The READ procedures perform formatted input into variables.
READ performs input from the standard input stream (IDL file unit 0), while READF requires a file unit to be explicitly specified.
A string or explicit expression (i.e, not a named variable) to be used as a prompt. This argument should not be included if the FORMAT keyword is specified. Also, if this argument begins with the characters "$(", it is taken to be a format specification as described below under "Format Compatibility".
Using the Prompt argument does not work well with IDL for Windows and IDL for Macintosh . The desired prompt string is written to the log window instead of the command input window. To create custom prompts compatible with these versions of IDL, use the PROMPT keyword, described below.
If FORMAT is not specified, IDL uses its default rules for formatting the input. FORMAT allows the format of the input to be specified in precise detail, using a FORTRAN-style specification. See Using Explicitly Formatted Input/Output .
The relation to be used when matching the supplied key with key field values (EQ = 0, GE = 1, GT = 2) when accessing data from a file with indexed organization. If this keyword is omitted, the equality relation (0) is used.
The value of a key to be found when accessing data from a file with indexed organization. This value must match the key definition that is determined when the file was created in terms of type and size--no conversions are performed. If this keyword is omitted, the next sequential record is used.
If the FORMAT keyword is not present and READ is called with more than one argument, and the first argument is a scalar string starting with the characters "$(", this initial argument is taken to be the format specification, just as if it had been specified via the FORMAT keyword. This feature is maintained for compatibility with version 1 of VMS IDL.