Tip 30: Use Apophenia to read in data and configuration info

30 November 11. [link] PDF version

level: Basic for anybody dealing with data
purpose: Use libraries!

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

This is a special case of Tip #2 about using pre-existing libraries wherever possible. After all, C's big edge is that it's been around for forty years; that's a lot of time for useful libraries to get written.

Reading in text is an especially difficult problem that everybdoy has to deal with so it is especially library-appropriate. Despite my self-conscious desire to not self-promote, I'm gonna tell you that Apophenia does a decent job with this.

First, let's generate a data set. I'll wrap it up as a here document, as per Tip # 8, so you can just paste this onto the command line:

cat > text_data << "."
2|5| 12
3|8|Galia est omnis divisa en partes tres

The sample data shows the first tip for the day: use pipes as field delimiters. Pipes really look like the bounds between fields, and they rarely appear in the data you're putting into a text file. The default for so many systems is commas or tabs, both of which are just asking for glitches.

Reading a data set to a matrix is pretty trivial via Apophenia. In this example, I'll stretch it out by first reading into the database (instead of directly using apop_text_to_data, which would save two lines of code but lose the non-numeric input). And remember Tip #9 about compiling C code via here document? It's how I test all the sample code I put here, and is still an easy way for you to try it all out.

#include <apop.h>

int main(){
    apop_text_to_db("text_data", "datatab");
    apop_data *indata = apop_query_to_data("select "
                                "left, middle from datatab");
    Apop_col(indata, 0, firstrow);
    Apop_col(indata, 1, secondrow);
    printf("first column sum: %Lg\n", apop_sum(firstrow));
    printf("second column sum: %Lg\n", apop_sum(secondrow));

If you installed the Apophenia library, then you also have the command-line apop_text_to_db, which just runs the C function in the second line of main.

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