calc: Making Selections
11.1.1 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. 
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. 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
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 (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.
Info Catalog
calc: Selecting Subformulas
calc: Selecting Subformulas
calc: Changing Selections