2.2.2 The gp history of results.
This is not to be confused with the history of your commands, maintained by readline. The
gp history contains the results they produced, in sequence.
The successive elements of the history array are called %1, %2, . . . As a shortcut, the latest
computed expression can also be called %, the previous one %‘, the one before that %‘‘ and so on.
When you suppress the printing of the result with a semicolon, it is still stored in the history,
but its history number will not appear either. It is a better idea to assign it to a variable for later
use than to mentally recompute what its number is. Of course, on the next line, you may just use
%.
The time used to compute that history entry is also stored as part of the entry and can be
recovered using the %# operator: %#1, %#2, %#‘; %# by itself returns the time needed to compute
the last result (the one returned by %).
Remark. The history “array” is in fact better thought of as a queue: its size is limited to 5000
entries by default, after which gp starts forgetting the initial entries. So %1 becomes unavailable as
gp prints %5001. You can modify the history size using histsize.
2.2.3 Special editing characters. A GP program can of course have more than one line. Since
your commands are executed as soon as you have finished typing them, there must be a way to tell
gp to wait for the next line or lines of input before doing anything. There are three ways of doing
this.
The first one is to use the backslash character \ at the end of the line that you are typing,
just before hitting <Return>. This tells gp that what you will write on the next line is the physical
continuation of what you have just written. In other words, it makes gp forget your newline
character. You can type a \ anywhere. It is interpreted as above only if (apart from ignored
whitespace characters) it is immediately followed by a newline. For example, you can type
? 3 + \
4
instead of typing 3 + 4.
The second one is a variation on the first, and is mostly useful when defining a user function
(see Section 2.7): since an equal sign can never end a valid expression, gp disregards a newline
immediately following an =.
? a =
123
%1 = 123
The third one is in general much more useful, and uses braces { and }. An opening brace {
signals that you are typing a multi-line command, and newlines are ignored until you type a closing
brace }. There are two important, but easily obeyed, restrictions: first, braces do not nest; second,
inside an open brace-close brace pair, all input lines are concatenated, suppressing any newlines.
Thus, all newlines should occur after a semicolon (;), a comma (,) or an operator (for clarity’s
sake, never split an identifier over two lines in this way). For instance, the following program
{
a = b
b = c
16
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