scan

The keyword scan causes a process function to be applied to each line of a table in a specific order, resulting in a new vector.

‘proc(expr1) scan expr2’, ordering of the process

The scan option can be applied to any Envision process to control the order of the operations over the processed table.

table T = with
  [| as N, as L |]
  [| 1,    "b"  |]
  [| 0,    "a"  |]
  [| 2,    "c"  |]

def process myConcat(t: text) with
  keep txt = ""
  txt = "\{txt}\{t}"
  return txt

T.Asc = myConcat(T.L) scan T.N
T.Desc = myConcat(T.L) scan -T.N

show table "" with T.N, T.L, T.Asc, T.Desc

The show statement results in the following table:

The first invocation of myConcat follows the ascending ordering of T.L’s elements according to T.N: first "a", then "b", then "c". The computation proceeds as follows:

1. txt = "a"
2. txt = "ab"
3. txt = "abc"

For the second invocation of myConcat, we follow the descending ordering: first "c", then "b", then "a". The computation proceeds as follows:

1. txt = "c"
2. txt = "cb"
3. txt = "cba"

The result of scan in both cases is a vector containing all the intermediate results of the computation, using the order in which respective elements were handled.

It is also possible to specify auto for the ordering, in which case the primary dimension of the table will be used. The primary dimension for table comprehension (table T = with) will comprise of distinct ordinal values assigned to respective lines of the table; however, it is unspecified what those ordinal values will be. In general, using auto for ordinal and enumeration types is considered a bad practice. For the sake of completeness, the following example demonstrates the use of auto:

table T = with
  [| as N, as L |]
  [| 1,    "b"  |]
  [| 0,    "a"  |]
  [| 2,    "c"  |]

def process myConcat(t: text) with
  keep txt = ""
  txt = "\{txt}\{t}"
  return txt

T.Aut = myConcat(T.L) scan auto

show table "" with T.N, T.L, T.Aut

As of Dec 7, 2024, the following table is produced:

However, the ordering of the elements in the Aut column may change arbitrarily due to 1) a new version of Envision or 2) any change in the above script.

See also

• process
• sort
• auto