title: Multiply 2 columns from two different data frames, matching rows by common factors
body: <p>I have 2 data frames.</p>
  <p>Df1:</p>
  <pre><code>Treatment Time  Species    Base      mn      h
     &lt;fct&gt;  &lt;fct&gt;  &lt;fct&gt;    &lt;fct&gt;     &lt;dbl&gt;  &lt;dbl&gt;
   1 TR      T1      A        C       0.340 0.00435
   2 TR      T2      A        C       0.329 0.00679
   3 TR      T1      B        C       0.336 0.00429
   4 TR      T2      B        C       0.328 0.00701
   5 UTC     T0     UTC       C       0.312 0.00542
   6 TR      T1      A        H       0.350 0.00256
   7 TR      T2      A        H       0.315 0.00525
   8 TR      T1      B        H       0.346 0.00181
   9 TR      T2      B        H       0.317 0.00400
  10 UTC     T0     UTC       H       0.344 0.00384
  11 TR      T1      A        L       0.18  0.00382
  12 TR      T2      A        L       0.219 0.00617
  13 TR      T1      B        L       0.198 0.00570
  14 TR      T2      B        L       0.223 0.00567
  15 UTC     T0     UTC       L       0.191 0.00601
  16 TR      T1      A        S       0.131 0.00117
  17 TR      T2      A        S       0.137 0.00530
  18 TR      T1      B        S       0.120 0.00406
  19 TR      T2      B        S       0.133 0.00310
  20 UTC     T0     UTC       S       0.153 0.00176
  </code></pre>
  <p>and Df2:</p>
  <pre><code>Species Variety Time    Value    Residual_p
      &lt;fct&gt;  &lt;fct&gt;  &lt;fct&gt;    &lt;dbl&gt;       &lt;dbl&gt;
  1    UTC   Blue    T0    0.00000000  1.0000000
  2     A    Blue    T1    0.03136850  0.9686315
  3     A    Blue    T2    0.17359611  0.8264039
  4     B    Blue    T1    0.03734234  0.9626577
  5     B    Blue    T2    0.10303113  0.8969689
  6    UTC   Green   T0    0.00000000  1.0000000
  7     A    Green   T1    0.04072433  0.9592757
  8     A    Green   T2    0.09362780  0.9063722
  9     B    Green   T1    0.03092473  0.9690753
  10    B    Green   T2    0.07051702  0.9294830
  </code></pre>
  <p>I create a third data frame where 1:4 columns are the same of Df1 by taping</p>
  <pre><code>Df_Blue&lt;-Df1[,1:4]
  </code></pre>
  <p>Then I have to create the column 5 in Df_Blue where rows are the product of multiplying Df1$mn by Df2$Residual_p but factors Species and Time must match and only rows of Df2$Variety == &quot;Blue&quot; should be considered.</p>
  <p>Hope it's clear.</p>
  <p>Thank you!</p>