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Lecture 1: Introduction and Basic Scheme
We were handed a piece of cardboard with 32 thumbtacks arranged in a cross pattern with one hole left empty in the very center. The object was to have only one thumbtack left in the middle of the cardboard after executing thumbtack jumps (similar in rules to jumping pieces on a checkerboard; no diagonal jumps, only in the cardinal directions, and the jumped tacks get removed). O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Below is a list of supplied smaller moves to aid us in the bigger puzzle. The circles represent the thumbtacks, while the dots represent holes in the board. Jumps may only be executed using the spaces shown.
Macros
The “T”
O O • O • • • O
O Æ • O O O Æ O • •
O • O O O O • •
O O • O O • O •
O O O Æ • • • • O O Æ • • O
O O O • • • O •
O O O • • •
Find patterns: which pattern when; composition of patterns simplifies problems.
There was a similar game on the back of the board. We were instructed to use the “T” macro and another macro listed below the image of the puzzle:
O O
O O O
O O O O
O O O O O
Additional Macro:
Emacs The mode line is the black highlighted line that has the characters **Edwin: scheme The minibuffer is the region where emacs asks questions and gets answers. The first in-class assignment was saved under class1.scm Some commands not listed in the handout (Edwin-cheat.txt), also commands that were mentioned out loud and may still be listed in handout:
Saving file C-x, C-s Asterisks indicate that the file has been modified since it was last saved Undo C–x, u Switching buffers C–x, b Evaluate C-x, C-e Escape Whatever C-g Emacs Tutorial C-h, t <= worth looking try out
Value
- Numbers - written as 4, .75, ¾ Operations performed on numbers: Add/Subtract/Mult./Div. Equality
- Booleans – “true” and “false”, written #t and #f Operations performed on Booleans: Not (e.g., not #t indicates “false”) And (e.g., #t and #t indicates “true”) Or (e.g., #t or #f indicates “true”)
- Strings – sequence of characters such as “yay” Æ strings are listed in quotes Operations performed on strings: Append strings Split strings Length of string Characters of strings Compare two strings
- Procedures – in scheme, macros are considered “things” (a procedure is considered a “thing”) Example of procedure: recipe that’s written, can physically be held in hand Questions asked of a recipe: What are inputs of recipe? Is it a recipe?
- Lists – e.g., (1 3 4). It is possible to have lists of lists.
Scheme
Expression – text input by the programmer and sent to the evaluator (with C-x, C-e) Value – result computed by the evaluator when it works on an expression
For every expression, there is a rule to evaluate it and produce a value. Almost every expression has a value (excepting cases covered by 6.001).
Two types of expressions: simple and compound.
(if #t 1 2) if true-ish, then 1, else 2 (not #f)
(if 3 1 2) => 1 (if 0 1 2) => 1 (if + 1 2) => 1
Experiments with names:
Name value foo 4
fooÆ 4 bar Æ fo Æerror Not in table, produces error D debug, q quit
