Merge data frames from a list with each other

Question

What I need: I have a huge data frame with the following columns (and some more, but these are not important). Here's an example: ``` user_id video_id group_id x y 1 1 0 0 39 108 2 1 0 0 39 108 3 1 10 0 135 180 4 2 0 0 20 123 ``` User, video and group IDs are factors, of course. For example, there are 20 videos, but each of them has several "observations" for each user and group. I'd like to transform this data frame into the following format, where there are as many `x.N`, `y.N` as there are users (`N`). ``` video_id x.1 y.1 x.2 y.2 … 0 39 108 20 123 ``` So, for video `0`, the x and y values from user 1 are in columns `x.1` and `y.1`, respectively. For user 2, their values are in columns `x.2`, `y.2`, and so on. What I've tried: I made myself a list of data frames that are solely composed of all the `x, y` observations for each `video_id`: ``` summaryList = dlply(allData, .(user_id), function(x) unique(x[c("video_id","x","y")]) ) ``` That's how it looks like: ``` List of 15 $ 1 :'dat…

Accepted Answer

`Reduce` does the trick:

```
reducedData <- Reduce(function(x,y) merge(x,y,by="video_id"),summaryList)
```

… but you need to fix the `names` afterwards:

```
names(reducedData)[-1] <- do.call(function(...) paste(...,sep="."),expand.grid(letters[24:25],names(summaryList)))
```

The result is:

```
   video_id  x.1 y.1  x.2 y.2  x.3 y.3  x.4 y.4  x.5 y.5  x.6 y.6  x.7 y.7  x.8
1         0   39 108  899 132   61 357  149 298 1105 415  148 208  442 200  210
2         1 1125  70  128 165 1151 390  171 587  623 623   80 643  866 310  994
```

Merge data frames from a list with each other

Solution

Problem