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Comments on How do I filter an array in C?
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How do I filter an array in C?
No, I'm not trying to get the full program written completely in C by you guys. I only need some way to implement the functionalities of each function I found confusing.
In this challenge in Code Golf CD, I have to make a program that computes the determinant of a two-dimensional array. It also has a program written in JavaScript that solves the challenge which looks like this:
function laplaceDet(matrix) {
if (matrix.length === 1) return matrix[0][0];
let sum = 0;
for (let rowIndex = 0; rowIndex < matrix.length; ++rowIndex) {
let minorMatrix = matrix.filter((_, index) => index !== rowIndex)
.map(row => row.slice(1));
sum += ((-1) ** rowIndex) * matrix[rowIndex][0] * laplaceDet(minorMatrix);
}
return sum;
}
I want to challenge myself into using C for solving the challenge. First, I'll need to reimplement the program, whereas the golfing part will be dealt with later, although I've already been golfing it. Here's what I got so far.
Why'd I stop? There's a function named filter() and I'm not so sure how to get it implemented, and since C has no maps (at least I think none exist in C), it's also difficult to complete the program without it.
So I'll have to improvise... How? How do I implement the filter() method, maybe without using a function to save bytes, and what's a C equivalent to the line of code matrix.filter((_, index) => index !== rowIndex).map(row => row.slice(1));?
Post
It looks like filter() effectively takes two arguments; the array to be filtered, and a predicate expression resulting in a boolean indicating whether the entry should be included in the output or not.
(I also note that the implementation in your question appears to be at least O(n^2) time complexity, which is pretty bad for anything but the smallest arrays.)
Beginning at the point of not concerning oneself with source code byte count, the tool in C's toolbox that I would reach for for this is either a specialized function for this one usage, or one that takes a pointer to a function that serves the purpose of the filter predicate. The latter is slightly more complex, but also generic as it can accept any predicate function.
The function pointer syntax in C takes some getting used to.
First, write a function that does whatever filtering you want. (For a real-world application, unless I had some highly specialized need, I would first see if there are already-debugged third-party library implementations of this rather than trying to come up with my own implementation.)
int* filter(int* input, bool (*predicate)(int))
{
int* output;
while (*input != -1)
{
if(predicate(*input))
{
// somehow include *input in the output
}
input++;
}
return output;
}
Then, write a predicate function which will be called once per entry:
bool my_predicate(int value)
{
return (value & 3) != 0; // or whatever
}
Then put them together:
int* filtered = filter(unfiltered, my_predicate);
Adding some boilerplate this compiles cleanly with gcc -Wall -std=c99 -pedantic-errors -Wextra -Werror.
Reducing the character count in the source code is kind of what code golfing is about as I understand it, so I will leave that part entirely up to you.
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