title: Creating data frame with variables of different sizes in R
body: <p>I have a data frame for each individual illustrated as rows.  I want to add a column that will calculate the portion of the year the employee was employed for each Year.  I do not want to add a column for each year but rather have a new variable Year that will contain the result of my formula.  the first element of this variable would be the first year of employment.  I have 25,000,000 rows, so looping is not an option.  Let assume a simple example as below:</p>
  
  <pre><code>  age Sex   DateHire   DateLeft
  1  45   M  6/30/1980 12/31/2006
  2  37   F  6/25/1994        N/A
  3  42   F 12/30/2015 12/30/2017
  </code></pre>
  
  <p>I want my output to look like this.</p>
  
  <pre><code>Year Exposure   Expected_Death
   1    3         0.0046
   2    3         0.0058
   3    3         0.0071
   4    2         0.008 
  </code></pre>
  
  <p>Exposure is the number of employees for a give Year where Year1 refers to 1st year of employment. 
  Expected_Death &lt;- Exposure * Probability_Death</p>
  
  <p>For each individual, I want to calculate the Exposure for the next 100 years after their employment.  I also want to read the Probability_Death from an Excel File.</p>
  
  <p>For each individual I wanted to define a variable Exposure and Probability_Death as such</p>
  
  <pre><code>  age Sex   DateHire   DateLeft       Exposure       Prob_Death
      1  45   M  6/30/1980 12/31/2006   1 1 1 1 1 ...  0.0035 0.0041 ...
      2  37   F  6/25/1994        N/A   1 1 1 1 1 ... 0.0009 0.0014 ...
      3  42   F 12/30/2015 12/30/2017   1 1 1 0 0 ... 0.0002 0.0003 ...
  </code></pre>
  
  <p>My Exposure is calculate from the date of hire and DateLeft. How can I define such calculated variables so my Expected Death for all years will be calculated simply by multiplying Exposure by Prob_Death ?
  thanks</p>