Debugging CMSC 201

Announcements Hw2, Hw3 grades returned if something doesn’t seem right, ask Midterm next Thurs (Oct 23) we’ll review on Tues no hw next week still have lab Questions?

Debugging Debugging is the process of removing errors from our code. It, like anything else in computer science, takes practice.

Errors There are three kinds of errors you’ll run into: Syntax errors—These are errors that occur from poorly formed python syntax. Exceptions—Exceptions occur when you try and do something that python can’t let you do, like access an element of a list that doesn’t exist. Logic Errors—Logic errors occur when your program terminates successfully, but just doesn’t do the right thing.

Syntax Errors These are the easiest kind of error to debug! 1. Look at what python is telling you the problem is. 2. Go to the line python is telling you the problem is on. 3. Fix the part of your program that is incorrect python. In some cases, you might have to look at your indentation, or the line above!

Exceptions Possible reasons for exceptions: File not found List index out of bounds Expected an integer but got a string Fixing these is also not a big deal! Python will tell you exactly what the error is, and what line it happened on. If a quick inspection of that line does not help you figure out what’s wrong, you might want to move on to the tricks we talk about for logic errors.

Logic Errors Logic errors are when your program simply does the wrong thing. These are the hardest to fix. Typically, with a logic error, all you know is that your program is producing incorrect output. The trick is to isolate where the error is happening.

Why We Have Functions When we’re trying to hunt down an error, functions are a helpful tool for this. Because functions take a small number of arguments and return a value, it’s easy to isolate them from the rest of the program to see if they’re working or not. def someCrazyFunction(input1, input2, input3): #All sorts of code return blah In order to see if our error is in this function, we can simply run it in main with a variety of different inputs and verify if it works or not.

How To Test A Function Imagine we have a sort function. What possible ways can we test it? sort([1, 10, 4, 5, 2]) Pretty standard! We know what the output should be. This is a general test case.

How To Test A Function Depending on how our program’s using it though, there could be other, weird cases! sort([]) sort([2]) These are called edge cases—the program should work, but it wasn’t necessarily what you had in mind.

How To Test A Function Notice that we can also have cases like these: sort([ “hats”, 1]) sort(10) However, these aren’t really worth testing! They will always break our sort function, and should! They violate what’s called the function’s precondition.

Precondtion A precondition is something that must be true about a function’s inputs in order for it to do its job. For sort(), an example would be “It must get a list of things that can all be compared to each other.” For a square root function, the precondition could be “Input must be a positive number.” Preconditions should be described in the function header comment.

Testing Functions So for functions, we want to test several general cases and several edge cases, and make sure that if something violates it’s precondition it reacts appropriately. Once a function has been thoroughly tested, you know that function’s not going to be a problem anymore!

Finding Errors Eventually, you’ll get to a function that is not producing the correct output, and your new job is to isolate the error within that function.

Example def longestSubSeq(myList): lastItem = myList[0] current = 0 best = 0 for item in myList: if(item == lastItem): best = best + 1 if best < current: best = current else: lastItem = item current = 1 return best What are some good test cases?

Test Cases def main(): print(longestSubSeq([1]) print(longestSubSeq([1, 2, 3]) print(longestSubSeq([1, 1, 1, 2]) print(longestSubSeq([1, 1, 2, 1]) print(longestSubSeq([1, 1, 2, 3, 2, 2, 2, 2]) Each of these targets a different place where something could go wrong!

Test Cases def main(): print(longestSubSeq([1]) print(longestSubSeq([1, 2, 3]) print(longestSubSeq([1, 1, 1, 2]) print(longestSubSeq([1, 1, 2, 1]) print(longestSubSeq([1, 1, 2, 3, 2, 2, 2, 2]) Output: File "oct14.py", line 17 print(longestSubSeq([1, 2, 3]) ^ SyntaxError: invalid syntax

Test Cases def main(): print(longestSubSeq([1])) print(longestSubSeq([1, 2, 3])) print(longestSubSeq([1, 1, 1, 2])) print(longestSubSeq([1, 1, 2, 1])) print(longestSubSeq([1, 1, 2, 3, 2, 2, 2, 2])) Output:

Test Cases We know something’s wrong! How do we find it?

Strategies Tracing by hand can be effective. Take the input, pretend to be the computer, and try and run the program yourself. Pros: Gives you a good idea of what your program is doing. Can help find simple errors. Cons: Can take awhile, especially for things with loops. Can be impractical for large inputs.

Strategies Print statements. Pros: Very informative, you can target specific variables to print. Cons: Have to know what to look at. Need careful labeling, or they get very confusing.

Example def longestSubSeq(myList): lastItem = myList[0] current = 0 best = 0 for item in myList: if(item == lastItem): best = best + 1 if best < current: best = current else: lastItem = item current = 1 return best Where are some good places for print statements?

WRONG def longestSubSeq(myList): lastItem = myList[0] current = 0 best = 0 for item in myList: print(item) if(item == lastItem): print(“hello”) best = best + 1 if best < current: best = current print(“here”) else: lastItem = item current = 1 print(“doing thing”) return best

Example def longestSubSeq(myList): lastItem = myList[0] current = 0 best = 0 for item in myList: print(“The current item is: “ + str(item) if(item == lastItem): print(“Item is part of sequence”) best = best + 1 if best < current: best = current print(“Longest sequence so far”) else: lastItem = item current = 1 print(“Sequence broken”) return best

Example Looking at the output, we can see right away that we never make it to longest sequence so far! That means best is never less than current… Looks like we forgot to increment current!

Example Still doesn’t fix it. Let’s try printing more stuff… def longestSubSeq(myList): lastItem = myList[0] current = 0 best = 0 for item in myList: print(“The current item is: “ + str(item)) if(item == lastItem): print(“Item is part of sequence”) print(“Best is “ + str(best)) best = best + 1 current = current + 1 if best < current: best = current print(“Longest sequence so far”) else: lastItem = item current = 1 print(“Sequence broken”) return best

Example Still doesn’t fix it. Let’s try printing more stuff… def longestSubSeq(myList): lastItem = myList[0] current = 0 best = 0 for item in myList: print(“The current item is: “ + str(item)) if(item == lastItem): print(“Item is part of sequence”) print(“Best is “ + str(best)) best = best + 1 current = current + 1 if best < current: best = current print(“Longest sequence so far”) else: lastItem = item current = 1 print(“Sequence broken”) return best

Example def main(): print(longestSubSeq([1])) print(longestSubSeq([1, 2, 3])) print(longestSubSeq([1, 1, 1, 2])) print(longestSubSeq([1, 1, 2, 1])) print(longestSubSeq([1, 1, 2, 3, 2, 2, 2, 2])) Output:

Example Debugging poorly will be your single biggest time sink in this class! Isolate the bug to a function or block of code, and put in well designed print statements at the start and end of the broken section, and narrow it down.

Exercise # Merges two sorted lists. # Precondition: list1 and list2 are lists of integers already sorted in ascending # order (smallest to largest) def merge(list1, list2): current1 = 0 current2 = 0 result = [] while current1 < len(list1) and current2 < len(list2): if(current1 == len(list1)): result.append(list2[current2]) current2 = current2 + 1 elif(current2 == len(list2)): result.append(list2[current1]) current1 = current1 + 1 elif(list1[current1] < list2[current2]): result.append(list2[current2]) current2 = current2 + 1 else: result.append(list1[current1]) current1 = current1 + 1 return result

Exercise def main(): main() Write some test cases, and then try debugging!