Understanding Arrays in C++: Yahtzee Game and Beyond

1. Chapter 5 Arrays: We've got their number

2. Yahtzee game • You throw 5 dice • You want them to be the same • If not the same, you are allowed to rethrow some or all the dice up to twice.

3. Game of Yahtzee • You can write: • int d1,d2,d3,d4, d5; • But can't you declare the dice in a single statement? • Yes • int d[5] • the dice are d[0],d[1],d[2],d[3], and d[4] • you always start counting at 0 and go up to the array length -1

4. Initialize the dice • arrays and for-loops go together like bacon and eggs int d[5]; //global array int dice(){ // returns the value of the dice return rand()%6+1; } rollAllDice(){ for(int i=0; i < 5; i++) d[i] = dice(); } rollSomeDice(int a[], int size){ for(int i=0; i < size; i++){ d[a[i]]=dice(); } }

5. Initializing and Printing an array int main(){ int a[] ={2,234,242,2,3}; for(int i=0; i < 5; i++){ cout << a[i] << " "; } }

6. Note about arrays • The size of an array needs to be a constant (known at compile time). It cannot be a variable that is not a constant. • Once an array is created, its size cannot be changed. • Make sure that you declare your array to be big enough. • Only access elements 0 up to size-1. Otherwise, the program will behave in unpredictable way. You will be seeing data you are not supposed to see.

7. Testing the random function • We want to generate 1 million random numbers between 0 and 9 and see if the distribution is even.

8. #include <math.h> #include <time.h> #include <stdlib.h> #include <iostream> using namespace std; int main(){ int a[]={0,0,0,0,0,0,0,0,0,0}; srand(time(NULL)); rand(); for(int i=0; i < 100000; i++){ int number = rand()%10; a[number]++; } for(int i=0; i < 10; i++){ printf("Number %d appears %d times\n",i, a[i]); } }

9. Working with strings char* names[] = {"Bob","Mike","Ann","Laura"}; for(int i=0; i < 4; i++){ cout << names[i]; } string functions: strcpy(s1,s2); //copies s2 into s1; char* s3 = strcat(s1,s2); //concatenates s1 and s2 strcmp(s1,s2); //returns 0 is strings are the same // positive integer if s1> s2; // negative integer is s1 < s2; strlen(s); //returns the length of s // strings are null (a.k.a. 0) terminated

10. Reading Strings char line[50]; int k = 0; while((line[k]=getchar())!= '\n') k++; line[k] = '\0' or alternative: gets(line);

11. Printing Strings for(int k=0;k<strlen(line)-1;k++) putchar(line[k]) putchar('\n'); alternative: puts(line);

12. Dealing cards #include <math.h> #include <time.h> #include <stdlib.h> #include <iostream> using namespace std; int main(){ char* suits[4] = {"clubs","diamonds","hearts", "spades"}; char* ranks[13] ={"two","three","four","five","six","seven","eight","nine","ten","jack", "queen", "king", "ace"}; srand(time(NULL));rand(); printf("%s of %s",ranks[rand()%13],suits[rand()%4]); }

13. Card Dealer #2 • This time, we want to generate a single random number. • We will calculate rank and suit from it. • We will store the calculated numbers in an array. • This will guarantee that every number that is generated is unique.

14. bool contains(int [], int , int ); int main(){ char* suits[4] = {"clubs","diamonds", "hearts", "spades"}; char* ranks[13] = {"two","three","four","five","six","seven","eight","nine","ten","jack", "queen", "king", "ace"}; srand(time(NULL));rand(); int cards[5]; for(int i=0;i< 5; i++){ int x = rand()%52; while(contains(cards,x,i)){ x = rand()%52; } cards[i]=x; } for(int i=0; i< 5; i++){ printf("%s of%s\n",ranks[cards[i]%13], suits[cards[i]/13]); }}

15. Method contains bool contains(int a[], int x, int size){ for(int i=0; i < size; i++){ if(a[i] == x) { return true; } } return false; }

16. Two-dimensional Array Data is commonly in table form Difficult to represent using one-dimensional array A collection of componentsstructured in two dimensions All elements should be the same type

17. Two-dimensional Array DataType ArrayName[Size1][Size2]; Each size has an integer value and specifies the number of components in that dimension Size1 : the number of rows Size2: the number of columns int data[5][10]; double sales[10][4];

18. Indexes in 2DArrays Individual array elements are accessed by a pair of indexes The first index represents the element’s row, and the second index represents the element’s column. int data[6][10]; data[2][7] = 4 ; // row 2, column 7

19. Accessing 2D Array int data[6][12]; data[2][7] = 4 ; // row 2, column 7 [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ]data [2] [7] [ 5 ] [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] row 2, column 7 4 3 2 8 5 9 13 4 8 9 8 0

20. Two-dimensional Array Initializing the element of a 2D array Elements of each row are enclosed within braces and separated by commas Rows are enclosed within braces int myArray[4][3]={{2,3,-1},{0,-3,5}, {2,6,3},{-2,10,4}};

21. Two-dimensional Array int scores[2][3] = {3, 6, 4, 7, 3, 9}; Fill elements row-by-row till available If not enough initializer, remaining elements set to 0(same as 1-D array case)

22. Two-dimensional Array Initializing the element of 2-D Array j int i, j, x[5][4]; … for (i =0; i <=4;i++) { for(j=0;j<=3;j++) { x[i][j]= i *j; } } i

23. Two-dimensional Array Printout array elements #include <stdio.h> int counter[5][5]; int total=0; void main() { for (int i = 0; i< 5; i++) { for (int j=0;j<5; j++) { counter[i][j] = j + (i *5) ; printf( " %d ", counter[i][j] ); total += counter[i][j]; } printf( " \n "); } printf(" The total is %d ", total); }

24. Two-dimensional Array Program output 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 The total is 300

25. Passing an array to a function int counter[5][5]; • In function prototype double maximum (double table[5][5], int x, int y) double max = maximum(counter, 5, 5);

26. Passing an array to a function void display(const int twoDArray[][50], //[][] int nrows, int ncols) //doesn't compile { for (int i=0; i < nrows; i++) { for (int j=0; j < ncols; j++ ) printf("%4d", twoDArray[i][j]); printf("\n"); } } const int cols = 50; const int rows = 50; int thisArray [rows][cols]; display( thisArray, 10, 30 );

27. Multidimensional Arrays Can define three-dimensional array or N-dimensional array (N can be any number) Syntax DataType ArrayName[Size1][Size2] … [SizeN]; Each size has a positive integer value and specifies the number of components in that dimension int data[5][10][15];