2. Functions

3. 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.

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

5. 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.

6. 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

7. 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))

8. 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

9. 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)

10. 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 !

11. 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'.

12. 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.

13. Function calls function x a = 10; x1(a) function x1(a1) a1 =20; x2(a1) function x2(a1) a1 =30; x3(a1 + 10) function x3(a3) a1 =20; a=10 a1=20 a1=30 a3=40 a1=20 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

14. 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

15. 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

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

17. 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 >> minVector(v) ans = 1 >> [a b] = minVector(v) a = 1 b = 4

18. 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

19. 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