# FORTRAN problem

need help in Fortran...

This is the main loop of the program..

``````do iStep=0,nStep
write(7,*)iStep

!* Compute new temperature using FTCS scheme.
do i=1,N
if( istep==0) then  !only for t=0
tt_new(i)=250
write(7,*)tt_new(i)
else
if(i==1) then
tt_new(i)=2*coeff*(tt(i+1)+35.494)-0.036*tt(i)
write(7,*)tt(i)
else
if(i==N) then
tt_new(i)=2*coeff*(tt(i-1)+35.494)-0.036*tt(i)
write(7,*)tt(i)
else
tt_new(i) = coeff*(tt(i+1) + tt(i-1)+33.333)+(1 - 2*coeff)*tt(i)
write (7,*) tt_new(i)
end if
end if
end if
end do

do i=1,N
tt(i) = tt_new(i)     ! Reset temperature to new values
enddo
end do
``````

this is the output....

``````0
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
2.5000000E+02
1
2.5000000E+02     <--
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.6666650E+02
2.5000000E+02   <--
``````

As you can see...the programm doesn't calculate the values for the first and last node...Can you tell me why???

For `i=1` and `i=N`, you are printing `tt(i)` instead of `tt_new(i)` - the calculation is being performed correctly, but the results will not be displayed properly. Stepping through the code with a debugger is quite helpful in cases like this.

I would also suggest restructuring your `if` statement, but I wouldn't go quite as far as gimel - I think the intent would be clearer as

``````if (iStep == 0) then
! Perform actions for time 0
else
! Perform actions for time > 0
if (i == 1) then
! Perform actions for first endpoint
else if (i == N) then
! Perform actions for last endpoint
else
! Perform actions for midsection
end if
end if
``````

since you have two types of special cases - a spatial constraint (how to handle the endpoints differently) and a temporal constraint (how to handle your initial condition).

Tim Whitcomb and gimel have given good answers. You should revise your code still more, though - since there is no need to use multiple write statements (or the blank lines, or the extra END IF statements).

``````if (istep==0) then  !only for t=0
tt_new(i)=250
else if (i==1) then
tt_new(i) = 2*coeff*(tt(i+1)+35.494)-0.036*tt(i)
else if (i==N) then
tt_new(i) = 2*coeff*(tt(i-1)+35.494)-0.036*tt(i)
else
tt_new(i) = coeff*(tt(i+1) + tt(i-1)+33.333)+(1 - 2*coeff)*tt(i)
end if
write(7,*)tt_new(i)
``````

Were it my code, I'd make more liberal use of spacing in the lines, too.

Try formatting your inner `IF ELSE` in a more readable way:

``````if( istep==0) then  !only for t=0
tt_new(i)=250
write(7,*)tt_new(i)
else if(i==1) then
tt_new(i)=2*coeff*(tt(i+1)+35.494)-0.036*tt(i)
write(7,*)tt(i)
else if(i==N) then
tt_new(i)=2*coeff*(tt(i-1)+35.494)-0.036*tt(i)
write(7,*)tt(i)
else
tt_new(i) = coeff*(tt(i+1) + tt(i-1)+33.333)+(1 - 2*coeff)*tt(i)
write (7,*) tt_new(i)

end if
end if //redundant
end if //redundant
``````

This way, you see that only one `END IF` is necessary, because leading `IF` (or `ELSE IF`) clauses are closed by the matching `ELSE`.

(EDIT: the leading 2 lines of code in the question are displayed OK - thanks to Jonathan Leffler.)