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Lab 19: Binary Numbers

As usual, create a directory to hold today's files. All programs that you write today should be stored in this directory. 

$ cd ~/cs120/labs
$ mkdir lab19
$ cd lab19

Parse Int

We have been using the input command for a few weeks now. And what you should have noticed is that it always returns a string. However, there are times where you want an actual integer instead of a string. We have been using the built in int to perform this conversion. You now possess the tools to write this function on your own. Let's practice string manipulation by writing your own function to parse integers.

Details

Create a function parse_int(integer_string) in a file called parse_int.py. This function takes as a parameter a string of numeric characters. It should return an integer, the decimal integer representation of the number in the string. Your function should assume that every character in the string is numeric. DO NOT USE THE BUILT IN int FUNCTION.

Make sure your program handles all necessary cases gracefully. What additional test cases should you check?

Sample Test Cases

Function Parameters Expected Output
"0" 0
"1" 1
"13" 13

Hint

  • You will need to use a for loop to iterate through the various characters of the string parameter. You can use either a for i in range... loop, or a for character in integer_string loop.

  • You can use the ord function to get the ASCII value of the numeric character. Because of the way the ASCII table is setup, we can simply subtract the ASCII value for '0' to get the actual digit this character represents

  • You will, once again, be using the accumulator pattern for this activity. However, for each character in the string, you need to shift the values in the integer one place to the left. You can do this by miltiplying the current values in the accumulator by 10.

Challenge

Your code above likely assumed, rightfully so, that the parameter string was positive. However, negative base-10 integers are still valid integers. Re-work your parse_int function so that it can handle a leading minus sign (-), and return the correct numeric value for the input.

Parse Binary

As we discussed in the lecture today, binary numbers are very important to the functionality of our computers today. Even in programs, it is very common to actually inspect or input a number in binary for use later in the program. Again, the int function can accomplish this already. However, we will practice again by doing the conversions manually.

Details

Create a function parse_binary(binary_string) in a file called parse_binary.py. This function takes as a parameter a string of '1' and '0' characters. It should return an integer, the decimal integer representation of the binary number in the string. DO NOT USE THE BUILT IN int FUNCTION.

Make sure your program handles all necessary cases gracefully. What additional test cases should you check?

Sample Test Cases

Function Parameters Expected Output
"0" 0
"1" 1
"11" 3

Hint

  • You are going to use the same format as parse_int
  • The only real change you need to make is to update the accumulator by 2 each iteration, as opposed to 10.

Challenge

The function assumes that all input strings represent a positive integer. Unlike decimal numbers, binary numbers typically do not use the minus sign, instead they use 0 and 1 and two's compliment. Modify the function to also convert strings that represent a negative number using two's complement.