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Multi-Dimensional Arrays

CS157 MultiDArrays

Two-Dimensional Arrays

• A two-dimensional array, or table, consists of both rows and columns of elements
• Similar to a spreadsheet or table
• Each row is an array

int val[3][4]; // 3 rows, 4 columns

Two-Dimensional Arrays

• val[1][3] means row 1, column 3. There’s a 6 there.

• It’s val[1][3], not val[1,3].
• Rows go across.
• Columns go up & down, like Roman columns.

Two-Dimensional Arrays

Initialization:

#define NUMROWS 3
#define NUMCOLS 4
int val[NUMROWS][NUMCOLS] = { { 8, 16,  9, 52},
                              { 3, 15, 27,  6},
                              {14, 25,  2, 10} };

First row is an array containing {8, 16, 9, 52}.

The inner braces can be omitted, if you have no soul:

int val[NUMROWS][NUMCOLS] =
    {8,16,9,52,3,15,27,6,14,25,2,10};

Initialization

• Initialization is done in row order
• The first row, 0, is filled first, element [0][0] first, then the next row, etc:

Two-Dimensional Arrays

#define NUMROWS 3
#define NUMCOLS 4
int val[NUMROWS][NUMCOLS] = {{8,16,9,52},
                             {3,15,27,6},
                             {14,25,2,10}};

printf("The hard way:\n");
printf(" %2d %2d %2d %2d\n",
       val[0][0], val[0][1], val[0][2], val[0][3]);
printf(" %2d %2d %2d %2d\n",
       val[1][0], val[1][1], val[1][2], val[1][3]);
printf(" %2d %2d %2d %2d\n",
       val[2][0], val[2][1], val[2][2], val[2][3]);

printf("The easy way:\n");
for (int i=0; i<NUMROWS; i++) {
    for (int j=0; j<NUMCOLS; j++)
        printf(" %2d", val[i][j]);
    printf("\n");
}

The hard way:
  8 16  9 52
  3 15 27  6
 14 25  2 10
The easy way:
  8 16  9 52
  3 15 27  6
 14 25  2 10

Two-Dimensional Arrays

#define NUMROWS 3
#define NUMCOLS 4
int val[NUMROWS][NUMCOLS] = {{8,16,9,52},
                             {3,15,27,6},
                             {14,25,2,10}};

/* Multiply each element by 10 and display it */
for (int i=0; i<NUMROWS; i++) {
    for (int j=0; j<NUMCOLS; j++) {
        val[i][j] *= 10;
        printf(" %3d", val[i][j]);
    }
    printf("\n");
}

  80 160  90 520
  30 150 270  60
 140 250  20 100

Two-Dimensional Arrays

#define NUMROWS 3
#define NUMCOLS 4

void display(int array[NUMROWS][NUMCOLS]);

int main() {
    int val[NUMROWS][NUMCOLS] = {{8,16,9,52},
                                 {3,15,27,6},
                                 {14,25,2,10}};
    display(val);
    return 0;
}

void display(int nums[NUMROWS][NUMCOLS]) {
    /* Multiply each element by 10 and display it */
    for (int i=0; i<NUMROWS; i++) {
        for (int j=0; j<NUMCOLS; j++) {
            nums[i][j] *= 10;
            printf(" %3d", nums[i][j]);
        }
        printf("\n");
    }
}

  80 160  90 520
  30 150 270  60
 140 250  20 100

Common Programming Errors

• Forgetting to declare the array
• Using a subscript that refers to a nonexistent array element
• Not using a large enough conditional value in a for loop counter to cycle through all the array elements
• Forgetting to initialize the array
• Forgetting that arrays start at 0, not 1

Summary of 1D arrays

• A single-dimensional array is a data structure that can store a list of values of the same data type
• Elements are stored in contiguous locations
  • Referenced using the array name and a subscript
• Single-dimensional arrays may be initialized when they are declared
• Single-dimensional arrays are passed to a function by passing the name of the array as an argument

Summary of 2D arrays

• A two-dimensional array is declared by listing both a row and a column size with the data type and name of the array
• Two-dimensional arrays may be initialized when they are declared
• Two-dimensional arrays are passed to a function by passing the name of the array as an argument

Even Worse

• You can have more than two dimensions on arrays.
• Here be dragons.

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