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String Variables

SM maintains a set of variables which are defined with the statement

DEFINE name value

or

DEFINE name { value_list 2

or

DEFINE name ( expression )

Value may be a word or a number. Value_list has no such restrictions and may contain many words. Note that due to the presence of the {2, variables are not expanded in value_list, whereas they are in value. In fact, the list can be delimited by <> rather than {2; see DEFINE for details.

The expression in DEFINE variable ( expr ) should be a scalar; if it is not, the first element of the vector will be used and you will be warned, if VERBOSE (@xref{VERBOSE}) is one or greater. Sometimes you just want to evaluate an expression and treat the answer as a string; in this case use the special vector form $(expr) which is replaced by the value of the expression -- for example echo e is $(exp(1)). Expressions are further discussed under `Vectors and Arithmetic'.

There are a number of special variables whose value is always the current value of some internal SM variable such as the current position or the point type. The variable "date" is also special and expands to give the current time and date, -- try typing echo $date. You can freeze these variables at their current value by saying define name | (see below).

Each time SM reads $name it replaces it by its value, considered as a character string. For example, @br

DEFINE hi hello
WRITE STANDARD $hi

DEFINE name Aelfred

XLABEL My name is $name

A variable can be deleted by DEFINE name DELETE so for example the macro

MACRO undef 1 { DEFINE $1 DELETE 2

undef name

There are also three special values, :, |, and ?. The command define name : means `get the value of name from the environment file'. If this fails, and if the variable is all uppercase, SM will then try to use the value of an environment (VMS: logical) variable of the same name. Using define name ? means `read the value of name from the keyboard'. You can specify a prompt to be used, see DEFINE for details. The form with | has changed a little with version 2.1.1. The variables that you can use with | have not changed, but their usage has slightly. They are all defined for you when SM starts and each is always correct, tracking the current value of the corresponding internal variable. For example, try echo $angle angle 45 echo $angle. If you now say define angle |, $angle will cease to track the internal value and will remain fixed (the same effect can be achieved with define angle 45). When you say define angle delete it will once more track the internal value. Your old code will continue to work, but in many cases it is possible to remove the explicit definition with |. This special sort of variable will not be SAVEd, and will not show up if you list the currently defined variables. A list of the | variables is given in the section on DEFINE.

So using the example `.sm' environment file listed in the previous section of the manual, DEFINE name : will define name to be Robert, DEFINE angle | will give the last value set by the ANGLE command, and DEFINE datafile ? will ask you for the value of `datafile', which can be useful in macros. For example, @br

DEFINE noise ? { Ring bell? 2 IF('$noise' != 'n') { bell 2

When writing macros, it is also sometimes useful to know if a variable has been defined. The variable $?name has the value 1 if name is defined, otherwise it is 0. For instance, there is a line@br

define term : if($?term) { termtype $term 2 

There are also commands to read the values of variables from data files defined with the DATA command.

DEFINE name READ i

or

DEFINE name READ i j

All currently defined variables may be listed with

LIST DEFINE [ begin end ]

Variables are usually not expanded within double quotes or { 2. If for some reason you need to force expansion within double quotes, it can be done with $!name. The macro `load' discussed under useful macros gives an example of this mechanism. If you need to expand a variable, with no questions asked (and even within {2), use $!!name.

Sometimes you may want to terminate a variable name where SM doesn't want to, and this can be done with a trick involving double quotes. Say you are writing a macro to find all the stars redder than B-V = 1.0 in a set of data vectors, and you want to rename them with a trailing "_red", so star goes to star_red. So you write a foreach loop,@br

FOREACH x ( U B V R I J K ) { SET $x_red = $x IF(B-V >1)2

Variables are string-variables, and are not primarily designed for doing arithmetic (that's what vectors are for). This is a common source of confusion so let's consider some examples (at the risk of anticipating some later sections of the manual).

DEFINE a 10

SET x=10

SET y=$a + x*12

DEFINE y $a+x*12

DEFINE y <$a+x*12>

DEFINE y ( $a+x*12 )

The fact that variables are simply strings can be used to build complex commands; consider for example the macro

readem          # read multiple lines columns with names in row 1
                READ ROW names 1.s
                DEFINE rc <$(names[(0)]) 1>
                DO i=2,DIMEN(names) {
                        DEFINE rc <$rc $(names[($i - 1)]) $i>
                }
                LINES 2 0
                READ < $rc >

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