Setting and Unsetting missing values

Set missing

!M v changes data values of v to missing values.
One of the operators < <= <> >= > may be inserted before v to modify the set of values changed to missing.
If used after !A or !I, value should refer to the encoded factor level rather than the value in the data file.
  Yield  !M-9  # Missing values were entered as -9

Unset missing

!NA value replaces missing data values with value and
!NA field replaces missing data with values from the named field.
 Rate !NA 0  # Change missing values to zero
 JuneWt !NA MayWt  # Insert May wt if June wt is missing

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