CODEWITHSUNDEEP
pythonfile-io
html92
html92

Reputation: 65

Reading from a file, storing values, and math operations

I am trying to read in from a file that contains city names, an odometer reading, and how much fuel is left. The contents of the text file are:

Kennewick 97840.4 40.0<br>
Benton_City 97861.3 37.6<br>
Prosser 97878.2 35.9<br>
Grandview 97886.5 35.0<br>
Sunnyside 97894.6 34.1<br>
Granger 97904.6 33.1<br>
Toppenish 97913.0 32.2<br>
Wapato 97921.7 31.3<br>
Union_Gap 97931.6 30.4<br>
Yakima 97935.2 30.0<br>
Selah 97939.4 29.6<br>
Ellensburg 97972.7 26.0<br>
Thorp 97981.1 25.1<br>
Rosyln 98002.8 22.6<br>
Easton 98016.1 21.1<br>
North_Bend 98058.1 16.9<br>
Preston 98067.7 16.0<br>
Issaquah 98073.6 15.4<br>
East_Gate 98082.9 14.4<br>
Mercer_Island 98088.5 13.7<br>
Seattle 98095.4 13.0<br>

Part of the assignment is subtracting the current read-through from the last read-through (both of those values stored in the variable miles).

def BusRoute():
    path = "C:\\"
    ext = ".txt"

    lines = ""
    stopname = ""
    miles = 0
    prevmiles = 0
    milest = 0
    fuel = 0.0
    mpg = 0.0
    i = 0

    filename = input("Please enter the file name. The file path [C:\...] and extension [*.txt] is not required: ")

    inFile = open(path + filename + ext, "r")

    for lines in inFile:
        stopname, miles, fuel = lines.split(" ")

        miles = float(miles)
        fuel = float(fuel)

        prevmiles = miles

        milest = miles - prevmiles

        miles += 1

        print("Since last stop:", float(milest), "miles")

My problem is, I am trying to store the last-read value of miles in a variable called prevmiles, and then subtract prevmiles (the last mileage read) from the CURRENT reading of miles, but it keeps subtracting the current miles from the current miles, obviously making the value that I want printed out 0.0.

Upvotes: 1

Views: 445

Answers (4)

D. Nusbaum
D. Nusbaum

Reputation: 65

I switched the two lines and got what I think you are looking for.

prevmiles = miles

milest = miles - prevmiles

Note - This code is for Python 2.5.4, so the syntax changes in a few parts, and because of the
on the end of the line the floats for fuel were giving errors so I commented that out.

def BusRoute():
    path = "C:\\"
    ext = ".txt"

    lines = ""
    stopname = ""
    miles = 0
    prevmiles = 0
    milest = 0
    fuel = 0.0
    mpg = 0.0
    i = 0

    filename = raw_input("Please enter the file name. The file path [C:\...] and extension [*.txt] is not required: ") #raw_input for text in Python 2.5

    inFile = open(path + filename + ext, "r")

    for lines in inFile:
        stopname, miles, fuel = lines.split(" ")

        miles = float(miles)
        ##fuel = float(fuel) (Gives me an error message)

        milest = miles - prevmiles

        prevmiles = miles            

        miles += 1

        print "Since last stop:", float(milest), "miles" #Print syntax is different in Python 2.5

BusRoute()

Gives the output:

Please enter the file name. The file path [C:\...] and extension [*.txt] is not required: name
Since last stop: 97840.4 miles
Since last stop: 20.9 miles
Since last stop: 16.9 miles
Since last stop: 8.3 miles
Since last stop: 8.10000000001 miles
Since last stop: 10.0 miles
Since last stop: 8.39999999999 miles
Since last stop: 8.7 miles
Since last stop: 9.90000000001 miles
Since last stop: 3.59999999999 miles
Since last stop: 4.2 miles
Since last stop: 33.3 miles
Since last stop: 8.40000000001 miles
Since last stop: 21.7 miles
Since last stop: 13.3 miles
Since last stop: 42.0 miles
Since last stop: 9.59999999999 miles
Since last stop: 5.90000000001 miles
Since last stop: 9.29999999999 miles
Since last stop: 5.60000000001 miles
Since last stop: 6.89999999999 miles

Why don't you try to print the miles from the file in the loop to make sure you are getting the right values?

Upvotes: 0

Ethan Furman
Ethan Furman

Reputation: 69041

Two things going on here:

  1. you need to initialize stopname, prevmiles, and fuel with the first line of the file, and
  2. you need to swap the two lines where you assign miles to prevmiles.

Here's the updated function:

def BusRoute():
    path = "C:\\"
    ext = ".txt"

    lines = ""
    stopname = ""
    miles = 0
    prevmiles = 0
    milest = 0
    fuel = 0.0
    mpg = 0.0
    i = 0

    filename = input("Please enter the file name (no extension)").strip()

    inFile = open(path + filename + ext, "r")

    stopname, prevmiles, fuel = inFile.readline().split()
    prevmiles = float(prevmiles)
    fuel = float(fuel)

    for lines in inFile:
        stopname, miles, fuel = lines.split(" ")

        miles = float(miles)
        fuel = float(fuel)

        milest = miles - prevmiles

        prevmiles = miles

        miles += 1

        print("Since last stop:", float(milest), "miles")

Upvotes: 1

Fran Borcic
Fran Borcic

Reputation: 716

You should just change the order of the operations. Do the substraction first and then assign a value to prevmiles.

Upvotes: 0

Personman
Personman

Reputation: 2323

You have 

prevmiles = miles

milest = miles - prevmiles

which guarantees that milest will be 0. Try just switching the order of those two lines.

Upvotes: 2

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