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Winter 2023

CS 35L

Discussion 1D

Week 2

Interesting Topics

  1. Log in to SEASnet without password
  2. Log in to SEASnet without username
  3. Open VS Code in Terminal (the Command Line)

ssh key pairs

Tip: alias

For Mac, Linux, and WSL users (idk how to use Windows Powershell)

  • use alias to simplify command

  • $ alias
alias ll='ls -alF'
alias vi='vim'

  • Executing $ ll is equivalent to runing $ls -alF

  • Which command do you use most frequently?

  • go to terminal

Another Example:
  • $ alias ln15="ssh username@lnxsrv15.seas.ucla.edu"
$ ln15
*****************************************************************************
                    lnxsrv15.seas.ucla.edu RHEL 8
*****************************************************************************

	* User processes older than 36 hours will be cleaned up

*****************************************************************************
*****************************************************************************
* SEASnet Computing Access                                                  *
*                                                                           *
* Priority is given both on the servers and in the student labs to those    *
* students doing coursework. Computing support for research is provided by  *
* each department.                                                          *
*****************************************************************************
* For assistance please contact help@seas.ucla.edu or call 206-6864.        *
*****************************************************************************
Activate the web console with: systemctl enable --now cockpit.socket

Last login: Mon Oct  3 23:06:29 2022 from 172.28.136.203
[classiwu@lnxsrv15 ~]$

  • Warning: potentially DANGEROUS !

  • $ unalias ln15  # remove aliasing

  • Make it Permanent

    $ cd ~
$ vim .bash_profile
  • [Optional]: '.bashrc' vs '.bash_profile'

  • .bash_profile: run commands that should run only once, such as customizing the $PATH environment variable

  • .bashrc: put the commands that should run every time you launch a new shell

  • Reference: https://linuxize.com/post/bashrc-vs-bash-profile/

Start VS Code on Terminal

Reference: https://code.visualstudio.com/docs/setup/mac

  • Open your VS Code

  • Command + Shift + P

  • Type shell and hit enter

    >shell

  • Done!

Not Working?

  • If you are using bash:

    cat << EOF >> ~/.bash_profile
# Add Visual Studio Code (code)
export PATH="\$PATH:/Applications/Visual Studio Code.app/Contents/Resources/app/bin"
EOF

  • If you are using zsh

    cat << EOF >> ~/.zprofile
# Add Visual Studio Code (code)
export PATH="\$PATH:/Applications/Visual Studio Code.app/Contents/Resources/app/bin"
EOF
Still Not Working?

  • google it

  • try troubleshoot by yourself

ELisp

Elisp

Q: When will you learn Lisp again?
  • CS 111 Operating System?
  • CS 131 Programming Languages?
  • CS 161 Artificial Intelligence?
  • CS 192 What is that?
Answer

CS 161 and CS 192 (maybe)!

Q: When was Lisp developed?
  • 1930s ?
  • 1950s ?
  • 1980s ?
  • 2010s ?
Answer

A: 1958 John McCarthy developed LISP, the first functional language

[Optional] Definition of Pure Functions:

According to Prof. Nachenberg's slide in CS 131:

  • A function that has two properties:

    1. Given a specific input x, the function always returns the same output y.
    2. It doesn't modify any data beyond initializing local variables required to compute its output.
  • # Pure
int f(int p){
  int q = 5 * p * p
  return q
}
  • # Impure
int z;
int f(int p){
  return p * z++;
}
[Optional] Functional languages:

  • Only composite of functions

  • Can't change variable state

    int f(int p) {
  p = p + 1    // NOT Allowed
  return p
}
    • No for loops; Only recursions
    • Why?
    for (int i = 0; i < 10; i++){
  ...  // i = 1    =>   i = 2    => ....  =>  i = 10
}

  • The order of execution is not important

    • What is a Pure function?

  • You will learn more about it in CS 131

Data types

Integer Type
-1               ; The integer -1.
1                ; The integer 1.
1.               ; Also the integer 1.
+1               ; Also the integer 1.

  • fixnum: small integers

    • Its range depends on your machine

    • Try use M-: to run (print most-positive-fixnum) in Emacs

    • Challenge: How many bits needed?

    • Answer

      (log most-positive-fixnum 2)

  • bignum: large integers

    • can have arbitrary precision

    • Which data structure might be used?

    • Answer

      Linked List

Floating-point type
1500.0         ;1500
+15e2          ;1500
15.0e+2        ;1500
+1500000e-3    ;1500
.15e4          ;1500
Character Type
?Q ⇒ 81  ?q => 113
  • A character in Emacs Lisp is nothing more than an integer.
Symbol Type
foo                 ; A symbol named ‘foo’.
FOO                 ; A symbol named ‘FOO’, different from ‘foo’.
1+                  ; A symbol named ‘1+’
                    ;   (not ‘+1’, which is an integer).
\+1                 ; A symbol named ‘+1’
                    ;   (not a very readable name).
\(*\ 1\ 2\)         ; A symbol named ‘(* 1 2)’ (a worse name).
  • A symbol in GNU Emacs Lisp is an object with a name
  • A symbol can serve as a
    • variable
    • function name
    • ...
  • Note: In Common Lisp, lower case letters are always folded to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct.
List Type:
(A 2 "A")            ; A list of three elements.
()                   ; A list of no elements (the empty list).
nil                  ; A list of no elements (the empty list).
("A ()")             ; A list of one element: the string "A ()".
(A ())               ; A list of two elements: A and the empty list.
(A nil)              ; Equivalent to the previous.
((A B C))            ; A list of one element
                     ;   (which is a list of three elements).
  • '(rose violet daisy buttercup)
'(+2 2)
("white rose")

  • or

  • '(rose
  violet
  daisy
  buttercup)

  • The elements of this list are the names of the four different flowers, separated from each other by whitespace and surrounded by parentheses

Numbers in List
  • (+ 2 2) ; a list with a plus-sign, followed by two ‘2’s, each separated by whitespace.
(+ 88 6)
List in List
  • '(this list has (a list inside of it))
(+ 2 (+ 2 2))

  • In Lisp, both data and programs are represented the same way

Evaluate List

  • send you an error message

    • Debugger entered--Lisp error: (invalid-function ...)

  • do nothing except return to you the list itself

    • '(rose violet daisy buttercup)

  • treat the first symbol in the list as a command to do something

    • (+ 2 2)
  • Key Binding to Evaluate: C-x C-e

  • go to terminal

Function Types
  • Functions are also Lisp objects
  • All functions are defined in terms of other functions, except for a few primitive functions that are written in C.
  • They are written in C so we can easily run GNU Emacs on any computer that has sufficient power and can run C.
  • A lambda expression can be called as a function even though it has no name;
Examples:

  • quote returns object, without evaluating it

    (quote (+ 2 2))
'(+ 2 2)  ; equivalent

  • car returns the first element in a list

    (car '(rose violet daisy buttercup))

  • cdr returns the rest of the list

    (cdr '(rose violet daisy buttercup))
(violet dasiy buttercup)

  • cons constructs lists

    (cons 'I  '(like lisp))
(cons (car '(rose violet daisy buttercup)) (cdr '(rose violet daisy buttercup)))

  • append attaches one list to another

    (append '(1 2 3 4) '(5 6 7 8))  ; (1 2 3 4 5 6 7 8)
(cons '(1 2 3 4) '(5 6 7 8))    ; ((1 2 3 4) 5 6 7 8)
Exercise:

Time to do Week 2 Worksheet

  • What do the following Lisp expressions output, when evaluated?
a. (quote (1 2 3))                    ;(1 2 3)
b. '(1 2 3)                           ;(1 2 3)
c. (list (+ 1 2) '(+ 1 2))            ;(3 (+ 1 2))
d. (cons (+ 1 2) '(3 4))              ;(3 3 4)
e. (+ 10 (car '(1 2 3)))              ; 11
f. (append ‘(1 2) ‘(3 4))             ; (1 2 3 4)
g. (reverse (append '(1 2) '(3 4)))   ; (4 3 2 1)
h. (cdddar (1 2 3 4 5 6 7))           ; error
How to define a function?
  1. Function name.
  2. A list of the arguments
  3. [Optional] Documentation describing the function
  4. [Optional] An expression to make the function interactive
  5. Body of the function
(defun function-name (arguments…)
  "optional-documentation…"
  (interactive argument-passing-info)     ; optional
  body…)
Interactive?
  • If a function is interactive, you can use it by typing M-x and then the name of the function
  • Or by a key defined by you
(defun multiply-by-thirty-five (number)
  "Multiply NUMBER by Thirty Five."
  (* 35 number))

  • Non-interactive

  • How to use this function?

    (multiply-by-thirty-five 2)
Exercise

Time to do our worksheet

Write a function called is-even that takes one argument and returns whether it is even.

  1. Example: (is-even 4) should evaluate to t.
  2. Example: (is-even 3) should evaluate to nil.

$$ x % 2 == 0 $$

;;; Solution
(defun is-even (number)
  "Return whether a number is even"
  (interactive "Return whether a number is even")
  (= (% number 2) 0))
Jargons in Emacs:
  • Point is the current location of the cursor.
    • More precisely, on terminals where the cursor appears to be on top of a character, point is immediately before the character.
    • In Emacs Lisp, point is an integer. The first character in a buffer is number one, the second is number two, and so on.
    • The function point returns the current position of the cursor as a number
  • Mark is another position in the buffer.
  • Use C-SPC (set-mark-command) to set a mark
  • Region is the part of the buffer between point and mark
save-excursion
  • It saves the location of point,
  • executes the body of the function,
  • and then restores point to its previous position if its location was changed.
  • save-excursion is often used to keep point in the location expected by the user.
(defun what-line ()
  "Print the current buffer line number and narrowed line number of point."
  (interactive)
  (let ((start (point-min))
	(n (line-number-at-pos)))
    (if (= start 1)
	(message "Line %d" n)
      (save-excursion
	(save-restriction
	  (widen)
	  (message "line %d (narrowed line %d)"
		   (+ n (line-number-at-pos start) -1) n))))))
Tips:
  • Get help C-h f ${function name}
  • Explore Source Code

Python

Installing

  • Mac/Linux:
$ which python
$ which python3
  • Windows:
$ where python
$ where python3.exe

Version

$ python3 --version  # or
$ python3 -V   # or

Conda

  • Conda is an open source package management system and environment management system that runs on Windows, macOS, Linux and z/OS. Conda quickly installs, runs and updates packages and their dependencies.

  • Reference: https://docs.conda.io/en/latest/

Tools you can use

  • Terminal

  • Jupiter Notebook

  • PyCharm

  • VS Code

  • Emacs

Elisp

Quote from my MATH 164 Professor Dr. Gleizer's shirt

$$ \text{I have } \ e^{\pi i} + 1 \text { friends} $$

>>> import cmath
>>> cmath.e ** (cmath.pi * i) + 1       # hopefully 0?

Sashwath Sundher, Assignment 6 Guide, Youtobe Videos