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Variables#

Variables are a way to define the constraints of your problem in a single place and reference them everywhere else.

The motivation for having variables are simple: imagine you've decided to change the constraints of your problem. Without variables, you'd have to change this constraint in the validator, (potentially) in the checker and in the statements. It's super easy to forget about these changes, increasing the likelihood of introducing a disastrous bug in your problem.

Defining variables#

Variables are defined in the vars section of your problem.rbx.yml file.

problem.rbx.yml
# ...
vars:
  MAX_N: 100000
  MAX_M: 200000

They're defined as key-value pairs. The keys should be valid Python identifiers and the values should be a bool, a string, an int or a float, all conforming to the YAML specification.

Besides that, it's possible to use Python expressions that evaluate to one of these types, using the py`...` syntax.

problem.rbx.yml
# ...
vars:
  MAX_N: py`10**5`
  MAX_M: py`2*10**5`

Using variables#

Variables can be used within validators, checkers and statements.

In this section, we go through the different ways to use variables in each of these.

Validators and checkers (C++)#

rbx automatically generates a rbx.h header file that contains the variables that were defined in the problem.rbx.yml file, right at the root of your package.

This header exposes a function getVar<T>(name) that can be used to get the value of a variable as a T-typed object. There are 4 overloads for this function: getVar<bool>(name), getVar<int>(name), getVar<float>(name) and getVar<std::string>(name).

This header can be directly included in your validator/checker files.

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#include "testlib.h"
#include "rbx.h"

int main(int argc, char* argv[]) {
    registerValidation(argc, argv);

    int n = getVar<int>("MAX_N");
    int m = getVar<int>("MAX_M");

    // Single line with two numbers.
    inf.readInt(1, n, "n");
    inf.readSpace();
    inf.readInt(1, m, "m");
    inf.readEoln();
    inf.readEof();
}

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#include "testlib.h"
#include "rbx.h"

int main(int argc, char* argv[]) {
    registerTestlibCmd(argc, argv);

    int n = getVar<int>("MAX_N");
    int m = getVar<int>("MAX_M");

    // ...
}

Validators (other languages)#

Validators also receive the variables as command-line arguments. This means that for the problem.rbx.yml above, your validator would be called roughly as follows:

./validator.exe --MAX_N=100000 --MAX_M=200000

You can freely parse those arguments in your language of choice.

Checkers

Checkers do not receive the variables as command-line arguments, as doing so is not compatible with any judging platforms.

If you want to use variables in your checkers, they must be in C++ and you have to follow the approach outlined in the previous section.

Statements#

rbxTeX statements can also use variables. This is done by using the \VAR command with the vars. prefix.

statement.rbx.tex
% ...
You're given a graph with \VAR{vars.MAX_N} vertices and \VAR{vars.MAX_M} edges.
% ...

Variables can also be used within any rbxTeX statements, including loops and conditionals.

statement.rbx.tex
% ...
%- if vars.MAX_N < 1000:
This problem is easy.
%- else:
This problem is hard.
%- endif
% ...

Also, rbx exposes a few transform builtins that can be used to change how a variable is rendered. One of them is the sci builtin, which formats a number with many trailing zeroes in scientific notation.

statement.rbx.tex
% ...
You're given a graph with \VAR{vars.MAX_N | sci} vertices
and \VAR{vars.MAX_M | sci} edges.
% ...

The sci builtin will make MAX_N and MAX_M be rendered as something like 10^5 and 2 x 10^5 respectively.

Stress tests#

Variables can also be used in generator expressions in stress tests with the <variable> notation.

rbx stress -g "gen [1..<MAX_N>]" -f "[sols/wa.cpp] ~ INCORRECT"