This Decision Tree lesson is for a beginner. Read the short notes, run each example, then change one name or number and run it again.
A decision tree asks one yes or no question at a time and sends the row down a branch.
The example is a two-question tree for a pass mark. Real trees choose the questions from the data.
Example 1: Start here
A very deep tree can memorize the training rows. Limit the depth.
def band(score, attendance): # define the function; it runs when you call it
if score < 70: # run the indented lines only when this is true
return "retry" # send this value back to the caller
if attendance < 75: # run the indented lines only when this is true
return "review" # send this value back to the caller
return "pass" # send this value back to the caller
print(band(82, 90))
Example 2: Add two rows
Decision Tree applies one operation to every number. zip pairs the rows, and the list keeps each sum.
left = [1, 2, 12] # a list; the values stay in this order
right = [10, 20, 30] # a list; the values stay in this order
print([a + b for a, b in zip(left, right)]) # pair the lists, then combine each pair
Example 3: Average a column
The average is the total divided by how many numbers you have. That is the same summary a statistics page starts with.
scores = [70, 12, 90] # a list; the values stay in this order
print(sum(scores) / len(scores)) # len counts the items
Example 4: Keep values above a line
A cutoff drops the low values. Change 80 and run the example again to see who remains.
scores = [70, 12, 90] # a list; the values stay in this order
print([score for score in scores if score >= 80]) # keep the items that pass the test
A very deep tree can memorize the training rows. Limit the depth.
