1. “The fastest I/O is no I/O.” Nils-Peter Nelson, Bell Labs
Output Streams
“The fastest I/O is no I/O.”
Nils-Peter Nelson, Bell Labs

2. Streams All input and output is accomplished via streams
A stream is a something from which data can be read and to which data can be written
You’ve likely worked with streams before in other programming languages

3. Standard Streams There are several streams built in, including:
*standard-input*
*standard-output*
*error-output*
*terminal-io*
*query-io*
*debug-io*

4. (not (equal #\B #\b)) → T
Characters
Strings are made up of characters
Examples: #\A, #\t, #\Newline
Self-quoting: #\A → #\A
Every character has an associated integer, usually its ASCII number
Case sensitive:
(not (equal #\B #\b)) → T

5. Codes and Chars
There are many built-in functions for working with characters
char-code returns the number associated with a character
code-char returns the character associated with a number
char<, char<=, char=, char>=, char> and char/=

6. Predicates for Characters
characterp
alpha-char-p
digit-char-p
alphanumericp
upper-case-p
lower-case-p
both-case-p
graphic-char-p
standard-char-p

7. Writing to Streams Plethora of possibilities
>(print "Lisp rocks my socks off!" *standard-output*)
"Lisp rocks my socks off!"
>(write-char #\S *standard-output*) S
#\S
>(write-string "Hello" *standard-output*)
Hello
"Hello“
In the above examples, *standard-output* is actually optional because it is the default output stream:
>(print "Lisp rocks my socks off!")

8. Writing From a Progn Block
>(progn (print "Hi")
(write-string "class"))
"Hi" class
"class"
(terpri)
"Hi"
class
terpri is a carriage return

9. More Writing Functions
prin1 – like print, but no newlines or spaces between calls
princ – like prin1, but no double quotes around output strings
pprint – like print, but doesn’t return the string printed (also makes output “pretty”)
fresh-line – goes to beginning of next line, unless you’re already at the beginning (terpri always goes to next line)

10. (format destination template S-expr*)
Most general and powerful output function
(format destination template S-expr*)
destination can be a stream, t or nil
If t, result is printed to currently selected output device
Returns NIL, unless destination is nil, in which case the string produced is returned and no stream is written to

11. Format (2)
Format has a complex set of directives that are close to a programming language in terms of expressive power
Format directives are indicated with a ~
Can indicate spacing information and printing specifications
Somewhat similar to printf in C

12. Basic Format Directives
Two primary directives: ~S and ~A
Not case sensitive
~S prints in a format that LISP can read
~A simply prints “nice” output
These will often look the same, but not always

13. Format Examples >(format t "This is a test") This is a test NIL
>(format t "This is a ~A" "test")
>(format t "This is a ~S" "test")
This is a "test"
>(format t "This is an ~S" 'example)
This is an EXAMPLE

14. Other Useful Directives
~~ - prints a ~
~& - like fresh-line
~% - like terpri
~D - decimal number
~B - binary number
~O - octal number
~X - hexadecimal number
~F - fixed-format floating point number
~E - exponential floating point number
~G - general floating point number
~$ - money format, meaning fixed floating point with two decimals