Fortran Program - Calculate the Function

[Haleemos]

Homework Statement

F(x) = exp(-a x^2) cos bx
for -100 =< x >= 100 , a=13 and b=7.

Homework Equations

...

The Attempt at a Solution

program main

implicit double precision (A-H,O-Z)
implicit integer (I-N)

DO I = -100,100
X = I * 1D0

F = DEXP(-A * X**2) * DCOS(B * X)

A = 13
B = 7

PRINT*,F
ENDDO

STOP
END

 

[DrClaude]

Have you tested the program to see if it works?

Because it will not give the right result. Check the order in which you are doing things.

 

[Haleemos]

the program works

but the answers all zero

 

[DrClaude]

but the answers all zero

Are you sure about that? Because it is not the case when I run it.

You only made one mistake, which is that a and b are set too late. And they shouldn't be inside the loop: set them at the beginning of the program. Might as well also set them directly as double precision

a = 13.d0
b = 7.d0

Also, instead of multiplying by 1D0, set I to double precision using

X = dble(I)

This is more efficient as there will not be an unnecessary multiplication every time.

I also suggest that you print out both x and F, which makes it easier to check the output and plot the results.