title: Finding running maximum by group
body: <p>I need to find a running maximum of a variable by group using R. The variable is sorted by time within group using <code>df[order(df$group, df$time),]</code>.</p>
  
  <p>My variable has some NA's but I can deal with it by replacing them with zeros for this computation.</p>
  
  <p>this is how the data frame df looks like:</p>
  
  <pre><code>(df &lt;- structure(list(var = c(5L, 2L, 3L, 4L, 0L, 3L, 6L, 4L, 8L, 4L),
                 group = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L),
                                   .Label = c("a", "b"), class = "factor"),
                 time = c(1L, 2L, 3L, 4L, 5L, 1L, 2L, 3L, 4L, 5L)),
            .Names = c("var", "group","time"),
            class = "data.frame", row.names = c(NA, -10L)))
  
  #    var group time
  # 1    5     a    1
  # 2    2     a    2
  # 3    3     a    3
  # 4    4     a    4
  # 5    0     a    5
  # 6    3     b    1
  # 7    6     b    2
  # 8    4     b    3
  # 9    8     b    4
  # 10   4     b    5
  </code></pre>
  
  <p>And I want a variable curMax as:</p>
  
  <pre><code>var  |  group  |  time  |  curMax
  5       a         1         5
  2       a         2         5
  3       a         3         5
  4       a         4         5
  0       a         5         5
  3       b         1         3
  6       b         2         6
  4       b         3         6
  8       b         4         8
  4       b         5         8
  </code></pre>
  
  <p>Please let me know if you have any idea how to implement it in R.</p>