Problem with 'while' loop and list index in Python

Question

I have a problem with my loop. I would like to set nominal diameter as value from nominal_diameters list. For example my calc_diameter=130 so I'd like that nominal_diameter=125. So far in this code I am getting nominal_diameter = 0 .

What should I change in my loop?

import math

print('Welcome in PipeDiameter program. It helps you to choose the right size of pipe for'
      ' sewerage pressure systems.')
flow=round(float(input('Please enter your sewage flow [(m^3)/s]:')),2)
speed_assumed = 1
calc_diameter=round(math.sqrt(((4*flow)/((math.pi)*speed_assumed))*1000),2)
print(str(calc_diameter)+' mm')

nominal_diameters=[90,110,125,160,180,200,225,250,315,355,400,450,500,560,630,710,800]
inner_diameters=[(90-2*5,4),(110-2*6,6),(125-2*7,4),(160-2*9,5),(180-2*10,7),(200-2*11,9),
                 (225-2*13,4),(250-2*14,8),(315-2*18,7),(355-2*21,1),(400-2*23,7),(450-2*26,7),
                 (500-2*29,7),(560-2*33,2),(630-2*37,4),(710-2*42,1),(800-2*47,4)]

nominal_diameter=0
i=0

while nominal_diameters[i]<=calc_diameter<=nominal_diameters[i+1]:
    nominal_diameter=nominal_diameters[i]
    i=i+1
print(nominal_diameter)

Answer 1

An alternative to while loop, since appears to be a select the problem:

nominal_diameters=[90,110,125,160,180,200,225,250,315,355,400,450,500,560,630,710,800]

calc_diameter = 130
select_diam = min([(abs(d-calc_diameter), d) for d in nominal_diameters ])[1]

print(select_diam) #=> 125

For calc_diameter = 195 it returns 200.

How it works

By list comprehension it maps the list of nominal_diameters to a list of tuples with this couple of values: the absolute (abs()) difference between the required diameter and the nominal diameter and the nominal diameter itself ((abs(d-calc_diameter), d)).

Then it pick the minimum (min()) tuple on the list, finally from the minimum pick the second element ([1]), which is the nominal diameter.

This is for example the map for calc_diameter = 130:

[(40, 90), (20, 110), (5, 125), (30, 160), (50, 180), (70, 200), (95, 225), (120, 250), (185, 315), (225, 355), (270, 400), (320, 450), (370, 500), (430, 560), (500, 630), (580, 710), (670, 800)]
                      ^^^^^^^^

Answer 2

The problem is that if the condition is not satisfied for the first value the loop is never entered, and hence your nominal diameter will evaluate to 0. You can change your code as below

while True:
        if nominal_diameters[i] <= calc_diameter <= nominal_diameters[i + 1]:
            nominal_diameter = nominal_diameters[i]
            break
        i = i + 1

This should solve the problem