(Python) One variable becomes zero while assigning the value to another variable

Question

As the title say, one variable is causing another to become zero for no apparent reason. Here is my code, shorted with a ton of print statements trying to figure out what's going on.

for i in range(1, number_of_layers):
    a_nt[i] = 0.5 * ((dia / 2) ** 2) * (theta_t[i] - sin(theta_t[i]))
    print("ant 1 ", a_nt[i])

I assign a value to a_nt and later decide how that value will be used in an if state to assign it to a_t. The code goes into the else portion.

else:
    print("ant 2 ", a_nt[i])
    a_t[i] = a_nt[i]
    print("at 1 ", a_t[i])
    y_t[i] = y_nt[i]
    print("at 2 ", a_t[i])
    theta_c[i] = 0
    print("at 3 ", a_t[i])

Python prints out:

ant 1 0.005738905060148785

ant 2 0.005738905060148785

at 1 0.005738905060148785

at 2 0.005738905060148785

at 3 0

For a reason that I cannot determine, a_t becomes zero after I assign theta_c a value of zero. I'm relatively new to python, but this is driving me insane!

No, I set theta_c[i] to zero and a_t becomes zero. There aren't connected to each other. — Andrew Gulbronson, Aug 21 '15 at 00:02
Are you doing something like `some_varaible = some_list` in your code somewhere? — Padraic Cunningham, Aug 21 '15 at 00:03
Specifically, are you doing something equivalent to: `theta_c = a_t`? — jmk, Aug 21 '15 at 00:07
@AndrewGulbronson Great! I added an answer to clarify; if that solved your problem you can mark it as accepted. — jmk, Aug 21 '15 at 00:56

score 0 · Accepted Answer · answered Aug 21 '15 at 00:16

If you make an assignment like theta_c = a_t, then you're not actually copying the array, you're merely referencing the same array with another variable.

Thus, when you change theta_c, you're also changing a_t. You should be probably taking a shallow copy instead:

theta_c = list(a_t)

Of course, depending on the situation, even a shallow copy may not be sufficient, but in this case it will probably do the trick.

(Python) One variable becomes zero while assigning the value to another variable

1 Answers1