HW 0

Running and Debugging Python Programs

Part one.  Debugging exercises

Download each of the following Python programs. Find out what the program does, and if it is buggy fix it.

• one.py: This is a text-based program. Your job is to find out what it does.
• two.py: This buggy program is supposed to ask the user to enter two numbers and then print their product.
• three.py: This is supposed to ask for your name and then print a message about your name.
• four.py: This is supposed to work just like program four.
• five.py: This is supposed to ask for your name and age and then print a message using these facts.

Part two.  A programming exercise

This program asks the user for two strings: a search string and a target string. It replies with a statement about whether the target string contains the search string, and if so the starting index of it. This is a very important problem -- the entire Human Genome project is based on recent algorithms that allow one to search for strings that are millions of characters long. Since this is your first program, we will do a very simple version making use of some built-in Python string methods.

Here is a typical run of your program. I have put in boldface what the computer types; the rest is input from the user.

Enter a search string: bob

Enter a target string: Nattering Nabobs

The target string 'Nattering Nabobs' contains the search string 'bob' starting at position 12

Here are the tools you can use for this program, in addition to the programing techniques we have discussed in class. Suppose variables s1 and s2 represent strings, such as "Bobalouie" or "snowball" or "CGGATTACGA". Then

• len(s1) and len(s2) are the lengths respectively of s1 and s2
• s1.find(s2) is the index of the start of string s2 within s1. This is -1 if s1 does not contain s2. Thus, if s is "go" and t is "bingo", then t.find(s) is 3, and s.find(t) is -1.
• s1.count(s2) is the number of times s2 appears within s1. So if s is "bobob" then s.count("bo") is 2.
• s1.lower() makes a new string identical to s1 except that all of the letters are converted to lower-case. So if s is the string "Hi Mom" then saying t = s.lower() makes t be the string "hi mom".
• s1.upper() makes a new string identical to s1 except that all of the letters are converted to upper-case.
• s1.capitalize() makes a new string identical to s1 except that the first letter is converted to upper-case.

For example, if search and target are your two strings, then target.find(search) is the index of string search within string target. If this is -1 then the search string isn't found; if it is 0 or more then the search string is found and this gives you its starting position.

After you have this working, try to modify it to make it case-insensitive. You can do this by making lower-case versions of your search and target strings: lc_search = search.lower() and lc_target = target.lower(). Do the analysis with lc_search and lc_target, then print out your answer using the original strings.