Debugging loops

Estimated Time: 1 hour

Usually it takes more than one try to write the code to solve a problem with loops. When the code isn’t working correctly, you need a strategy for figuring out what is happening, and to fix it.

Loop debugging: Printing each step

Printing values at each step is a strategy for debugging what’s happening in your loop. Let’s see what it looks like.

Here’s some broken code for solving a loop problem:

# Find the total of the even numbers from 2 to 12
total = 0
for i in range(1,12,2):
	total + i
print(total) # 0  <- Wrong, should not be 0!

You might be able to spot the bug in this code, but let’s try printing out the values to debug it.

total = 0
print("before the loop total is", total)
for i in range(1,12,2):
	print("i is", i, "total is", total)
	total + i
print("after the loop total is", total)
print(total)

Here’s the output:

before the loop total is 0
i is 1 total is 0
i is 3 total is 0
i is 5 total is 0
i is 7 total is 0
i is 9 total is 0
i is 11 total is 0
after the loop total is 0
0

Wow! It looks like there are actually 3 bugs!

• instead of the even numbers, i is getting set to the odd numbers
• instead of including 12, it’s stopping at 11
• total isn’t changing at all in the loop

Fixing the code

Since we can see the values, it’s much easier to tell what we need to fix.

• range needs to start at 2 (instead of 1)
• range needs to stop at 13 (instead of 12)
• it needs to be total += i instead of total + i

When we fix the code, we can leave the prints in to make sure our changes work.

total = 0
print("before the loop total is", total)
for i in range(2,13,2):
	print("i is", i, "total is", total)
	total += i
print("after the loop total is", total)
print(total)

Now the output is:

before the loop total is 0
i is 2 total is 0
i is 4 total is 2
i is 6 total is 6
i is 8 total is 12
i is 10 total is 20
i is 12 total is 30
after the loop total is 42
42

Now that we can see that the code is working, we can remove the extra print statements. Here’s the final code:

total = 0
for i in range(2,13,2):
	total += i
print(total)

Practice

total = 0
print("total before is", total)
for i in range(10,25,2):
  print("i is", i, "total is", total)
  total + i
print("total after is", total)
print(total)

# BUGS
# - supposed to be the odd numbers (range should start at 11)
# - supposed to include 25 (range should end at 26)
# - should be total += i

Loop debugging: Visual Tracing

When debugging more complicated code, it’s helpful to be able to see how the code executes step by step. You can use pythontutor.com to run your code step by step, and see the values of all your variables as the program runs.

Similar to printing out the values at each step, PythonTutor helps you see what’s happening when your code runs. That makes it easier to spot bugs.

Here’s the example from before in pythontutor. Try it out!