HP OpenVMS Systems Documentation

HP OpenVMS DCL Dictionary

Extracts the specified characters from the specified string.

Format

F$EXTRACT (start,length,string)

Return Value

Arguments

start

Specifies the offset of the starting character of the string you want to extract. Specify the start argument as an integer expression that is greater than or equal to zero.

The offset is the relative position of a character or a substring with respect to the beginning of the string. Offset positions begin with zero. The string always begins with the leftmost character.

If you specify an offset that is greater than or equal to the length of the string, F$EXTRACT returns a null string ("").

length

Specifies the number of characters you want to extract; must be less than or equal to the size of the string. Specify the length as an integer expression that is greater than or equal to zero.

If you specify a length that exceeds the number of characters from the offset to the end of the string, the F$EXTRACT function returns the characters from the offset through the end of the string.

string

Specifies the character string to be edited. Specify the string as a character string expression.

Examples

$ NAME = "PAOLO TESTA"
$ FIRST = F$EXTRACT(0,5,NAME)
$ SHOW SYMBOL FIRST
  FIRST = "PAOLO"

      

This portion of a command procedure uses the F$EXTRACT function to extract the first 5 characters from the character string assigned to the symbol NAME. The offset and length arguments are integers, and the string argument is a symbol. You do not need to use quotation marks (" ") around integers or symbols when they are used as arguments for lexical functions.

$ P1 = "MYFILE.DAT"
$ FILENAME = F$EXTRACT(0,F$LOCATE(".",P1),P1)

      

This portion of a command procedure shows how to locate a character within a string, and how to extract a substring ending at that location.

The lexical function F$LOCATE gives the numeric value representing the offset position of a period in the character string value of P1. (The offset position of the period is equal to the length of the substring before the period.)

This F$LOCATE function is used as an argument in the F$EXTRACT function to specify the number of characters to extract from the string. If a procedure is invoked with the parameter MYFILE.DAT, these statements result in the symbol FILENAME being given the value MYFILE.

Note that the F$LOCATE function in the above example assumes that the file specification does not contain a node name or a directory specification containing a subdirectory name. To obtain the file name from a full file specification, use the F$PARSE function.

$ IF F$EXTRACT(12,2,F$TIME()) .GES. "12" THEN GOTO AFTERNOON
$ MORNING:
$ WRITE SYS$OUTPUT "Good morning!"
$ EXIT
$ AFTERNOON:
$ WRITE SYS$OUTPUT "Good afternoon!"
$ EXIT

      

This example shows a procedure that displays a different message, depending on whether the current time is morning or afternoon. It first obtains the current time of day by using the F$TIME function. The F$TIME function returns a character string, which is the string argument for the F$EXTRACT function. The F$TIME function is automatically evaluated when it is used as an argument, so you do not need to use quotation marks.

Next, the F$EXTRACT function extracts the hours from the date and time string returned by F$TIME. The string returned by F$TIME always contains the hours field beginning at an offset of 12 characters from the start of the string.

The F$EXTRACT function extracts 2 characters from the string, beginning at this offset, and compares the string value extracted with the string value 12. If the comparison is true, then the procedure writes "Good afternoon!". Otherwise, it writes "Good morning!".

Note that you can also use the F$CVTIME function to extract the hour field from a time specification. This method is easier than the one shown in the above example.