Add day 13 debugging exercises and README

Add src/main.py with the six debugging exercises (describe the problem,
reproduce the bug, play computer, fix the errors, print is your friend,
use a debugger), each with its fix noted. Document them in the README.

README.md

@@ -1,2 +1,35 @@
 # 013-debugging
 
+Day 13 of Udemy's *100 Days of Code: Python Pro Bootcamp*: finding and fixing
+bugs in Python code.
+
+All exercises live in [`src/main.py`](src/main.py), each commented out with the
+fix noted beside it. Uncomment one at a time to run it.
+
+## Exercises
+
+| Exercise | Bug | Fix |
+| --- | --- | --- |
+| Describe the Problem | `range(1, 20)` never reaches 20, because the upper bound is excluded | Use `range(1, 21)` |
+| Reproduce the Bug | `dice_imgs[dice_num]` raises `IndexError` when the roll is 6 | Index with `dice_num - 1` |
+| Play Computer | A birth year of exactly 1994 matches no branch | Use `>=` in the `elif` |
+| Fix the Errors | `input()` returns a string, so `age > 18` fails; the `print` is not indented | Convert with `int()`; indent the `print` |
+| Print is Your Friend | `==` compares instead of assigning, so `word_per_page` stays 0 | Use `=` |
+| Use a Debugger | `b_list.append(new_item)` sits outside the loop, so only the last item is kept | Indent it into the loop |
+
+## Debugging approach
+
+1. Describe the problem in your own words.
+2. Reproduce the bug.
+3. Play computer: step through the code by hand.
+4. Fix the errors the interpreter reports.
+5. Use `print()` to inspect values.
+6. Use a debugger to step through execution.
+
+## Running
+
+Requires Python 3.
+
+```bash
+python src/main.py
+```

src/main.py

@@ -0,0 +1,51 @@
+############DEBUGGING#####################
+
+# Describe Problem
+# Answer: Range never reach 20 as range don't include 20. Only up to
+# def my_function():
+#   for i in range(1, 20):
+#     if i == 20:
+#       print("You got it")
+# my_function()
+
+# # Reproduce the Bug
+# from random import randint
+# dice_imgs = ["❶", "❷", "❸", "❹", "❺", "❻"]
+# dice_num = randint(1, 6)
+# # print(dice_imgs[dice_num]) # list index out of range
+# print(dice_imgs[dice_num - 1]) # The corrected code
+
+# # Play Computer
+# year = int(input("What's your year of birth?"))
+# if year > 1980 and year < 1994:
+#   print("You are a millenial.")
+# # elif year > 1994:
+# elif year >= 1994: # Answer corrected code (other solution, change the other if line)
+#   print("You are a Gen Z.")
+
+# # Fix the Errors
+# # age = input("How old are you?")
+# age = int(input("How old are you?")) # Answer: convert input to int
+# if age > 18:
+# print("You can drive at age {age}.")
+
+# #Print is Your Friend
+# pages = 0
+# word_per_page = 0
+# pages = int(input("Number of pages: "))
+# # word_per_page == int(input("Number of words per page: "))
+# word_per_page = int(input("Number of words per page: ")) # Answer
+# total_words = pages * word_per_page
+# print(total_words)
+
+# # Use a Debugger
+# def mutate(a_list):
+#     b_list = []
+#     for item in a_list:
+#         new_item = item * 2
+#         b_list.append(new_item) # Answer: indent code
+#     # b_list.append(new_item)
+#     print(b_list)
+#
+#
+# mutate([1, 2, 3, 5, 8, 13])