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Two-Dimensional Data • Class of 5 students • Each student has 3 test scores • Store this information in a twodimensional array • First dimension: which student 0, 1, 2, 3 or 4 • Second dimension: which test score 0, 1, or 2 Declaring a 2D Array • Give a second pair of square brackets to tell C++ you want a 2D array • Example: int grades[5][3]; Creating a 2D Array • Create array elements by telling how many ROWS and COLUMNS • Example: int grades[5][3]; grades is a two-dimensional array, with 5 rows and 3 columns. One row for each student. One column for each test. C++ arrays are row major, which means that we always refer to the row first. Initializing Elements // First student scores grades[0][0] = 78; grades[0][1] = 83; grades[0][2] = 82; Write assignment statements to fill-in the rest of the array. Declare & Create & Initialize Short Cut • Example: int grades[5][3] = { { 78, 83, 82 }, { 90, 88, 94 }, { 71, 73, 78 }, { 97, 96, 95 }, { 89, 93, 90 } }; A Two-D Array is an array of arrays. Each row is itself a One-D array. Row, Column Indices 0 1 2 0 78 83 82 1 90 88 94 2 71 73 78 3 97 96 95 4 89 93 90 Give both the ROW and COLUMN indices to pick out an individual element. The fourth student’s third test score is at ROW 3, COLUMN 2 What are the elements of the array table? int table[3][4]; x = 1; for (row = 0; row < 3; row++) for (col = 0; col < 4; col++) { table[row][col] = x; x++; } //for col column row Exercise: Average Overall • Find the average test score of all students’ test scores. • Use rows to tell you how many rows in the 2D array. • Use cols to tell you how many columns in the 2D array. Exercise: Average Overall int sum = 0; for(int r = 0; r < rows; r++) for(int c = 0; c < cols; c++) sum = sum + grades[r][c]; int avg = sum / (rows*cols); • Write a function that prints out the elements in a 2-D array (as they appear in the matrix) Exercise: print 2-D array void printArray(int matrix[][]) { for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[0].length; j++) cout << (matrix[i][j] << ” “); cout << endl; } } • Write a function that returns the maximum element in a 2-D array. • Things work pretty much the same way with 3-D arrays (and 4-D and …) Exercise: find maximum in a 2D array int findMax(int matrix[][COLS], int r, int c) { int max = matrix[0][0]; for(int i = 0; i < r; i++) for(int j = 0; j < c; j++) if ( matrix[i][j] > max) max = matrix[i][j]; return max; }