17.5.6 Calling Calc from Your Lisp Programs

A later section (see Calculator Internals) gives a full description of Calc’s internal Lisp functions. It’s not hard to call Calc from inside your programs, but the number of these functions can be daunting. So Calc provides one special “programmer-friendly” function called calc-eval that can be made to do just about everything you need. It’s not as fast as the low-level Calc functions, but it’s much simpler to use!

It may seem that calc-eval itself has a daunting number of options, but they all stem from one simple operation.

In its simplest manifestation, ‘(calc-eval "1+2")’ parses the string "1+2" as if it were a Calc algebraic entry and returns the result formatted as a string: "3".

Since calc-eval is on the list of recommended autoload functions, you don’t need to make any special preparations to load Calc before calling calc-eval the first time. Calc will be loaded and initialized for you.

All the Calc modes that are currently in effect will be used when evaluating the expression and formatting the result. Additional Arguments to calc-eval

If the input string parses to a list of expressions, Calc returns the results separated by ", ". You can specify a different separator by giving a second string argument to calc-eval: ‘(calc-eval "1+2,3+4" ";")’ returns "3;7".

The “separator” can also be any of several Lisp symbols which request other behaviors from calc-eval. These are discussed one by one below.

You can give additional arguments to be substituted for ‘$’, ‘$$’, and so on in the main expression. For example, ‘(calc-eval "$/$$" nil "7" "1+1")’ evaluates the expression "7/(1+1)" to yield the result "3.5" (assuming Fraction mode is not in effect). Note the nil used as a placeholder for the item-separator argument. Error Handling

If calc-eval encounters an error, it returns a list containing the character position of the error, plus a suitable message as a string. Note that ‘1 / 0’ is not an error by Calc’s standards; it simply returns the string "1 / 0" which is the division left in symbolic form. But ‘(calc-eval "1/")’ will return the list ‘(2 "Expected a number")’.

If you bind the variable calc-eval-error to t using a let form surrounding the call to calc-eval, errors instead call the Emacs error function which aborts to the Emacs command loop with a beep and an error message.

If you bind this variable to the symbol string, error messages are returned as strings instead of lists. The character position is ignored.

As a courtesy to other Lisp code which may be using Calc, be sure to bind calc-eval-error using let rather than changing it permanently with setq. Numbers Only

Sometimes it is preferable to treat ‘1 / 0’ as an error rather than returning a symbolic result. If you pass the symbol num as the second argument to calc-eval, results that are not constants are treated as errors. The error message reported is the first calc-why message if there is one, or otherwise “Number expected.”

A result is “constant” if it is a number, vector, or other object that does not include variables or function calls. If it is a vector, the components must themselves be constants. Default Modes

If the first argument to calc-eval is a list whose first element is a formula string, then calc-eval sets all the various Calc modes to their default values while the formula is evaluated and formatted. For example, the precision is set to 12 digits, digit grouping is turned off, and the Normal language mode is used.

This same principle applies to the other options discussed below. If the first argument would normally be x, then it can also be the list ‘(x)’ to use the default mode settings.

If there are other elements in the list, they are taken as variable-name/value pairs which override the default mode settings. Look at the documentation at the front of the calc.el file to find the names of the Lisp variables for the various modes. The mode settings are restored to their original values when calc-eval is done.

For example, ‘(calc-eval '("$+$$" calc-internal-prec 8) 'num a b)’ computes the sum of two numbers, requiring a numeric result, and using default mode settings except that the precision is 8 instead of the default of 12.

It’s usually best to use this form of calc-eval unless your program actually considers the interaction with Calc’s mode settings to be a feature. This will avoid all sorts of potential “gotchas”; consider what happens with ‘(calc-eval "sqrt(2)" 'num)’ when the user has left Calc in Symbolic mode or No-Simplify mode.

As another example, ‘(equal (calc-eval '("$<$$") nil a b) "1")’ checks if the number in string ‘a’ is less than the one in string ‘b’. Without using a list, the integer 1 might come out in a variety of formats which would be hard to test for conveniently: "1", "8#1", "00001". (But see “Predicates” mode, below.) Raw Numbers

Normally all input and output for calc-eval is done with strings. You can do arithmetic with, say, ‘(calc-eval "$+$$" nil a b)’ in place of ‘(+ a b)’, but this is very inefficient since the numbers must be converted to and from string format as they are passed from one calc-eval to the next.

If the separator is the symbol raw, the result will be returned as a raw Calc data structure rather than a string. You can read about how these objects look in the following sections, but usually you can treat them as “black box” objects with no important internal structure.

There is also a rawnum symbol, which is a combination of raw (returning a raw Calc object) and num (signaling an error if that object is not a constant).

You can pass a raw Calc object to calc-eval in place of a string, either as the formula itself or as one of the ‘$’ arguments. Thus ‘(calc-eval "$+$$" 'raw a b)’ is an addition function that operates on raw Calc objects. Of course in this case it would be easier to call the low-level math-add function in Calc, if you can remember its name.

In particular, note that a plain Lisp integer is acceptable to Calc as a raw object.

When it comes time to display the object, just use ‘(calc-eval a)’ to format it as a string.

It is an error if the input expression evaluates to a list of values. The separator symbol list is like raw except that it returns a list of one or more raw Calc objects.

Note that a Lisp string is not a valid Calc object, nor is a list containing a string. Thus you can still safely distinguish all the various kinds of error returns discussed above. Predicates

If the separator symbol is pred, the result of the formula is treated as a true/false value; calc-eval returns t or nil, respectively. A value is considered “true” if it is a non-zero number, or false if it is zero or if it is not a number.

For example, ‘(calc-eval "$<$$" 'pred a b)’ tests whether one value is less than another.

As usual, it is also possible for calc-eval to return one of the error indicators described above. Lisp will interpret such an indicator as “true” if you don’t check for it explicitly. If you wish to have an error register as “false”, use something like ‘(eq (calc-eval ...) t)’. Variable Values

Variables in the formula passed to calc-eval are not normally replaced by their values. If you wish this, you can use the evalv function (see Algebraic Manipulation). For example, if 4 is stored in Calc variable a (i.e., in Lisp variable var-a), then ‘(calc-eval "a+pi")’ will return the formula "a + pi", but ‘(calc-eval "evalv(a+pi)")’ will return "7.14159265359".

To store in a Calc variable, just use setq to store in the corresponding Lisp variable. (This is obtained by prepending ‘var-’ to the Calc variable name.) Calc routines will understand either string or raw form values stored in variables, although raw data objects are much more efficient. For example, to increment the Calc variable a:

(setq var-a (calc-eval "evalv(a+1)" 'raw)) Stack Access

If the separator symbol is push, the formula argument is evaluated (with possible ‘$’ expansions, as usual). The result is pushed onto the Calc stack. The return value is nil (unless there is an error from evaluating the formula, in which case the return value depends on calc-eval-error in the usual way).

If the separator symbol is pop, the first argument to calc-eval must be an integer instead of a string. That many values are popped from the stack and thrown away. A negative argument deletes the entry at that stack level. The return value is the number of elements remaining in the stack after popping; ‘(calc-eval 0 'pop)’ is a good way to measure the size of the stack.

If the separator symbol is top, the first argument to calc-eval must again be an integer. The value at that stack level is formatted as a string and returned. Thus ‘(calc-eval 1 'top)’ returns the top-of-stack value. If the integer is out of range, nil is returned.

The separator symbol rawtop is just like top except that the stack entry is returned as a raw Calc object instead of as a string.

In all of these cases the first argument can be made a list in order to force the default mode settings, as described above. Thus ‘(calc-eval '(2 calc-number-radix 16) 'top)’ returns the second-to-top stack entry, formatted as a string using the default instead of current display modes, except that the radix is hexadecimal instead of decimal.

It is, of course, polite to put the Calc stack back the way you found it when you are done, unless the user of your program is actually expecting it to affect the stack.

Note that you do not actually have to switch into the *Calculator* buffer in order to use calc-eval; it temporarily switches into the stack buffer if necessary. Keyboard Macros

If the separator symbol is macro, the first argument must be a string of characters which Calc can execute as a sequence of keystrokes. This switches into the Calc buffer for the duration of the macro. For example, ‘(calc-eval "vx5\rVR+" 'macro)’ pushes the vector ‘[1,2,3,4,5]’ on the stack and then replaces it with the sum of those numbers. Note that ‘\r’ is the Lisp notation for the carriage return, RET, character.

If your keyboard macro wishes to pop the stack, ‘\C-d’ is safer than ‘\177’ (the DEL character) because some installations may have switched the meanings of DEL and C-h. Calc always interprets C-d as a synonym for “pop-stack” regardless of key mapping.

If you provide a third argument to calc-eval, evaluation of the keyboard macro will leave a record in the Trail using that argument as a tag string. Normally the Trail is unaffected.

The return value in this case is always nil. Lisp Evaluation

Finally, if the separator symbol is eval, then the Lisp eval function is called on the first argument, which must be a Lisp expression rather than a Calc formula. Remember to quote the expression so that it is not evaluated until inside calc-eval.

The difference from plain eval is that calc-eval switches to the Calc buffer before evaluating the expression. For example, ‘(calc-eval '(setq calc-internal-prec 17) 'eval)’ will correctly affect the buffer-local Calc precision variable.

An alternative would be ‘(calc-eval '(calc-precision 17) 'eval)’. This is evaluating a call to the function that is normally invoked by the p key, giving it 17 as its “numeric prefix argument.” Note that this function will leave a message in the echo area as a side effect. Also, all Calc functions switch to the Calc buffer automatically if not invoked from there, so the above call is also equivalent to ‘(calc-precision 17)’ by itself. In all cases, Calc uses save-excursion to switch back to your original buffer when it is done.

As usual the first argument can be a list that begins with a Lisp expression to use default instead of current mode settings.

The result of calc-eval in this usage is just the result returned by the evaluated Lisp expression. Example

Here is a sample Emacs command that uses calc-eval. Suppose you have a document with lots of references to temperatures on the Fahrenheit scale, say “98.6 F”, and you wish to convert these references to Centigrade. The following command does this conversion. Place the Emacs cursor right after the letter “F” and invoke the command to change “98.6 F” to “37 C”. Or, if the temperature is already in Centigrade form, the command changes it back to Fahrenheit.

(defun convert-temp ()
    (re-search-backward "[^-.0-9]\\([-.0-9]+\\) *\\([FC]\\)")
    (let* ((top1 (match-beginning 1))
           (bot1 (match-end 1))
           (number (buffer-substring top1 bot1))
           (top2 (match-beginning 2))
           (bot2 (match-end 2))
           (type (buffer-substring top2 bot2)))
      (if (equal type "F")
          (setq type "C"
                number (calc-eval "($ - 32)*5/9" nil number))
        (setq type "F"
              number (calc-eval "$*9/5 + 32" nil number)))
      (goto-char top2)
      (delete-region top2 bot2)
      (insert-before-markers type)
      (goto-char top1)
      (delete-region top1 bot1)
      (if (string-match "\\.$" number)   ; change "37." to "37"
          (setq number (substring number 0 -1)))
      (insert number))))

Note the use of insert-before-markers when changing between “F” and “C”, so that the character winds up before the cursor instead of after it.