2015-03-13 21:00 CET

KATT 2015


2015-03-16 21:00 CET
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Language: Haskell

General information

You start out by finding a suitable problem to solve. Then you write code to solve the problem. After this, you submit the code to us for review. We will then compile your code and run it on some secret input. After some careful deliberation, you will get a judgement informing you whether your code behaved as expected or not.


Your program should read its input from standard input and produce output on standard output. This can for instance be done using interact.

Input will always follow the input specification (so you do not need to validate the input). Your output must follow the output specification.

Compiler settings

For Haskell, we use GHC version The Glorious Glasgow Haskell Compilation System, version 8.4.4 with the following flags: -O2 -ferror-spans -threaded -rtsopts {files}.

Runtime settings

For Haskell, we use the following runtime flags: +RTS -M{memlim}m -K8m -RTS.

Here {memlim} is the actual memory limit for the problem you are submitting to.

System libraries

You are allowed to use all standard libraries included with Haskell.


We are currently using Dell PowerEdge R230 servers for judging. These are equipped with an Intel Xeon E3-1220V6 CPU running at 3.0 GHz and 8 GB RAM. A 64-bit Linux kernel is used.


We will inspect the exit code of your program. If it is non-zero, we will judge your submission as Run Time Error.

Solving a problem

Now lets get down to business and write some code. The short tutorial below goes through the solution of A Different Problem.

  1. The problem
  2. Reading the input
  3. Computing the answer
  4. The solution

Step 1: The problem

You are tasked with writing a program that computes the difference between integers. Sounds simple, doesn't it? Well, as we will see, the problem still holds some small difficulties.

Step 2: Reading the input

One thing to note is that the integers can be fairly large, as large as 1015. This means that the Haskell Int may not be sufficient. Rather, Integer is needed. It is a good idea to test our solution on very large inputs to make sure it performs as expected.

Now that we have determined a suitable type, we just have to read the data. Reading is done from standard input. In this problem, we should read until the end of the file (in other problems, there might be an integer at the beginning of the input, specifying how much to read, or there might be a special indicator denoting that there is nothing more to read). The most straight forward pattern is to use the Haskell function interact transform where transform is some function that maps a String (the entire input) to another String (the entire output). In our case we use words to break the input into a list of strings, each string being the digits in a number. Then we use map read to turn it into a list of Integer:

readInput = (map read) . words -- "1 2\n3 5\n" --> ["1", "2", "3", "5"] --> [1, 2, 3, 5] main = interact (writeOutput . solve . readInput) --TODO: solve and writeOutput

Step 3: Computing the answer

Now that we've read the input as a list of Integer, we can process them in pairs, computing the absolute value of their difference.

solve [] = [] solve (a:b:rest) = abs(a - b):(solve rest)

Finally, it's time to print the result. Using map show to turn the list of integers (the result) into a list of String, and then apply unlines to turn this list into a single string with line breaks.

writeOutput = unlines . (map show) -- [1, 2] --> ["1", "2"] --> "1\n2\n"

Step 4: The solution

Now we are basically done, all that remains is to combine the above parts.


So are we using Int or Integer in the computations? It turns out that read defaults to Integer, so we are fine.

Frequently asked questions


I keep getting rejected but my solution works on the provided cases

The sample data provided in the problem statement is just there to help you make sure you understood what the problem asks for, and the input/output format. When you submit your solution, we will run it on an extensive set of additional test data to verify that it solves the problem correctly and efficiently.

When we run your solution, the first case(s) are always the sample case(s). If you fail on these, make sure that:

  1. You are not printing any output other than the one specified in the problem.
  2. You have not misspelled any part of the output (copy-paste is your friend).
  3. You are printing real-valued numbers with the precision requested in the problem.
If this does not help you get past the sample cases, make sure that there isn't a difference in system and compiler that causes your solution to behave differently when run on the judge machine.

I keep failing on testcase X. Can you please share it with me?

Sorry, no. We can't share the secret testdata.

There is an error in the sample data of the problem, can you please fix it?

The sample data is used to illustrate and clarify the problem. If you believe there is an error in the sample data, your interpretation of the problem is probably wrong. Consider if there is an alternative interpretation which matches the sample data.

How does judging work, and what do the different judgements mean, precisely?

See the judgements page.

Interacting with the judge system

Can I test my solution before I submit?

No, however we allow you to submit your solution multiple times so you can test your way to the right solution.

Do you store my submissions?

Yes, we store your submissions. Occasionally a problem is found with one of the problems (no pun intended) or a time limit is changed (this should not happen frequently) and then we need to rejudge all submissions on that problem. We also use the code to check for plagiarism.

I found a bug? / I have a question that is not answered in the documentation. / I think this or that would be much better if it worked like this instead.

Please contact us and tell us about it.