Python negative values in variables in multiplication

Question

I'm working on a program from Guttag's book, specifically altering the code from page 33 to work with negative numbers. The program is trying to find the square root of a number by using bisection search.

My problem in short:

>>> print(ans)
-12.0
>>> print(ans**2)
144.0
>>> print(-12**2)
-144
>>> -12.0**2
-144.0

Squaring a number (in this case -12) gives the correct answer that I expect when put in IDLE as a number. However, when it comes from a variable it suddenly turns positive. Any idea why this is happening and how to fix it?

My problem in the longer version in case I have understood something else wrong too:

x = float(input("Enter an integer, which square root you want to find: "))
epsilon = 0.01
numGuesses = 0
low = min(0.0, x)
high = max(1.0, x)
ans = (high + low)/2.0
while abs(ans**2 - x) >= epsilon:
    print("low =", low, "high =", high, "ans =", ans)
    numGuesses += 1
    if (ans**2) < x:
        low = ans
    else:
        high = ans
    ans = (high + low)/2.0
print("Number of guesses =", numGuesses)
print(ans, "is close to square root of", x)

So where the while loop is deciding whether to assign the variable ans to high or low, it can't do so if working with a negative number from the beginning. If I, for example, run this program with x = -25, it will print out the following:

Enter an integer, which square root you want to find: -25
low = -25.0 high = 1.0 ans = -12.0
low = -25.0 high = -12.0 ans = -18.5

Though ans (-12) squared should be -144, which would assign that to "low" rather than to high and making the program work.

All advice appreciated.

Operator precedence. `-12**2` means `-(12**2)`. Use `(-12)**2`. — byxor, Jan 23 '18 at 15:16
Squaring a negative number should produce a positive result, not negative. Why do you think `-144` is correct? — Barmar, Jan 23 '18 at 15:18

usernamenotfound · Answer 1 · 2018-01-23T15:50:24.310

1

When you're evaluating -12**2 you're essentially evaluating -(12**2).

And also, (-12) squared is always 144.

If you want to retain the sign use -(12**2). If you don't want the sign to be retained, use (-12)**2

If you're trying to evaluate the square root of a negative number. It is going to be the same as the sqrt of the positive number multiplied by imaginary i

Just evaluate negative numbers the same way you would positive numbers, and append i to it.

In your particular case, the min function is screwing things up when you evalute negative numbers

edited Jan 23 '18 at 15:50

answered Jan 23 '18 at 15:23

usernamenotfound

1,540
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"When you're evaluating -12**2 you're essentially evaluating -(12**2)." Thanks this really cleared things. However, now I don't really know how to fix this program so that it works with both negatives and positives... – Pyth0n N00b Jan 23 '18 at 15:31
what are you expecting to get as a result of `square root of -144`? Is it `12` or `-12` or `12i`. The correct answer is `12i` – usernamenotfound Jan 23 '18 at 15:33
@Usernamenotfound Or `12j` :-) – Stefan Pochmann Jan 23 '18 at 15:33
Ok that's alright. Or they could multiply with `1j`. After all, this is Python. – Stefan Pochmann Jan 23 '18 at 15:54

Python negative values in variables in multiplication

1 Answers1