modify

           modifies data point values (x/y/error)

           cafe

           modify, x|y|error|file=expression, [,range] [,subgroup][,group][,/debug]

          x,y,error - The column to modify (select one).
          file      - Change the file name.
         expression - Defines how to modify. This may be any IDL
                      expression.  Following variables have special meaning:
                      x     - the x column.
                      y     - the y column.
                      error - the error
                      dx    - the x bin size
                      model - apply the model at the x-column, insert
                              result.
                    selected- the select flag
                      def   - the data point defined flag
                      file  - the file name.
                      Case is important. When using external functions
                      containing x/y/err it is recomended to give
                      these functions upper case.
                      The variables may be appended with ":<group>"
                      to import the variables from different groups,
                      same subgroup. The number of rows must be the
                      same. Also the model group (model item above)
                      can be used to select the group.
                      The expression either must define an array which has
                      as much elements as the column to modify, or
                      contain scalar values.
                      It is possible to select array elements with
                      "[index]" but care must be taken when ranges
                      are used; the index refers to the range restricted array.
                      The error column can be deleted with the
                      expression "error=none".
           range    - (optional) Defines range of data points to modify. This can
                      be either:
                       - Data point range, denoted by the data
                         point number:
                         <n1>[-], while ,  are positive
                         numbers. Data point numbers start from zero.
                         Open intervals can be represented with a
                         "*" instead of the number.
                         If only one number is given, a single
                         data point will be referenced.
                         If <val2> is less than  all but the
                         interval <val2>.. are referenced.
                       - X value range, denoted by float point numbers:
                         <val1>-, while , 
                         represents X values defining the interval to
                         reference. Open intervals can be represented
                         with a *  instead of the value. If <val2> is
                         less than <val1> all but the interval
                         <val2>.. is referenced.
                       - Vector of data point indices. The vector is
                         denoted with [val,val,...,val] (indgen may
                         be used also).
                       - Boolean expressions with X/Y values. All
                         values for which this expression is true are
                         referenced. The expression may contain
                         algebraic formulas combined with comparison
                         operators "lt", "le", "eq", "gt", "ge" as used in IDL
                         comparisons. For each data point a keyword
                         is defined which has meaning and can be used
                         for the boolean expression:
                                 x - The x-column value.
                                 y - The y-column value.
                             error - The error column value.
                                dx - The x bin size (if given)
                          selected - True when the data point is selected.
                               def - True when the data point is defined.
                               x1  - Multi column first column value.
                               x2  - Multi column second column
                                     value.
                               x3  - Multi column third column value.
                         To avoid interference with functions which
                         also can be used a lower/upper case
                         distinction  is made. IDL funktions should
                         be upper case.
                      Default is the entire data set to modify.
           subgroup - (optional) The data set (subgroup) for which the expression
                      should be applied. This can be either the
                      subgroup number or the file name representing
                      the data set. It is possible to set more than
                      one subgroup, either with numbers in brackets
                      ([]) separated with ","; or denoting filenames
                      with wildcards ("*").
                      Default is the first valid subgroup in current used
                      group.
           group    - (optional) The data group for which the expression
                      should be applied.
                      Default is the primary group 0. Must be in
                      range [0..29].

           debug    - print modification expression for debugging
                      purposes.

           Changes data point values (no possibility to recover
           except reload!)

           > modify, y=2*x+0.5,0-100,0,1
                 -> modify all y values for data points 0..100,
                 group 1, subgroup 0 at linear x values.

           $Id: cafe_modify.pro,v 1.29 2005/01/05 09:51:45 goehler Exp $

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