Beyond the Mouse A Short Course on Programming

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Beyond the Mouse – A Short Course on

Programming

2. Fundamental Programming Principles I:

Variables and Data Types

Ronni Grapenthin

Geophysical Institute, University of Alaska Fairbanks

September 19, 2011

“The Uncomfortable Truths Well”, http://xkcd.com/568 (April 13, 2009)

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Today’s schedule . . .

1 How does (computer) programming work?

2 Variables and Datatypes

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Today’s schedule

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How does (computer) programming work?

Well, fist we should clearify terminology here!

What is a programming language?

What is a program?

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Alright, what is it then?

Definitions (broad sense)

Aprogramming language is an unambiguous artificial language that is made up of a set of symbols (vocabulary) and grammatical rules (syntax) to instruct a machine.

Aprogram is a set of instructions in one or multiple programming languages that specifies the behavior of a machine.

Compilation or interpretation is the verification of a program and its translation into machine readable instructions of a specific platform.

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Programming languages . . . continued

Two broad families can be identified:

1 Interpreted languages

An interpreter program is necessary to take in commands, check syntax and translate to machine language at runtime (e.g., Matlab, Unix Shell)

2 Compiled languages

Programs are translated and saved in machine language. At runtime no additional program is necessary (e.g., C/C++).

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Programming languages . . . continued

Two broad families can be identified:

1 Interpreted languages

An interpreter program is necessary to take in commands, check syntax and translate to machine language at runtime (e.g., Matlab, Unix Shell)

2 Compiled languages

Programs are translated and saved in machine language. At runtime no additional program is necessary (e.g., C/C++).

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Now, how does programming work?

1 opentext editor (vi, notepad, . . . , not MS Word!)

http://www.xkcd.com/378/ 2 translate your (mental) flowchart into a set of instructions

according to the rules of an applicable programming language

3 test your program for syntactical correctness (ask interpreter/compiler)

4 if errors, fix them and go back to (3)

5 test your program for semantical errors (the fun part!)

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Now, how does programming work?

http://www.xkcd.com/378/

2 translate your (mental) flowchart into a set of instructions according to the rules of an applicable programming language

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Now, how does programming work?

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Now, how does programming work?

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Now, how does programming work?

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Now, how does programming work?

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Now, how does programming work?

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Don’t even think that’s as simple as it sounds . . .

‘Hello World’ in Matlab

1 >> dsp ( h a l o o r l d

??? dsp ( h a l o o r l d

3 |

E r r o r : Unexpected MATLAB e x p r e s s i o n . 5

>> dsp ( ’ h a l o o r l d 7 ??? dsp ( ’ h a l o o r l d

|

9 E r r o r : A MATLAB s t r i n g c o n s t a n t i s n o t t e r m i n a t e d p r o p e r l y .

11 >> dsp ( ’ h a l o o r l d ’

??? dsp ( ’ h a l o o r l d ’

13 |

E r r o r : E x p r e s s i o n o r s t a t e m e n t i s i n c o r r e c t −−p o s s i b l y unbalanced ( , { , o r [ . 15

>> dsp ( ’ h a l o o r l d ’ )

17 ??? Undefined f u n c t i o n o r method ’ dsp ’ f o r input arguments o f type ’ char ’ .

19 >> disp ( ’ h a l o o r l d ’ ) h a l o o r l d

21

% S e m a t i c a l l y c o r r e c t , i f you want t o say ’ h i ’ t o t h e w o r l d : 23 %

>> disp ( ’ h e l l o w o r l d ’ ) 25 h e l l o w o r l d

Listing 2.1: hello_world.log

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Today’s schedule

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Variables (1)

Definitions – a selection

Donald Knuth: A quantity that may possess different values as a program is being executed.

Mehran Sahami: A box in which we stuff things – i.e. a box with variable content.

Wikipedia: User defined keyword that is linked to a value stored in computer’smemory (runtime).

The concept of avariable consists of:

name type value

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Variables (1)

Definitions – a selection

Donald Knuth: A quantity that may possess different values as a program is being executed.

Mehran Sahami: A box in which we stuff things – i.e. a box with variable content.

Wikipedia: User defined keyword that is linked to a value stored in computer’smemory (runtime).

The concept of avariable consists of:

name type value

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Memory interlude

“Depth”, http://xkcd.com/485 (September 16, 2009)

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Memory interlude

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Variables (2) – name

USE MEANINGFUL NAMES!

valid, follow programming language rules – MATLAB variable names mustbegin with a letter, followed by any combination of letters, digits, and underscores. MATLAB distinguishes

between uppercase and lowercase.No reserved keywords! USE MEANINGFUL NAMES, i.e. names that speak:

‘lengthGlacier’ or ‘glacier_length’NOT NOT NOT ‘a’ – avoid ambiguity

use consistent formatting, i.e.: ‘my_cool_var’ vs. ‘myCoolVar’ – supports reading

a gazillion style guides exist – punchline: use meaningful names, be consistent (that’s hard enough)!

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Variables (2) – name

between uppercase and lowercase.No reserved keywords!

USE MEANINGFUL NAMES, i.e. names that speak:

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Variables (2) – name

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Variables (2) – name

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Variables (2) – name

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Variables (2) – name

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Variables (3) – type

What is a type? – Think of sets of numbers in math: N, R, Z, . . . The type refers to how numbers are being represented in a computer’s memory, i.e. which bit has which meaning, and how many bits are necessary

Two kinds of Types

primitive, built in types – for MATLAB e.g.: ‘int32’, ‘double’,

‘boolean’ (important for *printf functions)

complex, home made types – (arrays,) structs, cell arrays (Matlab), classes

Types in Programming Languages

some languages, e.g. MATLAB, Shells, Perl are weakly typed: implicit type conversions (OR one type can be treated as another) this is nice at first, occasionally this leads to nasty/hard to fix problems (e.g. string interpreted as number, etc.)

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Variables (3) – type

Two kinds of Types

some languages, e.g. MATLAB, Shells, Perl are weakly typed: implicit type conversions (OR one type can be treated as another) this is nice at first, occasionally this leads to nasty/hard to fix problems (e.g. string interpreted as number, etc.)

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Variables (3) – type

Two kinds of Types

some languages, e.g. MATLAB, Shells, Perl are weakly typed:

implicit type conversions (OR one type can be treated as another) this is nice at first, occasionally this leads to nasty/hard to fix problems (e.g. string interpreted as number, etc.)

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Variables (4) – value

Value

a value of the type of the variable: 23, 3.1415926..., false i.e., the thing we stuff in the box

can/should change during the runtime of the program, otherwise use a constant (if possible)

Declaring a variable and Assigning a value:

In General: (type) name = value; or (type) name = expression;

Matlab: myNewVar = 10; TC-Shell (differs) set myNewVar = 10; Access to the values (de-referencing):

Matlab: use myNewVar; TC-Shell (differs) use ‘$’: $myNewVar What’s that?

myNewVar = myNewVar + 1;

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Variables (4) – value

Value

Matlab: myNewVar = 10; TC-Shell (differs) set myNewVar = 10;

Access to the values (de-referencing):

Matlab: use myNewVar; TC-Shell (differs) use ‘$’: $myNewVar

What’s that?

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Variables (4) – value

Value

Matlab: myNewVar = 10; TC-Shell (differs) set myNewVar = 10;

Access to the values (de-referencing):

Matlab: use myNewVar; TC-Shell (differs) use ‘$’: $myNewVar What’s that?

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Advanced Variables: Vectors and Matrices (1)

Array variables

are lists, vectors, matrices of data (1 to n dimensional – book keeping can become a hassle)

therefore instead of one value they hold alist of values linked to a chunk of memory (a sequence of boxes) access by index number

MATLAB treatseverything as a matrix Shells allow only vectors.

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Memory interlude (2)

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Memory interlude (2)

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Memory interlude (2)

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Memory interlude (2)

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Advanced Variables: Vectors and Matrices (2)

Example: Numeric Vector

index: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

vector: 12 23.3 23.3 nan nan 1 42 42.1 23 5 nan nan 0 0 0

Example: String

index: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

sting: h e l l o w o r l d ! ! ! !

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Advanced Variables: Vectors and Matrices (2)

Example: Numeric Vector

index: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

vector: 12 23.3 23.3 nan nan 1 42 42.1 23 5 nan nan 0 0 0

Example: String

index: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

sting: h e l l o w o r l d ! ! ! !

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Advanced Variables: Vectors and Matrices (3)

Setting up a numeric Matrix: Equinox marathon pacing tables index Mile

1 1

2 5

3 10

4 15

5 20

6 25

7 26.2

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Advanced Variables: Vectors and Matrices (3)

Setting up a numeric Matrix: Equinox marathon pacing tables index Mile record

1 1 0:05:55

2 5 0:30:01

3 10 0:59:56

4 15 1:35:01

5 20 2:04:59

6 25 2:32:19

7 26.2 2:40:00

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Advanced Variables: Vectors and Matrices (3)

Setting up a numeric Matrix: Equinox marathon pacing tables index Mile record well trained

1 1 0:05:55 0:08:42

2 5 0:30:01 0:44:06

3 10 0:59:56 1:28:01

4 15 1:35:01 2:19:33

5 20 2:04:59 3:03:34

6 25 2:32:19 3:43:43

7 26.2 2:40:00 3:55:00

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Advanced Variables: Vectors and Matrices (3)

Setting up a numeric Matrix: Equinox marathon pacing tables index Mile record well trained mildly trained

1 1 0:05:55 0:08:42 0:10:55

2 5 0:30:01 0:44:06 0:55:21

3 10 0:59:56 1:28:01 1:50:29

4 15 1:35:01 2:19:33 2:55:05

5 20 2:04:59 3:03:34 3:50:26

6 25 2:32:19 3:43:43 4:40:50

7 26.2 2:40:00 3:55:00 4:55:00

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Example: Equinox marathon pacing table in Matlab

1 % UAF / GI Beyond t h e mouse , f a l l 2010 , Ronni G r a p e n t h i n

% EXAMPLE : 2D m a t r i x ( Table ) , p r i n t s l i s t o f t i m e s t h a t can be used f o r o p t i m a l 3 % Equinox 2011 p r e p a r a t i o n

% parameter : m i l e s −− m i l e s you ’ ve run 5

f u n c t i o n p a c e _ t a b l e = p a c i n g _ t a b l e ( m i l e s ) 7

% Set up p a c i n g t a b l e : Give m i l e s as numbers and t i m e s as s t r i n g s ( r e q u i r e s a c e l l a r r a y , 9 % hence t h e c u r l y braces )

p a c e _ t a b l e = { 1 ’ 0 : 0 5 : 5 5 ’ ’ 0 : 0 8 : 4 2 ’ ’ 0 : 1 0 : 5 5 ’ ; 11 5 ’ 0 : 3 0 : 0 1 ’ ’ 0 : 4 4 : 0 6 ’ ’ 0 : 5 5 : 2 1 ’ ; 10 ’ 0 : 5 9 : 5 6 ’ ’ 1 : 2 8 : 0 1 ’ ’ 1 : 5 0 : 2 9 ’ ; 13 15 ’ 1 : 3 5 : 0 1 ’ ’ 2 : 1 9 : 3 3 ’ ’ 2 : 5 5 : 0 5 ’ ; 20 ’ 2 : 0 4 : 5 9 ’ ’ 3 : 0 3 : 3 4 ’ ’ 3 : 5 0 : 2 6 ’ ; 15 26.2 ’ 2 : 4 0 : 0 0 ’ ’ 3 : 5 5 : 0 0 ’ ’ 4 : 5 5 : 0 0 ’ } ;

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% Since I ’m l a z y and didn ’ t want t o t y p e a l l t h e m i l e s , a m i l e does n o t e qua l t h e index , 19 % hence we ’ l l have t o do some math . In dex i s rounded number o f m i l e s d i v i d e d by 5 . Since

% Matlab i n d i c e s s t a r t a t 1 , we have t o add a 1 . Otherwise e v e r y t h i n g s m a l l e r than 2 . 5 m i l e s 21 % would r e s u l t i n an e r r o r

i d x = round ( m i l e s / 5 ) + 1 ; 23

% lame o u t p u t 25 p a c e _ t a b l e ( i d x , : )

pause 27

% f a n c y o u t p u t : 29 disp ( ’ ’ ) ;

disp ( ’ m i l e s r e c o r d w e l l t r a i n e d m i l d l y t r a i n e d ’ ) ; 31 disp ( ’ −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−’ ) ;

disp ( p a c e _ t a b l e ( i d x , : ) ) ;

33 end Listing 2.2: pacing_table.m