title: Error: unsupported use of matrix or array for column indexing
body: <p>I have a list of variables name "comorbid_names". And I want to select people who have those comorbidities in "comorbidities". However, I want to select the variable names if they are true. </p>
  
  <p>For example patient 1 has "chd" only, therefore only that will be displayed as TRUE</p>
  
  <blockquote>
    <p>comorbid_names
     [1] "chd"                         "heart_failure"               "stroke"<br>
     [4] "hypertension"                "diabetes"                    "copd"<br>
     [7] "epilepsy"                    "hypothyroidism"              "cancer"<br>
    [10] "asthma"                      "ckd_stage3"                  "ckd_stage4"<br>
    [13] "ckd_stage5"                  "atrial_fibrilation"          "learning_disability"<br>
    [16] "peripheral_arterial_disease" "osteoporosis"<br>
    class(comorbid_names)
    [1] "character"</p>
  </blockquote>
  
  <p>comorbidities &lt;- names(p[, comorbid_names][p[, comorbid_names] == 1])</p>
  
  <p>At this point I get this error</p>
  
  <p>Error: Unsupported use of matrix or array for column indexing</p>
  
  <p>I am not entirely sure why, but I think it's to do with comorbid_names being character</p>
  
  <p>Does anyone have an advice?</p>