1. CSC 107 - Programming for Science Lecture 27: Arrays as Parameters, Searching, & Sorting

2. Question of the Day Three people check into a hotel for which they pay the manager $30. The manager finds out the rate is $25 and gives $5 to the bellboy to return. To make it easier, the bellboy pockets $2 and gives $1 back to each person. Each person paid $10 and got back $1. So they paid $9 each, totaling $27. The bellboy has $2, totaling $29. Where is the remaining dollar? $25 (manager) + $2 (bellboy) + $3 (customers) = $30

3. Today’s Goal After lecture should be familiar with using arrays in functions  Passing arrays as parameters  Returning arrays from parameters  Using the values stored in the array  Searching for data in an array Higher dimension arrays on Friday

4. Arrays Variable holding multiple pieces of data  Really contains many different locations  Locations are numbered sequentially from 0 Each location is like an individual variable  Initial value is unknown  New value set at assignment But location is not independent of array  Can only be used by accessing array variable

5. Declaring an Array Variable Declaration includes type, name, and size  Size should be literal value  Variable names rules still apply Variable is an array of the type  Each location (element) is of that type int sampleArray[10]; float armada[200] = {0.0f}; FILE* inputs[3] = {NULL, NULL, NULL}; char text[] = {‘h’,‘i’,‘ ’,‘m’,‘o’,‘m’};

6. Using An Array Use array location like a variable  But must access through array variable Select element using braces ([ ])  Value inside braces must be an integer int i, fib[20]; fib[0] = 1; fib[1] = 1; for (i = 2; i < 20; i++) { fib[i] = fib[i – 1] + fib[i – 2]; }

7. Passing an Array Location Can use array elements like a variable  Can pass to function like any other value sqrt(values[8]); pow(acceleration[4], 2); printf(“Speed = %lf”, 9.8 * v[t]); Element’s value cannot change  No different than any other value Function only gets single value  Cannot access or use any other elements

8. Variable Number of Parameters va_args allows variable number of parameters  Important for scientific & statistical coding Using va_args not simple or easy  Parameter must be processed separately  Cannot go back after going to next parameter  Real limit to usefulness of parameter

9. Passing Array As Parameter Could instead store data in an array  But all values must be of same type Then pass the array as a parameter Will also need to pass array’s size  Remember: program cannot determine size To pass array, put variable as argument  Matching parameter must also be array  Passes all locations of the array

10. Array As Parameter Parameter(s) must be listed as arrays type parameterName[]  type is type of each element within array  parameterName can be any legal name  Cannot list size – C does not know it Argument must also be array  Cannot pass single variable or value Can have zero or more array parameters  Appear in any order among the parameters

11. Examples of Passing an Array double average(double data[], int n); double standardDev(double mean, double data[]); int[] add(int vector1[], int vector2[], int n); double holder; double a[5] = {3}; double b[7] = {1, 3, 3, 5, 3, 1, 5}; int gravity[3] = {0, 0, -10}; int cannonBall[3] = {60, 60, 60}; holder = average(a, 5) + average(b, 7); print(“Sigma = %lf\n”, standardDev(3, b)); add(gravity, {60, 60, 60}, 3); add(gravity, cannonBall, 3);

12. Functions with Array Parameters CANNOT change value of array variable  This is like other variables, but… CAN change value of locations in array  Functions can have “side-effects”  Must be careful when passing & using arrays void makeRandomVector(int v[], int n) { int i; for (i = 0; i < n; i++) { v[i]=rand(); } }

13. Searching through an Array Computers are fast, have vast memories, and dumber than a post  Bad for coming up with new designs, theories  Helpful when testing new designs, theories Perform lots of experiments or experiments generating lots of data  Store results in very large arrays  Search for data patterns

14. Searching Through Array Must begin with ordered data  Otherwise, will need to examine each location Data ordered from smallest to largest  Start at location 0 and keep increasing by 1  If we find the item, stop & return location  At location with value larger than datum, datum is not in array  stop searching  Reach end of array  stop searching May end up examining every item!

15. Searching Through Array #define NOT_FOUND -1 int findAndGetLoc(int[] arr, int n, int findIt) { int j; for (j = 0; j findIt) { break; } } return NOT_FOUND; } int data[] = {0, 3, 5, 32, 12912, 34834, 92928}; int findMe = rand() % 92928; int location = findAndGetLoc(data, 7, findMe); if (location != NOT_FOUND) { printf(“Wow: data[%d] = %d\n”, location, data[location]); }

16. Smarter Way To Search Circle target by guessing midpoints  Guess is too high, ignore locations above current midpoint  Guess is too low, ignore locations with even smaller values  Guess just right, return it!  Each guess halves possible locations Stop when out of possible locations

17. Speeding Up Searching #define NOT_FOUND -1 int fastFindLoc(int[] arr, int n, int findIt) { int low = 0, high = n, mid; while (low findIt) { high = mid - 1; } else if (arr[mid] < findIt) { low = mid + 1; } else { return mid; } } return NOT_FOUND; }

18. Your Turn Get back into groups and do activity

19. For Next Lecture Continue lab #11 Continue week #11 weekly assignment Work on Programming Assignment #3 Prepare for midterm #2  Will be Monday – 11/13/06