My program isnt correctly comparing values in python

Question

I wrote this code and it's supposed to compare the set of line values with the nominal value. The user is supposed to input a percentage value that will compare the lineValue to the nominal value. If the lineValue is within the percentage given for the nominal value it will pass as true.

My program will only return true only if the lineValue number is exactly the nominal value. All other values are failures even if it is within the percentage that the user inputs. Does anyone see an error in my code that would prevent the numbers from being registered as true?

nominalValue=470
print "Nominal Resistor Value: " , nominalValue
lineValue = [470, 358, 324, 234, 687,460]


user_Input=raw_input("Please Enter a Tolerance %: ")
if user_Input.isdigit():
    tolerance = int(user_Input)
    if tolerance <=20 and tolerance >=1:
        print "Tolerance Level:", user_Input
        percentageHigh = (tolerance/100.0 + 1.00)
        percentageLow = (1.00 - tolerance/100.0)
        print percentageHigh
        print percentageLow
        highNominal = nominalValue*percentageHigh
        lowNominal = nominalValue*percentageLow
        print highNominal
        print lowNominal
        for seriesInput in lineValue:
            if (percentageHigh*seriesInput) <= highNominal and (percentageLow*seriesInput) >= lowNominal:           
                print seriesInput,"Pass"
                print percentageHigh*seriesInput

            else: 
                print seriesInput,"Fail"
                print percentageLow*seriesInput
    else:
        print "Please enter a value between 1-20"
else:
    print "Please enter a number for a percent value"

Could you include in your question an example run of the program? — Bill Lynch
– Bill Lynch, Commented Oct 6, 2014 at 21:27
Python has a great syntax for comparisons: 1 <= tolerance <= 20 means the same thing as you have written for checking the tolerance number. — Greg Hewgill
– Greg Hewgill, Commented Oct 6, 2014 at 21:34
Er, you are multiplying both sides of your comparison by percentageHigh (and Low) -- you don't need to do that... — Jesse W at Z - Given up on SE
– Jesse W at Z - Given up on SE, Commented Oct 6, 2014 at 21:35

Mark Tolonen · Accepted Answer · 2014-10-06 21:36:10Z

2

You've already computed highNominal and lowNominal, so you want this line:

if seriesInput <= highNominal and seriesInput >= lowNominal:

or as @GregHewgill pointed out:

if lowNominal <= seriesInput <= highNominal:

answered Oct 6, 2014 at 21:36

Mark Tolonen

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2 Comments

Greg Hewgill Over a year ago

Again, lowNominal <= seriesInput <= highNominal means the same thing and might be easier to read.

Mark Tolonen Over a year ago

@GregHewgill, easier to read but I always seem to type it wrong the first time.

CDspace · Accepted Answer · 2014-10-06 21:40:47Z

In your check

if (percentageHigh*seriesInput) <= highNominal and (percentageLow*seriesInput) >= lowNominal:

you are checking if the range of the seriesInput is within the range around the nominal value, which by definition it will never be. You want to simply check if the value of the seriesInput is within the range around the nominalValue, like so

if seriesInput <= highNominal and seriesInput >= lowNominal:

Put more visually, you are checking this:

nominalValue range
|-----x-----|

seriesInput range, never going to be inside the nominalValue range
    |----y----|

seriesInput value, within range of nominalValue
|-----x--y--|

Jesse W at Z - Given up on SE · Accepted Answer · 2014-10-06 22:16:46Z

1

Your current code is asking:

if (percentageHigh*seriesInput) <= highNominal and 
   (percentageLow*seriesInput)  >= lowNominal:

But

highNominal = nominalValue*percentageHigh
lowNominal = nominalValue*percentageLow

So your comparison is equivalent to:

if (percentageHigh*seriesInput) <= nominalValue*percentageHigh and 
   (percentageLow*seriesInput)  >= nominalValue*percentageLow:

Which simplifies down to:

if seriesInput <= nominalValue and 
   seriesInput => nominalValue:

As should be clear from seeing it that way, that can only be true when seriesInput == nominalValue, as seriesInput can't be both larger and smaller than nominalValue!

edited Oct 6, 2014 at 22:16

answered Oct 6, 2014 at 21:39

Jesse W at Z - Given up on SE

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2 Comments

Mark Tolonen Over a year ago

That's not it. OP is comparing highNominal and lowNominal not nominalValue.

Jesse W at Z - Given up on SE Over a year ago

highNominal is nominalValue*percentageHigh. The comparison is (percentageHigh*seriesInput) <= highNominal which is equivalent to (percentageHigh*seriesInput) <= (nominalValue*percentageHigh), which when you divide by percentageHigh on both sides, gives you what I said.

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