Python code

import re

def convert(operation, value):
    value = float(value)
    if operation == "1": #Checks what conversion the user has entered, calculates it and prints out the result.
        print("\n")
        print(value, "°C is equal to ", c_to_f(value), "°F\n")
    elif operation == "2":
        print("\n")
        print(value, "°F is equal to ", f_to_c(value), "°C\n")
    elif operation == "3":
        print("\n")
        print(value, " miles is equal to", m_to_km(value), " kilometers\n")
    elif operation == "4":
        print("\n")
        print(value, " kilometers is equal to", km_to_m(value), " miles\n")
    elif operation == "5":
        print("\n")
        print(value, " pounds is equal to", lb_to_kg(value), " kilograms\n")
    elif operation == "6":
        print("\n")
        print(value, " kilograms is equal to", kg_to_lb(value), " pounds\n")
    elif operation == "7":
        print("\n")
        print("At the time of writing, ", value, "Pounds was equal to ", gbp_to_eur(value), " Euros\n")
    elif operation == "8":
        print("\n")
        print("At the time of writing, ", value, "Euros was equal to ", eur_to_gbp(value), " Pounds\n")
    else:
        print("Converter - please enter a number between 1 and 8.")
    #This would be a lot more elegant if python had a switch function...

def checkValid(argument): #This is used by main() to make sure what the user enters is a number, not an empty string or a letter.
    if argument.isalpha() or argument == "" or re.search(r"^[\w\d'-]+$", argument):
        return False
    else:
        return True

def checkNumber(argument):
    for i in range(1, len(argument)):
        if argument[i].isalpha():
            return False
            print("Ran option 1")
        else:
            return True
            print("Ran option 2")

def c_to_f(c): #Converts degrees Celsius to Farenheit
    f = (c * 1.8) + 32
    return f

def f_to_c(f): #Converts degrees Farenheit to Celsius
    c = (f - 32) * (5/9)
    return f

def m_to_km(m): #Converts miles to kilometres
    km = m / 0.62137
    return km

def km_to_m(km): #Converts kilometres to miles
    m = km * 0.62137
    return m

def lb_to_kg(lb): #Converts pounds to kilograms
    kg = lb / 2.2
    return kg

def kg_to_lb(kg): #Converts kilograms to pounds
    lb = kg * 2.2
    return lb

def gbp_to_eur(gbp): #Converts Pounds to Euros, using exchange rate as of 5th June 2015
    eur = gbp * 1.36028
    return eur

def eur_to_gbp(eur): #Converts Euros to Pounds, using exchange rate as of 5th June 2015
    gbp = eur / 1.36028
    return gbp

def main():
    #Prints out the list of available conversions. The number in brackets allows the user to select a conversion.
    print("[1] °C to °F \n[2] °F to °C \n[3] Miles to Kilometers \n[4] Kilometers to Miles")
    print("[5] Pounds to Kilograms \n[6] Kilograms to Pounds \n[7] Pounds to Euros \n[8] Euros to Pounds")
    operation = input("Enter a number to choose your conversion: ") #Allows the user to select what conversion they want.
    value = input("Enter a value to convert: ") #Allows the user to enter a value to convert.
    if checkValid(operation):
        print("Please enter a number between 1 and 8!") #If the input is not valid, it prints out an error.
    elif checkNumber(value) == False:
        print("Please enter a whole number!")
    else:
        convert(operation, value) #If no problem is detected, it runs the conversion.
    main()
main()