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The following is the (semi) finished version of a simple Mils to Degrees conversion calculator I built in Python 2.7 as a learning exercise. I am new to Python and still working out the kinks. The premise is, the user chooses to convert mils to degrees or vice versa. The user enters an azimuth of the chosen unit and it is converted to the other. If the user chooses an invalid menu option he is notified as such. If he enters an azimuth outside of the range (1-360 for degrees, 1-6400 for mils) he is notified and must start over. The problem is that if the user enters an invalid azimuth, say 365 degrees, he is kicked back to the menu. Is there a way to return to the previous step in the if/else loop? I've searched the forums and documentation and it doesn't appear to be possible. Also, since I'm new to Python, how can make this code more efficient? Do I need to have the if and elif statements in the function definitions or can I combine them without creating errors or redundant output? I've not been successful in doing so. All input is welcome and appreciated.

#Mil-Deg.py
#Simple Mils / Degrees Conversion Calculator
#6 JAN 2013

#Note: 6400 mils / 360 degrees in a circle. 17.78 mils to one degree.

import sys
import math
import winsound

#Define menu function
def menu():                     

    print 
    print " Mils / Degrees Conversion Calculator"
    print "-" * 38
    print 
    print "Options: "
    print "1. Degrees to Mils"
    print 
    print "2. Mils to Degrees"
    print 
    print "3. Quit"
    print "-" * 20
    print 
    return input ("Choose your option: ")
    print 

#Define mils to degrees function    
def m2d(a):
    if a <= 6400 and a >= 1:    #get user input within given range      
        b = 17.78
        c = round((a / b),0)    #convert and round to nearest degree  
        if c > 359 or c < 1:    #change 0 degrees to 360               
            c = 360
            #Output
            print
            print a, "mils =", int (c), "degrees"    
        else:
            print
            print a, "mils =", int (c), "degrees"
    elif a > -1 and a < 1:      #change 0 mils to 6400
        a = 6400
        b = 17.78
        c = round((a / b), 0)   #math, same as above
        if c > 359 or c < 1:    #0 to 360, same as above              
            c = 360
            #Output
            print
            print 6400, "mils =", int (c), "degrees"   
        else:
            print
            print a, "mils =", int (c), "degrees"
    else:
        #warning, mulligan
        winsound.Beep(440, 500)
        print 
        print "*** There are only 6400 mils in a circle. Try again. ***"
    print
    print "-" * 38
    print

#define degrees to mils function        
def d2m(b):
    if b <= 360 and b > 0:          #get user input within given range
        a = 17.78
        c = round((b * a),0)        #convert and round to nearest mil
        if c >= 6400:               #set limit to 6400 mils
            c = 6400
            #Output
            print
            print b, "degrees =", int (c), "mils"
        else:
            print
            print b, "degrees =", int (c), "mils"
    elif b > -1 and b < 1:          #change 0 to 360 degrees
        b = 360
        a = 17.78
        c = round((b * a),0)        #math, same as above
        if c >= 6400:
            c = 6400
            #Output
            print
            print 360, "degrees =", int (c), "mils"
        else:
            print
            print b, "degrees =", int (c), "mils"
    else:
        #warning
        winsound.Beep(440, 500)
        print 
        print "*** There are only 360 degrees in a circle. Try again. ***"
    print
    print "-" * 38
    print


#Begin program
loop = 1
choice = 0
while loop == 1:
    choice = menu()                 #Menu function call
    if choice == 1:                 #If user chooses degrees to mils:
        #Output
        print
        print "Enter your azimuth in degrees (1 - 360)"
        print 
        d2m(input("Degrees: "))     #function call

    elif choice == 2:               #If user chooses mils to degrees:
        #Output
        print
        print "Enter your azimuth in mils (1 - 6400)"
        print 
        m2d(input("Mils: "))        #function call

    elif choice == 3:               #If user chooses quit, exit program
        loop = 0

    elif choice != 1 and choice != 2 and choice != 3:   #if user makes invalid menu choice:
        #warning
        winsound.Beep(440, 500)
        print
        print
        print
        print choice, "is not a valid choice."
        print
        print "Please choose from the available options."
        print
        print

print 
print "-" * 38
print 
print "Thank you for using Mil-Deg.py"
print
print
outhwest
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Betastate
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  • `if-else` is not a loop, try `while` loop here. – Ashwini Chaudhary Jan 09 '13 at 05:34
  • You have a `print` after your `return` statement, which will never get executed. You should also use `raw_input`, it is a lot safer than `input`. `raw_input` will take the input and convert it to a string. – Burhan Khalid Jan 09 '13 at 06:07
  • @AshwiniChaudhary, thanks. I have a little C experience. I should have realized that. I had tried while but I put it in the function definitions instead of the execution block and it resulted in an infinite loop. Now that I put it in the right place it works as intended. Thank you. – Betastate Jan 09 '13 at 17:15
  • @BurhanKhalid, I changed input to raw_input as you suggested and made the necessary changes so it recognizes the proper menu choice. However, now I get an unsupported operand error when I enter the azimuth. I understand the reasoning, I just need to figure out the implementation. Thank you. – Betastate Jan 09 '13 at 17:18
  • Got it! a = int(a) prior to the if statement. – Betastate Jan 09 '13 at 18:17

2 Answers2

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As Ashwini mentioned in the comment, your if-else conditional block isn't actually a loop, so using while loops should answer the program flow part of your question.

In terms of elif's and general elegance, consider this method of handling multiple options using a dictionary of functions; it's certainly not necessary with your handful of options here, but it might give you some practice with how Python handles dictionaries and functions as classes.

Stylistically, I'd recommend the use of the line-break escape sequence \n instead of all these extraneous print statements.

outhwest
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  • Cameron, the while loop did indeed solve that issue as I mentioned above. I'm still working on the rest of your suggestions. I need to read over the dictionary thing a few more times. Thanks for the advice. I appreciate it. – Betastate Jan 09 '13 at 17:25
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I cleaned up your code a bit and made the following changes:

  1. Fixed your print statements
  2. Fixed your method names. Method names should be descriptive. Characters are free to type, so don't be stingy with typing out complete words.
  3. Your methods were printing output and duplicating logic, so I cleaned them up to just return the values instead. This way you can use these methods in other programs later on.
  4. Added a helper method to check user's input (to stop repeating/copy-pasting the same code again).
  5. I used the format function of strings to make it easy to print the calculations.

Here is the result:

import sys
import math
import winsound

#Define menu function
def menu():                     
    '''This function prints the main menu''' # this is a docstring for your method
    print "\nMils / Degrees Conversion Calculator"
    print "-" * 38
    print "\nOptions: "
    print "1. Degrees to Mils\n"
    print "2. Mils to Degrees\n"
    print "3. Quit\n"
    print "-" * 20
    print "\n"


#Define mils to degrees function    
def mils_to_degrees(a): # use method names that make sense
    '''This function takes mils and converts them into degrees'''
    b = 17.78
    c = round((a / b),0)    #convert and round to nearest degree  
    if c > 359 or c < 1:    #change 0 degrees to 360               
        c = 360
    return (a,int(c))

#define degrees to mils function        
def degrees_to_mils(b):
    '''This method converts degrees to mils'''
    a = 17.78
    if b == 0:
        b = 360
    c = round((b * a),0)        #math, same as above
    if c > 6400:               #set limit to 6400 mils
        c = 6400
    return (b,c)

def check_input(user_input):
    '''Checks if the input is a number'''
    try:
        converted_number = int(user_input)
    except ValueError:
        return -1
    return converted_number

#Begin program
loop = True
while loop:
    menu()
    choice = int(raw_input("Choose your option: "))
    if choice not in [1,2,3]:
        winsound.Beep(440, 500)
        print choice, "is not a valid choice.\n"
        print "Please choose from the available options.\n"
        menu()
        choice = int(raw_input('Choose your option: '))

    if choice == 1:
        print "\nEnter your azimuth in degrees (0 - 360)\n"
        degrees = check_input(raw_input('Degrees: '))
        if degrees > 360 or degrees < 0:
            print 'Please enter a value between 0 and 360'
        else:
            result = degrees_to_mils(degrees) # function call
            print '{} degrees = {} mils'.format(*result)
            print "-" * 38

    elif choice == 2:               #If user chooses mils to degrees:
        print "\nEnter your azimuth in mils (1 - 6400)\n"
        mils = check_input(raw_input('Mils: '))
        if mils > 6400 or mils < 1:
            print 'Please enter a value between 1 and 6400'
        else:
            result = mils_to_degrees(mils)        #function call
            print '{} mils = {} degrees'.format(*result)
            print "-" * 38

    elif choice == 3: 
        loop = False

print "\n"
print "-" * 38
print "\nThank you for using Mil-Deg.py\n\n"
Burhan Khalid
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  • wow. That is a huge improvement over my original code. I'm sure it was very easy for you but you've given me quite a bit to study and absorb. Thanks a lot, man. I really appreciate the help! – Betastate Jan 13 '13 at 05:24