<1e-6) then
           print*,'impossible'
           stop

  
        else out
           br:if (abs(x-10)>=1e-6) then
        
             y3=sqrt(x-10)
        print*,y3
        
        
        end if br
        end if out
        end if in
        end if ex
        end if pr
        end program

 

[SteamKing]

Homework Statement

. [/B]Hello all ! I am currently trying to make my fortran95 code run in vain.I would really appreciate it if someone could lend my a hand since I am a starter in programming.So the problem goes like that:Create a program capable of calculating the following function: f(x)=1/x when x<3 , x+2(x)^2 when 3≤x≤8 and finally sqrt(x-10) when x>8
The problem is that my code will pop up a window 'Error' and it is indicated that the problem is with the square root .Thanks in advance!

The Attempt at a Solution

.
[/B]
Mod note: Added code tags.

program function
implicit none
integer::st=0
real::x,y1,y2,y3
  print*,'Welcome to the function  y=1/x , x<3 &
& 2.*x**2.+x   3<=x<=8               and sqrt(x-10),x>8 '
  print*,'input to x'
  read (*,*,iostat=st) x
        pr:if (st /=0) then
          print*,'possible error'
     stop
          else pr
         ex:if (abs(x-3)<1e-6 .or. abs(x)<1e-6) then
    
      y1=1./x
        print*,y1

        else ex

      in: if (abs(x-3)>=1e-6 .and. abs(x-8)<=1e-6) then
        y2=(2*x)**2+x
        print*,y2

    else in
  
           out:if (abs(x-8)>1e-6 .and. abs(x-10)<1e-6) then
           print*,'impossible'
           stop        else out
           br:if (abs(x-10)>=1e-6) then
      
             y3=sqrt(x-10)
        print*,y3
      
      
        end if br
        end if out
        end if in
        end if ex
        end if pr
        end program

When including source code with your post, please include the source code inside [ CODE ] [ /CODE ] tags (omit the spaces before and after the word CODE.

Your problems with this program stem from the arithmetic of the assignment, not so much the code.

You want to evaluate the following expressions:

1. f(x)=1/x when x<3 ,
2. x+2(x)^2 when 3≤x≤8 and
3. sqrt(x-10) when x>8

Do you notice anything about expressions 1 and 3 and their respective ranges of x which might cause a problem in evaluation? Did you add any statements to your program which might detect these problems and alert the user?

The way you have coded expression 2 will evaluate f = x + (2*x)² rather than f = x + 2 x², which is a significant difference.

Also, the first statement in your code, PROGRAM FUNCTION, might cause confusion in the compiler, since FUNCTION is a reserved word in Fortran. Better to rename the program to something besides FUNCTION.

 

[Mark44]

I have added code tags around your code. Please use them, as SteamKing had described in his reply.

Your code can be greatly simplified as follows:

1. Ask user for an input value
2. If input value < 3, use first branch of function definition; i.e., f(x) = 1/x. Be sure to check for improper input to that function.
3. Else if 3 <= input value <= 8, use second branch of function definition.
4. Else if input value > 8, use third branch.
5. Display calculated function value.

You really need to get rid of all those labels (e.g. in: ).

 

[NicolasPan]

Thank you very much both SteamKing and Mark44 I really appreciate your help!,and excuse me for not adding tags around my code