Checkpoint 1: I finished planning.md

planning.md

@@ -38,18 +38,57 @@ return digit_names[number]
 If the integer is under 10, then use the procedure described above. 
 Otherwise, ... (this is where you take over!)
 
-## Integers under 100
+We can again make a list of digit names:
 
+```
+digit_names = [ 
+    "zero", "one", "two", "three", "four", 
+    "five", "six", "seven", "eight", "nine",
+    "ten", "eleven", "twelve", "thirteen",
+    "fourteen", "fifteen", "sixteen",
+    "seventeen", "eighteen", "nineteen"
+]
+return digit_names[number]
+```
+
+## Integers under 100
+If the integer is under 20, then use the procedure described above.
+Otherwise, 
+
+First, create a list of tens digit strings:
+```
+tens_names = [
+    "twenty", "thirty", "forty", "fifty",
+    "sixty", "seventy", "eighty", "ninety"
+]
+```
+Second, determine the leading digit, using integer division by 10, and the ones digit using mod 10 and look up the correct names.
+return tens_names[number / 10 - 2] + digit_names[number % 10]
 
 ## Integers under 1000
+If the integer is under 100, then use the procedure described above. We'll call that procedure tens_number().
+Otherwise,
 
+First, create a list of hundreds digit srings:
+```
+hundreds_names = [
+    "one hundred", "two hundred", "three hundred",
+    "four hundred", "five hundred", "six hundred",
+    "seven hundred", "eight hundred", "nine hundred"
+]
+```
+Second determine the hundreds digit using integer division by one hundred, the tens digit by finding the number mod 100 and applying tens_number(), and the ones digit by finding the number mod 10.
+return hundreds_name[number / 100] + tens_number(number % 100) + digit_names[number % 10]
 
 ## Integers under 1000000
+If the integer is under 1000, then use the procedure described above. We'll call that procedure hundreds_name().
 
+return hundreds_name(number / 1000) + "thousand" + hundreds_name(number % 1000)
 
 ## Negative integers down to -1 million
 We won't deal with negative integers in this problem set, 
 but how would you deal with a negative integer, using the 
 functions above?
 
-
+If the number is positive, use the above method.
+Otherwise, return "negative " + number_words(-1 * number)