## Quarters and modulo arithmetic

This is another post that’s mainly for my benefit when I inevitably forget. I’m working with dates in PHP which, unlike MySQL, does not have a built in quarter function for extracting the quarter from a year.

Even if it did, one would have to be very careful with it because quarters are actually defined differently in different countries. In the UK, where I am, April to June is the first quarter (and January to March the last). In other countries, including (I think) the US, the first quarter is January to March, and October to December the last.

With no quarter function, and the hypothetical threat of an unpredictable one, my next recourse is to divide the month by 3 in order to give me a quarter number. This is done very simply as:

```
echo ceil(date("m") / 3);

```

This will return 1 when the month is 1 to 3 (January to March), 2 when it’s 4 to 6 (April to June) etc. But as I mentioned before UK quarters don’t work like this. We don’t have quarters in a sequence 1, 2, 3, 4. They go in a sequence like 4, 1, 2, 3. Now I could write code that deals with each of those cases individually, converting 1 to 4, 2 to 1, 3 to 2, etc., but I thought it was better to do it properly (and store up generalisability for the future) by converting the 1, 2, 3, 4 sequence with modulo arithmetic. I confess, I still can’t work out how to do this other than by trial and error but it definitely involves modulo 4 because the sequence is of period 4.

To convert 1, 2, 3, 4 to 4, 1, 2, 3 one need only do the following, where x is the input value 1, 2, 3, 4:

4 – (5 – x) %% 4

As far as I can figure it the first constant (4) tells the sequence how big it should be (4, 1, 2, 3 or 10, 7, 8, 9) and the second constant (5) tells the series where it should flip back to the start (4, 1, 2, 3 or 3, 4, 1, 2).

That’s all I know, I’m really busy and don’t have time to think about it any more. Hopefully this will help someone/ me in two years’ time.

## Consuming REST APIs with PHP and CURL

I wasted such a lot of time on this that I must commit it to the internet on the off chance that it helps someone else in the same situation.

If you are using PHP to consume a RESTful API via CURL and you want to manipulate the data you get back it’s very important that you set CURLOPT_RETURNTRANSFER to true. This allows you to collect the response from the server in a variable. If you don’t set this option it will just echo the return to the screen, which is obviously of no use whatsoever.

While I’m here I may as well mention as well that if you want json_decode to return an array you need to use json_decode(\$result, true); otherwise you get an object back. The final code I wrote looks like this:

```
\$ch = curl_init();
curl_setopt(\$ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt(\$ch, CURLOPT_URL, "http://YOUR_URL_HERE");
\$result = curl_exec(\$ch);

curl_close(\$ch);

\$result = json_decode(\$result, true); // giving true to json_decode returns array

```

## Converting a grouped plyr::ddply() to dplyr

I’m going full tidyverse at the moment and so I’m converting my old plyr code to dplyr. It’s been pretty steady going so far, although I had a bit of difficulty converting an instruction using ddply which carried out a function based on a subgrouping within the data. I wrote a toy example to get it right, I may as well share it with the internet in case it helps someone else. If you can’t figure out what I’m talking about with what the function does, just run the code. You’ll see what it is. Easier than explaining in words. The following two functions carry out the same task, the latter is a translation of the former.

```
library(plyr)
library(tidyverse)

plyr::ddply(mtcars, "cyl", mutate, mean.mpg = mean(mpg))

mtcars %>%
dplyr::group_by(cyl) %>%
dplyr::mutate(mean.mpg = mean(mpg))

```