Python - Average of three values in a while true loop

Question

I'm developing a python script on a raspberry pi with a distance sensor. My code is working and i can reach the distance but I want to avoid some false positive/negative so I would like to get the average of the last three data.

Here is my code:

while True:
        distance= misura() # GETTING THE REAL DISTANCE IN CM
        print "Distanza : %.2f" % distanza

        avg_distance = <something> # <- HERE I NEED SOMETHING ELEGANT

        if (avg_distance > 30):
            print "NOT PRESENT"
        else:
            print "PRESENT"

        time.sleep(1.5)

I would like some function (maybe based on a list?) that returns the average of the last three (or n) distances

IMPORTANT: I don't want to store ALL the values because this script will run for days and days

Answer 1

I would use a collections.deque.

from collections import deque
SAMPLE = 3
data = deque(maxlen=SAMPLE)
while True:
    distance = misura() # GETTING THE REAL DISTANCE IN CM
    print "Distanza : %.2f" % distanza

    data.append(distance)
    avg_distance = sum(data)/SAMPLE # <- HERE I NEED SOMETHING ELEGANT

    ... # rest as before

The deque is fixed length so you're only sampling the latest three. If you want to change the sampling period you only have to change one variable.

If you want to run this in a tight loop (ie, removing the 1.5s sleep), you could hold the sum in a variable to remove the O(n) operation each loop (even if it's not really an issue for only 3 elements). Something like:

total -= data.popleft()
total += distance

data.append(distance)
average = total / 3 

Answer 2

If it is your objective to only remember the last three measurements and find their average, consider using a collections.deque object so that when you add to this object, it will only remember the last three elements that were added to it. You can find the average of the contents in the deque at each iteration:

from collections import deque
l = deque(maxlen=3) # New
while True:
    distance= misura() # GETTING THE REAL DISTANCE IN CM
    print "Distanza : %.2f" % distanza
    l.append(distance) # New
    #avg_distance = <something> # <- HERE I NEED SOMETHING ELEGANT
    avg_distance = sum(l) / len(l) # Compute average of the last three eleemnts

    if (avg_distance > 30):
        print "NOT PRESENT"
    else:
        print "PRESENT"

    time.sleep(1.5)

The nice thing about this is that when the deque is full and you append an item to it, it will remove the oldest added element in the deque and add in the newest element you're trying to add.

Answer 3

Well maybe you need just create a list like list = [misura(),misura(),misura()] to get three values respectively and pass it to a function like

def mean(list) :
    Total = 0
    For item in list :
        Total += item
    Return total /len(list)

Answer 4

You could take advantage of the fact that mutable default arguments are only created once, to store the last three values measured:

def last_three_average(val, last_three=[]):
    last_three.append(val)
    last_three = last_three[-3:]
    return sum(last_three) / len(last_three)

for idx in range(1, 10, 1):
    print(f'{idx}: {last_three_average(idx)}')

output:

1: 1.0
2: 1.5
3: 2.0
4: 3.0
5: 4.0
6: 5.0
7: 6.0
8: 7.0
9: 8.0

Or, using a deque i/o a list:

from collections import deque

def last_three_average(val, last_three=deque(maxlen=3)):
    last_three.append(val)
    return sum(last_three) / len(last_three)

Answer 5

You can apply some math and recalculate average each new iteration.

passed = 0
avg_distance = 0
while True:
    passed += 1
    distance = misura()
    print("Distanza : %.2f" % distanza)

    avg_distance = ((passed - 1) * avg_distance + distance) / passed

    if (avg_distance > 30):
        print("NOT PRESENT")
    else:
        print("PRESENT")

    time.sleep(1.5)

Answer 6

If type(distance) is a list then you can simply calculate the average for last 3 points by

distance = distance[-3:] # This will remove the extra data and get only last 3 values

average = sum(distance) / len(distance)

Answer 7

First create a list (outside the loop):

distances = [misura(),misura(),misura()]

Then your loop should be like this:

while True:
    distances.append(misura())
    print "Distanza : %.2f" % distances[-1]

    distances = distances[1:] # this way you keep only the last 3 values
    avg_distance = sum(distances) / len(distances)

    if (avg_distance > 30):
        print "NOT PRESENT"
    else:
        print "PRESENT"

    time.sleep(1.5)

Answer 8

See another version with the logic of just saving the distance with only 3 elements in a loop. The last value is 'pop' out so we only have 3 elements.

my mockup below;

distancedata=[0,0,0]
for i in range(20):
        if len(distancedata) >= 3: 
            a=distancedata.pop(0)
        distancedata.append(float(i))
        print i,  distancedata,sum(distancedata) / len(distancedata)

The results here:

0 [0, 0, 0.0] 0.0
1 [0, 0.0, 1.0] 0.333333333333
2 [0.0, 1.0, 2.0] 1.0
3 [1.0, 2.0, 3.0] 2.0
4 [2.0, 3.0, 4.0] 3.0
5 [3.0, 4.0, 5.0] 4.0
6 [4.0, 5.0, 6.0] 5.0
7 [5.0, 6.0, 7.0] 6.0
8 [6.0, 7.0, 8.0] 7.0
9 [7.0, 8.0, 9.0] 8.0
10 [8.0, 9.0, 10.0] 9.0
11 [9.0, 10.0, 11.0] 10.0
12 [10.0, 11.0, 12.0] 11.0
13 [11.0, 12.0, 13.0] 12.0
14 [12.0, 13.0, 14.0] 13.0
15 [13.0, 14.0, 15.0] 14.0
16 [14.0, 15.0, 16.0] 15.0
17 [15.0, 16.0, 17.0] 16.0
18 [16.0, 17.0, 18.0] 17.0
19 [17.0, 18.0, 19.0] 18.0

Answer 9

try to restrict the length of the list

_distance = []
if _distance.size == 3:
       _distance = _distance[-2:]
       _distance.append(distance)
else:
   _distance.append(distance)

 avg_distance = sum(distance[-3:]) / len(distance[-3:])