Fortran 95 code won't run

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1. Oct 23, 2015

NicolasPan

1. The problem statement, all variables and given/known data. 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!

3. The attempt at a solution.
Code (Fortran):

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'
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

Last edited by a moderator: Oct 23, 2015
2. Oct 23, 2015

SteamKing

Staff Emeritus
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.

3. Oct 23, 2015

Staff: Mentor

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: ).

4. Oct 23, 2015

NicolasPan

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