How can we go back a previous line of code in Python like we could do in Fortran

Question

I try to convert Fortran code into Python language since it's easy to find some python tutorials online rather than with Fortran. However, the Fortran code I work with has a goto function to go back to the previous line, to make an iteration (a loop). I used the while loop Python instead of the goto in but the result value is not the same. For easy understanding, my code can be simply explained by a simple math problem, which is to calculate the square root of 2. In Fortran, it wrote:

    a = 2
    x = 1
    iter0 = 0
100 continue
    iter0 = iter0 + 1
    x1 = a / x
    err = abs(x1 - x) / abs(x)
    x = x + 0.7 * (x1 - x)
    if(err.gt.1.0e-6) goto 100

    print*, x

In Python, I wrote

a = 2
x = 1
x1 = a / x
err = abs(x1 - x) / abs(x)
iter0 = 0
while err > 1.0e-6:
    iter0 = iter0+1
    x = x + 0.7 * (x1 - x)
    x1 = a / x
    err = abs(x1 - x) / abs(x)
print(x)

So for you, is the structure of the Python code would work the same way the Fortran one did, isn't it? I found the result is not the same

Answer 1

Your result isn't the same because you're doing things in a different order to what you were doing in your original code.

There are no GOTOs in python, you've done the right thing of using a while loop

you want

a = 2
x = 1
iter0 = 0
err = 1 #do this so we enter the while loop
while err > 1.0e-6:
    iter0 = iter0+1
    x1 = a / x
    err = abs(x1 - x) / abs(x)
    x = x + 0.7 * (x1 - x)
    
print(x)

This gives 1.4142133304064006

Answer 2

Unfortunately you can't do that in python, but instead you could put all your code in a function and call the function. Or if you want to repeat something, you could of course use loops. So basically go around the "issue".