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PHP: Determinacy

Regardless of the programming language, functions possess certain fundamental properties. Knowing these properties makes it easier to predict the behavior of functions, as well how they can be tested and used. These properties include determinacy. A function is called deterministic when it always returns the same result for the same input arguments. For example, the function that flips a string is deterministic.


strrev('cat'); // tac
strrev('cat'); // tac

No matter how many times we call it with 'cat', it will always return 'tac' (though you could technically write it differently, but that would make no sense and cause more problems). In turn, a function that returns a random number isn't deterministic, because we'll always get different results for the same input (even if it's empty, i.e., no arguments are passed). How it differs doesn't matter, even if at least one in a million calls returns something different, this function is deemed non-deterministic.


rand(); // 827606195
rand(); // 635369726

So what use is knowing that to us? Determinism significantly affects many aspects. Deterministic functions are easy to work with, easy to optimize, and easy to test. If you can make the function deterministic, then you should absolutely do so.


Display a random number from 1 to 6. To do this, you'll need the rand() function, which can accept two numbers: a start number and an end number

The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

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