Stress Test - Find the Test and Debug

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Do curso por University of California San Diego

Algorithmic Toolbox

3589 classificações

University of California San Diego

Algorithmic Toolbox

3589 classificações

Curso 1 de 6 em Specialization Data Structures and Algorithms

The course covers basic algorithmic techniques and ideas for computational problems arising frequently in practical applications: sorting and searching, divide and conquer, greedy algorithms, dynamic programming. We will learn a lot of theory: how to sort data and how it helps for searching; how to break a large problem into pieces and solve them recursively; when it makes sense to proceed greedily; how dynamic programming is used in genomic studies. You will practice solving computational problems, designing new algorithms, and implementing solutions efficiently (so that they run in less than a second).

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Programming Challenges

Welcome to the first module of Data Structures and Algorithms! Here we will provide an overview of where algorithms and data structures are used (hint: everywhere) and walk you through a few sample programming challenges. The programming challenges represent an important (and often the most difficult!) part of this specialization because the only way to fully understand an algorithm is to implement it. Writing correct and efficient programs is hard; please don’t be surprised if they don’t work as you planned—our first programs did not work either! We will help you on your journey through the specialization by showing how to implement your first programming challenges. We will also introduce testing techniques that will help increase your chances of passing assignments on your first attempt. In case your program does not work as intended, we will show how to fix it, even if you don’t yet know which test your implementation is failing on.

Solving the Sum of Two Digits Programming Challenges (screencast)6:30

Solving the Maximum Pairwise Product Programming Challenge: Improving the Naive Solution, Testing, Debugging13:37

Stress Test - Implementation8:22

Stress Test - Find the Test and Debug7:49

Stress Test - More Testing, Submit and Pass!8:45

Conheça os instrutores

Alexander S. Kulikov
Visiting Professor
Department of Computer Science and Engineering
Michael Levin
Lecturer
Computer Science
Neil Rhodes
Adjunct Faculty
Computer Science and Engineering
Pavel Pevzner
Professor
Department of Computer Science and Engineering
Daniel M Kane
Assistant Professor
Department of Computer Science and Engineering / Department of Mathematics

In this video we will launch the stress test to find a test

on which our main solution and alternative solutions differ,

use this test to debug our main solution and fix it.

So we save the file.

Now we will compile this file because we will want to run it.

And to compile this file, we go to the section Solving Programming Assignments.

We look at the C++ Flags, copy them, and then use them to compile our file.

Also, of course, specify which file want to compile and

also the name for the output file will be stress_test, without .cpp.

So we compile our stress test and then we launch it and see what happens.

So we see, our screen was filled with a lot of lines.

Actually, those lines were words, OK, and some numbers,

like number n from 2 to 11, and input numbers, which are big.

And there are a lot of tests for which our solutions didn't differ, and

that all went pretty fast.

And then in the end, we have number 11, which is n, and

then 11 big numbers, which is the input test, and then the words,

Wrong answer and two very big numbers which are different, which are the answers

correspondingly of the first and the second solution we implemented.

So now,

we already have a test on which at least one of our solution runs incorrectly.

But it is very hard to work with this test because we have 11 numbers and

they are very big and the answer to this problem is even bigger.

So let us try to find a smaller test on which our solution still fails.

Usually we can create a very small test with this technique.

So let's go back to our code, stress_test.cpp.

And let's decrease the restrictions on n and on the numbers.

So, for example, we want our number n to be no more than 5,

then we first generate a random number modulo 4 so it will be from 0 to 3.

And then, after adding 2, it will be from 2 to 5.

And also, we want our numbers to be less than 10.

So we just generate numbers modulo ten.

And that's everything we change.

Now we save the file.

We recompile with the same options.

And we again launch our stress test.

Now, we found a test on which solutions differ even faster.

Only maybe five or seven tests were OK.

And now we have a test with five numbers, which are 2, 9, 3, 1, and 9,

on which our solutions differ.

And the answer of the first solution is 81.

And the answer of the second solution is 27.

Which one of those is correct?

Of course, the first one is correct, because we have numbers 9 and

second 9, which are the biggest in the input, and their product is 81.

So our first naive solution gives a correct answer, but

our second fast solution gives an incorrect answer.

And let's think why that can be.

Well, that can be if it multiplies wrong numbers from the array.

So, let's go back in the code and check which are those numbers.

We can go into the code and

we look into the code for function MaxPairwiseProductFast.

It's very convenient that we have a separate function for

this solution because the error should be localized somewhere here.

So to simplify our goal of debugging,

let's output the indices of the maximum number and of the second

maximum number that at least this function thinks are two maximums in the array.

So we output them to cout.

First the first index, then a space to separate them,

then the second index, and then newline character.

So we save our program.

We again recompile it, and we again launch the stress test.

So note that although we generate random tests, it is safe to

launch our stress test again because those are pseudo random.

The sequence of generated random values is the same.

So this is a reproducible test.

So now, again, we have the same test in which two solutions differ

with five numbers, the same five numbers.

And now, under the line with five numbers, we have another line which says 1 2.

This is actually our debug output, which means that the index of the first number,

which is maximum, is 1 and it corresponds to number

9 because we enumerate our numbers from 0 in our C++ array.

And the second number has index 2, and this is number 3, again,

because we enumerate from 0.

So, our fast solution thinks that the second maximum number

in the array is the number 3 instead of number 9, which is in position four.

So let's go back to the code and see why it happened.

Again, we open the file with the code.

We go to the function MaxPairwiseProductFast.

We ignore the code for

the max_index1 and we're very interested in the code for max_index2.

So now let's look at this complex if expression,

where we try to determine whether a number at position j

may be a new maximum, which is different from the maximum we've already found.

And let's look more closely at this particular first condition.

So here, what we do is we check that number at position number j is

different from the maximum we've already found.

But that's exactly the problem.

What we need is instead, that j

is different from max_index1 because we don't want to find the same number, but

we can find number which is equal to the first found maximum.

So, instead of this comparison, what we actually need is to compare j,

To max_index1.

If we fix our code this way, then we will be at least able

to find the second number 9 in our input array 2 9 3 1 9, which we saw.

So let's see how it goes after we fix this.

So we save the file, we again recompile it with the same options and

we launch our stress test.

So now what we see is a screen which is very quickly filling with

some numbers, some words OK, and then some numbers again and words OK.

And we should wait for some time, for maybe 10 seconds or

even 30 seconds to be sure that we cannot find a test in which our solutions differ.

But, by now, it seems that that is the case.

And in the next video, we will do more testing and

then submit our main solution to the system and see how it goes.

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