String Scanning
One of the powerful features of Icon is string scanning. The scan string operator, ? saves the current string scanning environment and creates a new string scanning environment. The string scanning environment consists of two keyword variables, &subject and &pos. Where &subject is the string being scanned, and &pos is the cursor or current position within the subject string.
For example
s := "this is a string" s ? write("subject= pos=")
would produce subject= pos=
Built-in and user defined functions can be used to move around within the string being scanned. Many of the built in functions will default to &subject and &pos (for example the find function). The following, for example, will write all blank delimited "words" in a string.
s := "this is a string" s ? { # Establish string scanning environment while not pos(0) do { # Test for end of string tab(many(' ')) # Skip past any blanks word := tab(upto(' ') | 0) # the next word is up to the next blank -or- the end of the line write(word) # write the word } }
A more complicated example demonstrates the integration of generators and string scanning within the language.
procedure main s := "Mon Dec 8" s ? write(Mdate | "not a valid date")
end
# Define a matching function that returns
# a string that matches a day month dayofmonth
procedure Mdate
# Define some initial values
static dates
static days
initial { days := dates := ["Jan","Feb","Mar","Apr","May","Jun", "Jul","Aug","Sep","Oct","Nov","Dec"]
}
every suspend (retval <- tab(match(!days)) || # Match a day =" " || # Followed by a blank tab(match(!dates)) || # Followed by the month =" " || # Followed by a blank matchdigits(2) # Followed by at least 2 digits ) & (=" " | pos(0) ) & # Either a blank or the end of the string retval # And finally return the string
end
# Matching function that returns a string of n digits
procedure matchdigits(n) suspend (v := tab(many(&digits)) & *v <= n) & v
end
The idiom of expr1 & expr2 & expr3 returns the value of the last expression
Read more about this topic: Icon (programming Language)
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