Myujth

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.")