Language: OCaml

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.

Input/Output

Your program should read its input from standard input and produce output on standard output. This can for instance be done using read_line() / Printf.printf. Anything written on standard error (Pervasives.stderr) will be ignored. This can be used for debugging your program during development (i.e., you do not have to remove debug output before submitting if you use standard error for debug output). Of course, writing to standard error will take some runtime.

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 OCaml, we use OCaml version 4.08.1 with the following flags: unix.cmxa str.cmxa bigarray.cmxa {files}.

System libraries

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

Hardware

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.

Exiting

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.

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.

One thing to note is that the integers can be fairly large, as large as 1015, which is a lot larger than the maximum value of an int (which is 231−1). Luckily, there is a 64 bit integer type in OCaml, Int64.

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). Using read_line, we can do e.g.:

(* Get the i^th entry in the list, and convert it to an Int64. *) let nth l i = (Int64.of_string (List.nth l i));; try while true; do let line = read_line () in let tokens = Str.split (Str.regexp " ") line in let a = (nth tokens 0) in let b = (nth tokens 1) in let diff = Int64.abs (Int64.sub a b) in begin (* print the result here *) end; done; with End_of_file -> ();;

Now that we've read the input, it's time to actually solve the problem. Since 0 ≤ a, b ≤ 1015, we have that −(1015) ≤ ab ≤ 1015, which means that there is no danger of overflow involved in just subtracting the two numbers a and b. Then, we can just take the absolute value by using the Int64.abs function.

Finally, it's time to print the result. Using Printf.printf:

Printf.printf "%Ld\n" diff

Step 4: The solution

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

Here is a version of the complete solution.

different.ml

Judging

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.