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Functions

Learn about scripts, workspace, global variables, passing data to scripts, and functions in MATLAB. Includes examples and explanations.

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Functions

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  1. Functions

  2. Scripts and Workspace • scripts are just a collection of commands, running a script is same as running the statements in command window. • Your scripts and command window shares the same set of variables, also called the workspace.

  3. Passing a variable to a script script.m : disp(num2str(x)) x = 5; Command Window: >>x=2; >>script 2 >>x x = 5

  4. Global Variables • Variables in scripts and command window are shared, in other words, they are global. • Global variables have been found to be problematic, since your program could erase a value needed by some other script, or the values you depend on might be changed by some other program. These side-effects are difficult to find • In large systems, name clashes would be very common.

  5. Passing Data to a Script • Let’s say we have a script that sums the values in a vector : sum_vect.m : sum = 0; for x=1:length(v) sum = sum + v(x) end

  6. Passing Data to a Script • We have another program where we are given three vectors: a,b, & c. We want to take the average of all elements in them. How can we make use of sum script? v = a; sum_vect, suma = sum; v = b; sum_vect, sumb = sum; v = c; sum_vect, sumc = sum; average = (suma+sumb+sumc)/(length(a) + length(b)+length(c))

  7. A better way? • Take the rem function • Do we call it like: >>numerator=7,denominator = 3; rem; disp(num2str(result)) • We do: >>disp(num2str(rem(7,3))); • We just send our data as parameters, and the expression will be evaluated to return value of our function

  8. Horizontal Line Example • Rather than getting the input from user every time, we can write our own function horizontalLine: function horizontalLine(a, b, y, g) for x=a:b-1 putPixel(x, y, g); end >>>> horizontalLine(10, 300, 20, g)

  9. Drawing a Rectangle function drawRectangle(x1, y1, x2, y2, g) x = y1; while (x <= y2) horizontalLine(x1, x2, x, g) x = x + 1; end what about x? horizontalLine also have a variable x !

  10. Functions have private workspaces >> whos Name Size Bytes Class fr 1x1 javax.swing.JFrame g 1x1 sun.awt.windows.WGraphics mines 400x400 160000 logical array Grand total is 160002 elements using 160000 bytes >> horizontalLine(10, 200, 20, g) >> x ??? Undefined function or variable 'x'.

  11. Functions as Black Boxes • Functions are machines that get some input and produce some output. • We cannot see, or shouldn’t care to see the inner workings of a function, the local variables used by the function and so on. • functions cannot see or use variables that exist in the context of the caller. • when you pass input arguments to functions, you are not passing variables, just simple values.

  12. Function calls a=10 the only information that goes from a function to another is the values of parameters. The callee cannot see or know which variables the caller has function x1(a1) a1 =20; x2(a1) function x a = 10; x1(a) a1=20 function x2(a1) a1 =30; x3(a1 + 10) a1=30 function x3(a3) a1 =20; a3=40 a1=20

  13. A closer look at putPixel function suite • initWindow : fr = javax.swing.JFrame('Drawing Pad'); fr.setSize(400, 400); mines = rand(400, 400) > 0.99; fr.show; g = fr.getContentPane.getGraphics; • it creates global variables mines and g

  14. PutPixel function function putPixel(x, y, g) g.drawLine(x,y, x,y) • Since putPixel cannot see the global variable g, we have to pass it ourselves • Note that we could also write: function putPixel(x, y, graphicsObject) • You can give any variable name you want

  15. Functions that Return values function minValue = minVector(v) minValue = v(1); for i = 2:length(v) if (v(i) < minValue) minValue = v(i); end end >> v = [5 3 7 1 9 11] v = 5 3 7 1 9 11 >> minVector(v) ans = 1

  16. Function that Returns multiple values function [minValue, minIndex] = minVector(v) minValue = v(1); minIndex = 1; for i = 2:length(v) if (v(i) < minValue) minValue = v(i); minIndex = i; end end >> minVector(v) ans = 1 >> [a b] = minVector(v) a = 1 b = 4

  17. Complete Form function [minValue, minIndex] = minVector(v) %MINVECTOR function that returns minimum of a vector % it returns two values, one the minimum % value in the vector, the other, the index of the minimum value minValue = v(1); minIndex = 1; for i = 2:length(v) if (v(i) < minValue) minValue = v(i); minIndex = i; end end

  18. help and lookfor >> lookfor minVector MINVECTOR function that returns minimum of a vector >> >> help minVector MINVECTOR function that returns minimum of a vector it returns two values, one the minimum value in the vector, the other, the index of the minimum value

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