title: R: Multiplying columns by a constant in a data table
body: <p>I am trying to correct my data table so my columns have the same units. Here's an example of what I have.</p>
  
  <pre><code>hh:mm   A   V   W   kA  V   kW  A   kV  kW
  11:00   13.84   470.16  6509.88 14.89   467.85  6964.38 15.74   464.01  7303.13
  11:05   12.54   475.17  5959.22 13.40   474.52  6358.89 13.34   473.13  6311.80
  11:10   9.73    476.20  4632.14 10.36   473.38  4905.86 10.38   472.73  4907.14
  11:15   9.20    479.30  4410.89 9.65    482.79  4659.67 9.73    479.09  4659.33
  11:20   11.28   482.22  5437.78 12.03   484.95  5835.33 12.24   476.36  5829.44
  11:25   11.66   481.64  5614.56 12.76   479.95  6124.56 12.88   476.86  6139.33
  11:30   10.38   475.13  4934.00 11.99   480.96  5760.44 11.50   478.77  5515.13
  </code></pre>
  
  <p>As you can see, some columns are in "A","V", or "W", while others are in "kA","kV" or "kW". What I am trying to do is to change all these to the same units by multiplying the "kA", "kV" and "kW" columns by 1000. Here's what I want to get</p>
  
  <pre><code>hh:mm   A   V   W   A   V   W   A   V   W
  11:00   13.84   470.16  6509.88 14890   467.85  6964380 15.74   464010  7303130
  11:05   12.54   475.17  5959.22 13400   474.52  6358890 13.34   473130  6311800
  11:10   9.73    476.20  4632.14 10360   473.38  4905860 10.38   472730  4907140
  11:15   9.20    479.30  4410.89 9650    482.79  4659670 9.73    479090  4659330
  11:20   11.28   482.22  5437.78 12030   484.95  5835330 12.24   476360  5829440
  11:25   11.66   481.64  5614.56 12760   479.95  6124560 12.88   476860  6139330
  11:30   10.38   475.13  4934.00 11990   480.96  5760440 11.50   478770  5515130
  </code></pre>
  
  <p>I tried doing this:</p>
  
  <pre><code>units&lt;-which(names(dt)=="kA") # Gives me a vector with the positions needed.
  dt[,units:=units*1000] #Multiplies the vector by 1000
  names(dt) &lt;- gsub("kA", "A", names(dt)) # Changes "kA" to "A"
  
  units&lt;-which(names(dt)=="kV") # Gives me a vector with the positions needed.
  dt[,units:=units*1000] #Multiplies the vector by 1000
  names(dt) &lt;- gsub("kV", "V", names(dt)) # Changes "kV" to "V"
  
  units&lt;-which(names(dt)=="kW") # Gives me a vector with the positions needed.
  dt[,units:=units*1000] #Multiplies the vector by 1000
  names(dt) &lt;- gsub("kW", "W", names(dt)) # Changes "kW" to "W"
  </code></pre>
  
  <p>On my second line, I receive this error:</p>
  
  <pre><code>Warning message:
  In `[.data.table`(x2, , `:=`(units, units * 1000)) :
    Supplied 48 items to be assigned to 286 items of column 'units' (recycled leaving remainder of 46 items).
  </code></pre>
  
  <p>Can anyone help me with my syntax please?</p>
  
  <p>PS: Here's my dput for the input, it looks a little funny though.</p>
  
  <pre><code>&gt; dput(c)
  structure(list(`hh:mm` = c("11:00", "11:05", "11:10", "11:15", 
  "11:20", "11:25", "11:30"), A = c(13.84, 12.54, 9.73, 9.2, 11.28, 
  11.66, 10.38), V = c(470.16, 475.17, 476.2, 479.3, 482.22, 481.64, 
  475.13), W = c(6509.88, 5959.22, 4632.14, 4410.89, 5437.78, 5614.56, 
  4934), kA = c(14.89, 13.4, 10.36, 9.65, 12.03, 12.76, 11.99), 
      V = c(467.85, 474.52, 473.38, 482.79, 484.95, 479.95, 480.96
      ), kW = c(6964.38, 6358.89, 4905.86, 4659.67, 5835.33, 6124.56, 
      5760.44), A = c(15.74, 13.34, 10.38, 9.73, 12.24, 12.88, 
      11.5), kV = c(464.01, 473.13, 472.73, 479.09, 476.36, 476.86, 
      478.77), kW = c(7303.13, 6311.8, 4907.14, 4659.33, 5829.44, 
      6139.33, 5515.13)), .Names = c("hh:mm", "A", "V", "W", "kA", 
  "V", "kW", "A", "kV", "kW"), row.names = c(NA, -7L), class = c("data.table", 
  "data.frame"), .internal.selfref = &lt;pointer: 0x00000000003b0788&gt;)
  </code></pre>