While loop not existing as expected

Question

I have the code below where in the function neural_net_trainer(p, a), the while-loop is meant to end/exit when the value of p is equal to the value of a. However, the loop keeps running infinitely.

Instead of while-loop I tried for-loop, to loop it 30 times (running the 'update' function each time), and 'p' eventually becomes equal to 'a'. But I need to use while-loop instead because I don't know how many times the loop will run. It's whenever 'p' will become equal to 'a'.

# 'p' represent prediction. 'a' represent actual output of Neural network.
#-- improving Neural Network prediction (using derivative of cost function) --
def slope(p, a):
    return 2 * (p - a)

# --- Create the Update rule function (to correct prediction) --- 
def update(p, a): 
    p = p - 0.1 * slope(p, a)
    return p

# -- Train network - minimising the cost function until prediction is equal to actual output
# - In while loop, if 'p' is not equal to 'a', then 'update' function will increment/decrement 'p' accordingly, until 'p' == 'a'. While-loop should then exit.  
def neural_net_trainer(p, a):
    print('prediction = ' + str('{:.2f}'.format(p)) + '. Actual Output = ' + str('{:.2f}'.format(a)))
    while p != a:
        p = update(p, a)
        print('prediction = ' + str('{:.2f}'.format(p)))
    else:
        print('Prediction = ' + str('{:.2f}'.format(p)) + ', and actual output = ' + str('{:.2f}'.format(a)) + '. Our Neural Network is trained on this dataset.')

# - Testing 'neural_net_trainer' function
a = 4
p = 3
neural_net_trainer(p, a)

Basically the code simulates functions of a very simple neural network, where the functions receive two values - p (the predicted output) and a (the actual output). In the code - the slope function is a formula needed to correct the prediction (to become equal to the actual output). update function does the updating/correction. And neural_net_trainer function uses a while-loop to run update function enough times, until p (prediction) is equal to a (actual output). At which point the while-loop should exit.

Any help would be really appreciated.

Alessandro Cosentino · Accepted Answer · 2019-01-27 18:50:33Z

3

The problem here is that you're checking for exact equality and p - a never reaches the exact 0. Try a weaker condition as a guard of the while loop, for example:

THRESHOLD = 1e-5

while abs(p - a) > THRESHOLD:

answered Jan 27, 2019 at 18:50

Alessandro Cosentino

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Daniel Roethlisberger · Accepted Answer · 2019-01-27 18:48:45Z

2

p is not reaching 4.0, it is merely reaching 3.999999999999999..., which gets rounded to 4.00 when you use {:.2f} to format it. p != a remains True because 4.0 != 3.999999999999999.

This is more of an algorithmic problem than a python problem, but perhaps round() will come in handy to solve this based on p approaching a instead of a fixed number of iterations.

answered Jan 27, 2019 at 18:48

Daniel Roethlisberger

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2 Comments

Hazzaldo Over a year ago

Super, thank you very much Daniel, and for the very quick reply. It's great to know the round function exists. I did use the other answer from Alessandro, because I find it easier to control the threshold number for equality. Though I'll experiment with round() as well. I wanted to mark both of your answers as valid, but SO doesn't seem to allow me.

Daniel Roethlisberger Over a year ago

I like his reply better too, his threshold approach is much better than using round()

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