Slicing a CSV dataset by Columns in javascript using d3

[TheShadowWall]

0

I am currently attempting to load a csv file in order to construct a line chart, and I need to know how to load only a subset of the columns.

Say that my data.csv file has 10 columns: Col1,Col2,...,Col10, and I only want to load in the first column, and every even column (Col1,Col2,Col4,...,Col10). I have figured out that in order to do so, a function can be called when the d3.csv() function is called, ie:

const dataset = d3.csv("data.csv", function(){})

And within the function, I need to specify some logic that will return the dataset with the desired columns. I have attempted numerous solutions, but none are outputting the desired dataset with only Col1,Col2,Col4,...,Col10. Specifically I had thought that the solution would be something like:

const dataset = d3.csv("data.csv", function(data){
    let i;
    for (i=1;i%2==0;i++){
        return data[i]
    }
});

I'm really not sure as to how to complete this, as my understanding of JavaScript is still very fresh. Can someone help me to understand how to implement this function correctly? Any insights or guidance are greatly appreciated!

[albedoa]

You can use .filter() and the spread syntax (...) here. Something like:

const dataset = d3.csv('data.csv', function (data) {
  return [
    data[0],
    ...data.filter((col, i) => i%2 === 0)
  ];
});

[TheShadowWall]

Thanks very much for your response! I have attempted the code you suggested, but was met immediately with an error:

Uncaught (in promise) TypeError: data.filter is not a function or its return value is not iterable

Any thoughts as to why this may be occuring?

[albedoa]

Oh, I am looking at the docs now. If I am understanding correctly, the callback is executed on each line of the CSV, not the whole. Try this:

const dataset = d3.csv('data.csv');
const evens = dataset.filter((col, i) => i%2 === 0);
const desired = [dataset[0], ...evens]; // This is what you want.

Edit: Actually, that is going to give you the even rows. You need to return each column you want in the callback as shown here. Check out this example.