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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

	Column 0	Column 1	Column 2	Column 3
Row 0	8	16	9	52
Row 1	3	15	28	6
Row 2	14	25	2	10

• 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:

	Column 0	Column 1	Column 2	Column 3
Row 0	first	second	third	fourth
Row 1	fifth	sixth	seventh	eighth
Row 2	ninth	tenth	eleventh	twelfth

Two-Dimensional Arrays

#include <stdio.h>
#define NUMROWS 3
#define NUMCOLS 4
int main() {
    int val[NUMROWS][NUMCOLS] = {{8,16,9,52},{3,15,27,6},{14,25,2,10}};
    printf("Display of val by explicit element:\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("Display of val using loops:\n");
    for (int i=0; i<NUMROWS; i++) {
        for (int j=0; j<NUMCOLS; j++)
            printf(" %2d", val[i][j]);
        printf("\n");
    }
    return 0;
}

Two-Dimensional Arrays

The display produced by the previous program is:

    Display of val by explicit element:
      8 16  9 52
      3 15 27  6
     14 25  2 10
    Display of val using loops:
      8 16  9 52
      3 15 27  6
     14 25  2 10

Two-Dimensional Arrays

#include <stdio.h>
#define NUMROWS 3
#define NUMCOLS 4
int main() {
    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");
    }

    return 0;
}

Two-Dimensional Arrays

#include <stdio.h>
#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");
    }
}

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.