Wednesday, 2 October 2013

Matlab Getting Started

Valuable Commands

The following commands are generally useful, particularly when first learning Matlab
  • help - Provides help on specified topics. May be followed by a specific topic, e.g. "help plot"
  • helpwin - Opens up a GUI window to the help system. May also be followed by a specific topic.
  • lookfor - ( not in book ) Find functions based on keywords in description
  • clear - clears ( deletes ) variables. You can also clear specific variables.
  • clc - clears command window
  • who, whos - Lists variables, with and without size information
  • which - Indicates whether an identifier is a variable, built-in Matlab function, user-defined function, or other
  • what - Lists files in the current directory

Predefined Variables

  • ans - The last value calculated
  • i, j - square root of -1, UNLESS they have been re-defined
  • pi - 3.14159 . . .
  • Inf - Infinity
    • inf( N ) creates an N x N array of infinity
    • inf( M, N ) creates an M x N array of infinity
  • NaN - Not a Number, for example zero divided by zero
    • Can also generate arrays of NaN
  • eps - Epsilon, the smallest number the computer can add to 1.0 and distinguish the sum from 1.0. I.e. the smallest value for which the computer can tell the difference between ( 1.0 + epsilon ) and 1.0.

Special Characters

  • % ( percent ) - Indicates the start of a comment, which extends to the end of the line
  • ; ( semicolon ) - Suppresses output. Ordinarily every Matlab command generates a result, and that result is displayed in the command window. Placing a semicolon at the end of the line causes it to do the work without displaying the result. In a script, it causes the command to not be displayed at all.
    • Also used to separate rows in a matrix
  • ' ( apostrophe ) - Used to quote strings of text.
    • Also takes the transpose of a matrix
  • ... ( ellipses ) - Placed at the end of a line ...
    to indicate that the command continues on the next line
  • ( ) - Used for sevaral purposes:
    • forcing precedence, e.g. ( 3 + 4 ) / 2
    • calling functions
    • accessing elements of a matrix. ( i.e. for matrix subscripts )
  • [ ] - Used for creating matrices
  • , ( comma ) - Separates arguments in a function or values in a matrix
  • : ( colon ) - Generally used to indicate a range of values, e.g. 5:10. See also basic matrix operations

Mathematical Operators

  • May be different for scalars versus arrays versus combinations of both
  • +, -
    • Addition and subtraction.
    • Two matrices must have the same number of rows and columns to be added or subtracted.
    • A scalar can be added to or subrracted from an array, in either order. ( Array - scalar or scalar - array ) The opeation is applied element by element.
  • *, /
    • Multiplication and division
    • To multiply two matrices requires that the number of columns in the first matrix match the number of rows in the second.
      • An ( M x C ) matrix * a ( R x N ) matrix yields a ( M x N ) result, where R must equal C
    • To divide two matrices requires that the have the same number of columns. The numerator should have at least as many rows as the denominator.
    • Scalars can be multiplied by or divided by arrays in either order. The operation is applied element-by-element.
  • \
    • Left division is only relevant to matrix operations:
      • A x = b
      • x = A \ b
      • Requires that the number of columns in the numerator and denominator match. The numerator should have at least as many rows.
  • ^
    • Exponentiation, e.g. area = pi * radius^2
    • When applied to a matrix, the matrix must be square. I.e. it must be possible to multiply the matrix by itself.
  • Elementwise - .*, ./, .^ - Acts on matrices element by element, as opposed to normal matrix operations.
    • For example, array.^2 would square each element of the array, as opposed to multiplying the array by itself.
    • .* and ./ require that the matrices have the same sizes, i.e. the same number of both rows and columns.
  • Modulus is the function "mod", not an operator.

Basic Input and Output

  • input
    • variable = input( 'Prompt string' ); - Issues the prompt, reads in the user's response, and stores the result in the variable
    • variable = input( 'Prompt string', 's' ); - To specify a text ( string ) input instead of numbers.
  • disp
    • disp 'A quoted string to be displayed'
    • disp ' ' % Displaying a single space creates a blank line
    • disp [ 'X = ', num2str( x ) ]; % Print an array of two strings as a sentence.
  • format - long, short, bank, e, +

Advanced Output Formatting ( Optional )

  • fprintf( 'format string with % codes', variables )
    • %f - Fixed decimal number
    • %e - Exponential notation
    • %g - E or F, whichever fits best
    • %d - decimal integer
    • width.precision - E.g. %5.2f prints a floating point number in a space of width 5, with two digits right of the decimal point.
    • \n - newline
    • \t - tab
    • Note: When printing arrays with Matlab:
      • Format specifiers are used repeatedly until all data values are printed
      • Array data is accessed column-wise, not row-wise!
      • fprintf example printing arrays

Creating and running scripts

  • Any command that can be typed in the command window can also be run from a script file, which is just an ordinary text file containing Matlab commands.
  • Matlab has a built-in text editor, launchable from an icon on the left end of the Matlab toolbar, or by clicking on an existing file in the Matlab directory window.
  • Commands can either be typed into the file directly, or dragged from either the command or history windows.
  • Save with a ".m" extension - Automatic when using Matlab's editor.
    • ( Files must be plain text only. Do not use a word processor or other program unless you are careful to save the file as plain text. )
  • Run the file by typing the name, or "run name", or run( 'name' ), or right-clicking, or from the Matlab editor.