title: Add value (from 2nd dataframe) in new column (in 1st dataframe) based on equality value of another column from both dataframe in R
body: <p>I am trying to add the value from each row of the column ConversionFactor (from df2) to new column ($value) of df1 when the value of same row of column (NameSize) from df1 and df2 are equals.</p>
  <p>See the code:</p>
  <p>df1$value[df2$NameSize == df1$NameSize] &lt;- df2$ConversionFactor[df1$NameSize == df2$NameSize]</p>
  <p>the two dataframe doesnt have same lenght, I have this warning message:</p>
  <p>Warning messages:
  1: In data_merged_2018_2019_1$NameSize == final_carbonfactor$NameSize :
  longer object length is not a multiple of shorter object length
  2: In if (data_merged_2018_2019_1$NameSize == final_carbonfactor$NameSize) { :
  the condition has length &gt; 1 and only the first element will be used</p>