names of a dataset returns NULL- R version 3.2.4 Revised-Ubuntu 14.04 LTS
<p>I have a small issue regarding a dataset I am using. Suppose I have a dataset called <code>mergedData2</code> defined using those command lines from a subset of <code>mergedData</code>: </p>

<pre><code>mergedData=rbind(test_set,training_set)
lookformean&lt;-grep("mean()",names(mergedData),fixed=TRUE)
lookforstd&lt;-grep("std()",names(mergedData),fixed=TRUE)
varsofinterests&lt;-sort(c(lookformean,lookforstd))
mergedData2&lt;-mergedData[,c(1:2,varsofinterests)]
</code></pre>

<p>If I do <code>names(mergedData2)</code>, I get: </p>

<pre><code> [1] "volunteer_identifier" "type_of_experiment"    
 [3] "body_acceleration_mean()-X" "body_acceleration_mean()-Y"                     
 [5] "body_acceleration_mean()-Z"  "body_acceleration_std()-X"     
</code></pre>

<p>(I takes this 6 first names as MWE but I have a vector of 68 names)</p>

<p>Now, suppose I want to take the average of each of the measurements per <code>volunteer_identifier</code> and <code>type_of_experiment</code>. For this, I used a combination of <code>split</code> and <code>lapply</code>:</p>

<pre><code>mylist&lt;-split(mergedData2,list(mergedData2$volunteer_identifier,mergedData2$type_of_experiment))
average_activities&lt;-lapply(mylist,function(x) colMeans(x))
average_dataset&lt;-t(as.data.frame(average_activities))
</code></pre>

<p>As <code>average_activities</code> is a list, I converted it into a data frame and transposed this data frame to keep the same format as <code>mergedData</code> and <code>mergedData2</code>. The problem now is the following: when I call <code>names(average_dataset)</code>, it returns <code>NULL</code> !! But, more strangely, when I do:<code>head(average_dataset)</code> ; it returns : </p>

<pre><code> volunteer_identifier type_of_experiment body_acceleration_mean()-X body_acceleration_mean()-Y 
                    1                  1    0.2773308                -0.01738382
                    2                  1    0.2764266                -0.01859492
                    3                  1    0.2755675                -0.01717678
                    4                  1    0.2785820                -0.01483995
                    5                  1    0.2778423                -0.01728503
                    6                  1    0.2836589                -0.01689542
</code></pre>

<p>This is just a small sample of the output, to say that the names of the variables are there. So why <code>names(average_dataset)</code> returns <code>NULL</code> ?</p>

<p>Thanks in advance for your reply, best </p>

<p>EDIT: Here is an MWE for mergedData2:</p>

<pre><code> volunteer_identifier type_of_experiment body_acceleration_mean()-X body_acceleration_mean()-Y
   1             2                  5                  0.2571778                -0.02328523
   2             2                  5                  0.2860267                -0.01316336
   3             2                  5                  0.2754848                -0.02605042
   4             2                  5                  0.2702982                -0.03261387
   5             2                  5                  0.2748330                -0.02784779
   6             2                  5                  0.2792199                -0.01862040
 body_acceleration_mean()-Z body_acceleration_std()-X body_acceleration_std()-Y body_acceleration_std()-Z
1                -0.01465376                -0.9384040                -0.9200908                -0.6676833
2                -0.11908252                -0.9754147                -0.9674579                -0.9449582
3                -0.11815167                -0.9938190                -0.9699255                -0.9627480
4                -0.11752018                -0.9947428                -0.9732676                -0.9670907
5                -0.12952716                -0.9938525                -0.9674455                -0.9782950
6                -0.11390197                -0.9944552                -0.9704169                -0.9653163
 gravity_acceleration_mean()-X gravity_acceleration_mean()-Y gravity_acceleration_mean()-Z
1                     0.9364893                    -0.2827192                     0.1152882
2                     0.9274036                    -0.2892151                     0.1525683
3                     0.9299150                    -0.2875128                     0.1460856
4                     0.9288814                    -0.2933958                     0.1429259
5                     0.9265997                    -0.3029609                     0.1383067
6                     0.9256632                    -0.3089397                     0.1305608
 gravity_acceleration_std()-X gravity_acceleration_std()-Y gravity_acceleration_std()-Z
1                   -0.9254273                   -0.9370141                   -0.5642884
2                   -0.9890571                   -0.9838872                   -0.9647811
3                   -0.9959365                   -0.9882505                   -0.9815796
4                   -0.9931392                   -0.9704192                   -0.9915917
5                   -0.9955746                   -0.9709604                   -0.9680853
6                   -0.9988423                   -0.9907387                   -0.9712319
</code></pre>

<p>My duty is to get this average_dataset (which is a dataset which contains the average value for each physical quantity (column 3 and onwards) for each volunteer and type of experiment (e.g 1 1 mean1 mean2 mean3...mean68
2 1 mean1 mean2 mean3...mean68, etc)</p>

<p>After this I will have to export it as a txt file (so I think using write.table with row.names=F, and col.names=T). Note that for now, if I do this and import the dataset generated using read.table, I don't recover the names of the columns of the dataset; even while specifying col.names=T.</p>