title: Merge columnwise from file_list
body: <p>I have 96 files in <code>file_list</code></p>
  
  <pre><code>file_list &lt;- list.files(pattern = "*.mirna")
  </code></pre>
  
  <p>They all have the same columns, but the number of rows varies. Example file:</p>
  
  <pre><code>&gt; head(test1)
                         seq          name freq             mir start end mism   add t5  t3       s5       s3    DB
  1    TGGAGTGTGATAATGGTGTTT seq_100003_x4    4  hsa-miR-122-5p    15  35 11TC     0  0   g GCTGTGGA TTTGTGTC miRNA
  2 TGTAAACATCCCCGACCGGAAGCT seq_100045_x4    4  hsa-miR-30d-5p     6  29 17CT     0  0  CT TTGTTGTA GAAGCTGT miRNA
  3   CTAGACTGAAGCTCCTTGAAAA seq_100048_x4    4 hsa-miR-151a-3p    47  65    0 I-AAA  0  gg CCTACTAG GAGGACAG miRNA
  4   AGGCGGAGACTTGGGCAATTGC seq_100059_x4    4   hsa-miR-25-5p    14  35    0     0  0   C TGAGAGGC ATTGCTGG miRNA
  5    AAACCGTTACCATTACTGAAT seq_100067_x4    4    hsa-miR-451a    17  35    0  I-AT  0 gtt AAGGAAAC AGTTTAGT miRNA
  6   TGAGGTAGTAGCTTGTGCTGTT seq_10007_x24   24   hsa-let-7i-5p     6  27 12CT     0  0   0 TGGCTGAG TGTTGGTC miRNA
       precursor ambiguity
  1  hsa-mir-122         1
  2  hsa-mir-30d         1
  3 hsa-mir-151a         1
  4   hsa-mir-25         1
  5 hsa-mir-451a         1
  6   hsa-let-7i         1
  </code></pre>
  
  <p>second file </p>
  
  <pre><code>&gt; head(test2)
                        seq           name freq             mir start end mism  add t5 t3       s5       s3    DB
  1   ATTGCACTTGTCCTGGCCTGT seq_1000013_x1    1  hsa-miR-92a-3p    49  69 14TC    0  t  0 AAAGTATT CTGTGGAA miRNA
  2   AAACCGTTACTATTACTGAGA seq_1000094_x1    1    hsa-miR-451a    17  36 11TC  I-A  0 tt AAGGAAAC AGTTTAGT miRNA
  3  TGAGGTAGCAGATTGTATAGTC seq_1000169_x1    1   hsa-let-7f-5p     8  28  9CT  I-C  0  t GGGATGAG AGTTTTAG miRNA
  4    TGGGTCTTTGCGGGCGAGAT seq_100019_x12   12 hsa-miR-193a-5p    21  40    0    0  0 ga GGGCTGGG ATGAGGGT miRNA
  5  TGAGGTAGTAGATTGTATAGTG seq_100035_x12   12   hsa-let-7f-5p     8  28    0  I-G  0  t GGGATGAG AGTTTTAG miRNA
  6 TGAAGTAGTAGGTTGTGTGGTAT seq_1000437_x1    1   hsa-let-7b-5p     6  26  4AG I-AT  0  t GGGGTGAG GGTTTCAG miRNA
        precursor ambiguity
  1 hsa-mir-92a-2         1
  2  hsa-mir-451a         1
  3  hsa-let-7f-2         1
  4  hsa-mir-193a         1
  5  hsa-let-7f-2         1
  6    hsa-let-7b         1
  </code></pre>
  
  <p>I would like to create a unique ID consisting of the columns <code>mir</code> and <code>seq</code>:</p>
  
  <p>hsa-miR-122-5p_TGGAGTGTGATAATGGTGTTT</p>
  
  <p>Then I would like to merge all the 96 files based in this ID and take the column <code>freq</code> form each file. </p>
  
  <pre><code>ID                                    freq_file1     freq_file2     ...
  hsa-miR-122-5p_TGGAGTGTGATAATGGTGTTT  4           12
  </code></pre>
  
  <p>If an ID is not pressent in a specific file the <code>freq</code> should be NA</p>