10.1.1 Making Selections

To select a sub-formula, move the Emacs cursor to any character in that sub-formula, and press j s (calc-select-here). Calc will highlight the smallest portion of the formula that contains that character. By default the sub-formula is highlighted by blanking out all of the rest of the formula with dots. Selection works in any display mode but is perhaps easiest in Big mode (d B). Suppose you enter the following formula:

           3    ___
    (a + b)  + V c
1:  ---------------
        2 x + 1

(by typing ' ((a+b)^3 + sqrt(c)) / (2x+1)). If you move the cursor to the letter ‘b’ and press j s, the display changes to

           .    ...
    .. . b.  . . .
1*  ...............
        . . . .

Every character not part of the sub-formula ‘b’ has been changed to a dot. (If the customizable variable calc-highlight-selections-with-faces is non-nil, then the characters not part of the sub-formula are de-emphasized by using a less noticeable face instead of using dots. see Displaying Selections.) The ‘*’ next to the line number is to remind you that the formula has a portion of it selected. (In this case, it’s very obvious, but it might not always be. If Embedded mode is enabled, the word ‘Sel’ also appears in the mode line because the stack may not be visible. see Embedded Mode.)

If you had instead placed the cursor on the parenthesis immediately to the right of the ‘b’, the selection would have been:

           .    ...
    (a + b)  . . .
1*  ...............
        . . . .

The portion selected is always large enough to be considered a complete formula all by itself, so selecting the parenthesis selects the whole formula that it encloses. Putting the cursor on the ‘+’ sign would have had the same effect.

(Strictly speaking, the Emacs cursor is really the manifestation of the Emacs “point,” which is a position between two characters in the buffer. So purists would say that Calc selects the smallest sub-formula which contains the character to the right of “point.”)

If you supply a numeric prefix argument n, the selection is expanded to the nth enclosing sub-formula. Thus, positioning the cursor on the ‘b’ and typing C-u 1 j s will select ‘a + b’; typing C-u 2 j s will select ‘(a + b)^3’, and so on.

If the cursor is not on any part of the formula, or if you give a numeric prefix that is too large, the entire formula is selected.

If the cursor is on the ‘.’ line that marks the top of the stack (i.e., its normal “rest position”), this command selects the entire formula at stack level 1. Most selection commands similarly operate on the formula at the top of the stack if you haven’t positioned the cursor on any stack entry.

The j a (calc-select-additional) command enlarges the current selection to encompass the cursor. To select the smallest sub-formula defined by two different points, move to the first and press j s, then move to the other and press j a. This is roughly analogous to using C-@ (set-mark-command) to select the two ends of a region of text during normal Emacs editing.

The j o (calc-select-once) command selects a formula in exactly the same way as j s, except that the selection will last only as long as the next command that uses it. For example, j o 1 + is a handy way to add one to the sub-formula indicated by the cursor.

(A somewhat more precise definition: The j o command sets a flag such that the next command involving selected stack entries will clear the selections on those stack entries afterwards. All other selection commands except j a and j O clear this flag.)

The j S (calc-select-here-maybe) and j O (calc-select-once-maybe) commands are equivalent to j s and j o, respectively, except that if the formula already has a selection they have no effect. This is analogous to the behavior of some commands such as j r (calc-rewrite-selection; see Selections with Rewrite Rules) and is mainly intended to be used in keyboard macros that implement your own selection-oriented commands.

Selection of sub-formulas normally treats associative terms like ‘a + b - c + d’ and ‘x * y * z’ as single levels of the formula. If you place the cursor anywhere inside ‘a + b - c + d’ except on one of the variable names and use j s, you will select the entire four-term sum.

The j b (calc-break-selections) command controls a mode in which the “deep structure” of these associative formulas shows through. Calc actually stores the above formulas as ‘((a + b) - c) + d’ and ‘x * (y * z)’. (Note that for certain obscure reasons, by default Calc treats multiplication as right-associative.) Once you have enabled j b mode, selecting with the cursor on the ‘-’ sign would only select the ‘a + b - c’ portion, which makes sense when the deep structure of the sum is considered. There is no way to select the ‘b - c + d’ portion; although this might initially look like just as legitimate a sub-formula as ‘a + b - c’, the deep structure shows that it isn’t. The d U command can be used to view the deep structure of any formula (see Normal Language Modes).

When j b mode has not been enabled, the deep structure is generally hidden by the selection commands—what you see is what you get.

The j u (calc-unselect) command unselects the formula that the cursor is on. If there was no selection in the formula, this command has no effect. With a numeric prefix argument, it unselects the nth stack element rather than using the cursor position.

The j c (calc-clear-selections) command unselects all stack elements.