A Terminal Session

1
A Terminal Session

• Log In Username and Password
• A window appears
• Check the prompt
• some computer name other stuff gt
• styx.cse.buffalo.edu is the computer, there are
  many.
• can be anything
• gt is where you type
• 4. set prompt / gt

2
Creating Folders (or Subdirectories, or Work
Areas)

• You type
• set prompt/ gt
• eng/home/mikebgt mkdir Lab1
• eng/home/mikeb gt cd Lab1
• eng/home/mikeb /Lab1 gt
• What just happened?
• eng/home/mikeb /Lab1 gt cd
• eng/home/mikeb gt

3
Work Areas on the main computer disk

mkdir Lab1 cd Lab1
/
Lab1/
/Lab1 Work Area is the location of edited
files
4
What is a file?

• Dear John,
• I think we should just be
• friends

Created and Saved in an editor eng gt
/util/bin/jgrasp Dearjohn.txt Could be a Java
program eng /Lab1 gt /util/bin/jgrasp
myProgram.java
5
so far

• We address the computer through a terminal window
  in a primitive language called Unix
• Unix allows us to set aside private areas of the
  disk and go to them to do our work
• Once there, we can open, edit, and save files
  which will later become programs
• Computer Scientists model some aspect of the real
  world in the computer program
• Computer programs make extensive use of changing
  values called variables.

6
Concatenation (Sequencing)

• Computers perform one small step at a time, in a
  stated order.
• z 0
• x 15
• y 12
• z x y
• print z

7
Variables

• x, y, z
• i, j, k
• TotalCountOfNewEmployees
• Geekish_Designations_0123
• They Stand for Something

8
The ESSENCE of a variable

• a 15
• b 12
• c ab What is C?
• b 1288
• c ab What is C now?
• b, and c, are both CHANGEABLE

9
Variable Types (primitive, common)

• Integers meant to be a whole number (used for
  counting)
• Floating Pt. meant to be a detailed number
  (used for measuring) called a DOUBLE
• Character has no meaning other than to be read
  by a human
• Strings of Characters have no meaning other
  than to be read by a human

10
x 14

• Could mean 14 (not likely)
• Could mean 13.999995
• Could mean 14.000001
• Could mean 20 (honestly), might not be base-10,
  more later
• Could mean Fourteen, which has no meaning other
  than to be read by humans

11
Typing a variable

• Integer X
• X 14
• Therefore, 14 means 14
• Float X (or Real X, or Single X, or Double X)
• X 14.0000
• Therefore, X has meaning, 14 to 4 places

12
More

• Char X, Y
• X 1
• Y 4
• X with Y 14 (Concatenation)
• String Z
• Z cat

13
Legality

• Variable value and variable type must be the
  same.
• Deterministic behavior - All computers yield the
  same result with the same variables

14
Computers check our programs

• Called Compilation
• Must be successful before a program can run

15
Illegality

• Compiler Errors however, some languages and
  compilers allow mis-designations of variables
• Indeterminate results
• Run-time Errors

16
Compilation
will it work?

17
Illegal

• Integer Y
• Y 14.001
• Char Q
• Q abcdef
• Double z
• z hello

18
Why Type Variables

• Deterministic behavior of variables and the
  numbers they represent.
• New definition of Legality We will never
  perform mathematical or logical operations on
  variables of different types.

19
Illegal, part 2

• Integer X
• Double Y
• Char Z
• X 12
• Y 16.001
• Z X Y

20
Legal Operation

• String K
• Char I, J
• I H
• J i
• K I with J (what is K?)

21
New Types

• Boolean TRUE or FALSE (really means presence or
  absence), used to signify a condition, or
  state of being.
• Boolean X, Y, Z
• X TRUE (means X is present)
• Y FALSE (means Y is NOT present)
• Z X OR Y (what is Z?, i.e. does Z signify
  some presence?)
• Z X AND Y (what is Z?)

22
floating point?

• 14.6567
• 128.1231
• 154367.001423
• 3.14159
• 987987465986543.

23
3 categories of ILLEGAL

• Variables set to a value wrong for its type
• Integer X
• X TRUE
• 2. Variables of different types used in the same
  operation
• Integer X, Y
• Char Z
• Z X Y ( multiply)
• 3. Operation wrong for variable type
• String Z cat
• Q X Z

24
Constants

• constant integer X
• or
• final integer X
• X 16
• X 14 illegal!

25
Algorithms

• Series of mathematical or variable manipulations
• Integer a,b,c
• a 12
• b 22
• c a b (what is c?)

26
Complicated, Concatenated Algorithms

• integer Total, LoopCounter
• Total 0
• LoopCounter 0
• Start
• Add 2 to Total
• Add 1 to LoopCounter
• Go Back to Start Until Total 12
• What is LoopCounter?

two variables
a loop
done
27
Written in Code

• int LoopCounter, Total
• LoopCounter 0
• Total 0
• while (Total lt 12)
• Total Total 2
• LoopCounter LoopCounter 1

semicolon
no semicolon (because brackets follow)
open bracket
odd-looking algebra
close bracket
28
Real code

• public class Looper
• public static void main( String args
  )
• int LoopCounter, Total
• LoopCounter 0 Total 0 while
  ( Total lt 12 ) Total Total
  2 LoopCounter LoopCounter 1
  System.out.println( LoopCounter )
  

file
program
loop
Output!
29
Summary

• Concatenation
• Typing
• Legality
• Algorithm
• Code

30
Code summary

• public class filename
• public static void main( String arg )
• surround functional paragraphs
• semicolon