Nrthugu

I'm trying to create a scientific calculator for an assignment. I'm looking for a bit of help with python syntax, and I think my design and pseudo code are doing well, but for whatever reason python isn't having any of my syntactical issues. Here is the code I have for converting binary to decimal.

I need the code to reprompt when the input is invalid, but when it does reprompt, it gets stuck in a loop of reprompting and won't give me any way out.

def bintodec(var):

    power = (len(var) + 1)

    value = ' '

    while True:



        var = input('Give a number to convert from binary to decimal: ')

        for x in range(len(var)):



            if (ord(var[x]) == 49):

                power -= 1

                value += x * (2 ** power)



            if (ord(var[x]) == 48):

                power -= 1

                value += x * (2 ** power)



            if power == -1:

                break



            else:

                boo = True



    return value

Any help is greatly appreciated!

This is pretty classic. Reading in a base is easier than writing in one.

def bintodec(var):

    assert set(var) <= set("01") # Just check that we only have 0s and 1s

    assert isinstance(var, str) # Checks that var is a string



    result = 0

    for character in var: # character will be each character of var, from left to rigth

        digitvalue = ord(character) - ord("0")

        result *= 2

        result += digitvalue



    return result

Ok, how does it work ?

Well, it reads the value from left to right. digitvalue will contain 1 or 0. For each digit we read, if it is 0, there is nothing to add the result (so result += digitvalue adds indeed 0), but we still need take into account that there is one more 0 at the end of the number.

Now, in base 10, adding a zero to the end makes a number 10 times as big. This is the same in base 2. Adding a zero at the end makes a number twice as big. This is why we multiply it by 2.

Finally, if digitvalue is 1 instead of 0, we need to add 1 to the number and result += digitvalue does it.

Note: Just for things to be clear, the two for loops below are equivalent.

for character in var:

    pass # pass does nothing



for i in range(len(var)):

    character = var[i]

    pass

@JayF.:

Is there any way to reprompt without using assert?

I suppose you want to reprompt if the input is incorrect. You need to use a loop for that:

while True:

    var = input()

    if set(var) <= set("01"):

        print(bintodec(var))

        break # Remove this `break` statement if you want to reprompt forever

    else:

        # print("The input must consist of only 0s and 1s.")

        pass # `pass` does nothing.

If you leave the asserts in the bintodec function, it can be done in a more pythonic way, using exception handling:

while True:

    var = input()

    try:

        print(bintodec(var))

        break

    except AssertionError:

        print("The input must consist of only 0s and 1s.")