Program not entering if statement

Question

In my python program, an if statement is not being entered. I have simplified the code to the following:

x = -5
while x < 5:
    if (x == 0):
        prin         


        

Answer 1

Behold the power of the print statement...

Let us insert a print statement...

x = -5
while x < 5:
    if (x == 0):
        print 0
    x += .01
    print x

Running this program, and inspecting the output around 0 reveals the problem:

...
-0.13
-0.12
-0.11
-0.1
-0.0900000000001
-0.0800000000001
-0.0700000000001
-0.0600000000001
-0.0500000000001
-0.0400000000001
-0.0300000000001
-0.0200000000001
-0.0100000000001
-6.23077978101e-14
0.00999999999994
0.0199999999999
0.0299999999999
0.0399999999999
0.0499999999999
0.0599999999999
0.0699999999999
0.0799999999999
0.0899999999999
0.0999999999999
0.11
0.12
0.13
...

Oh boy, it is never actually equal to zero!

Solutions:

1. Use integers. Most reliable.

   x = -500    # times this by a 100 to make it an integer-based program
   while x < 500:
     if (x == 0):
       print 0
     x += 1
   

2. Never test equality with floating point arithmetic, but rather use a range:

   delta = 0.00001 #how close do you need to get
   point = 0 #point we are interested in
   if  (point-delta) <= x <= (point+delta):
       # do stuff