Two Dimensional Array- Definition and Example
Two-Dimensional Arrays
C/C++ supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array. A two-dimensional array is, essentially, an array of one-dimensional arrays. To declare a two-dimensional integer array d of size 10,20, you would write
int d[10][20];
Pay careful attention to the declaration. Some other computer languages use commas to separate the array dimensions; C/C++, in contrast, places each dimension in its own set of brackets.
Similarly, to access point 1,2 of array d, you would use
d[1][2]
The following example loads a two-dimensional array with the numbers 1 through 12 and prints them row by row.
#include
int main(void)
{
int t, i, num[3][4];
for(t=0; t<3; ++t)
for(i=0; i<4; ++i)
num[t][i] = (t*4)+i+1;
/* now print them out */
for(t=0; t<3; ++t) {
for(i=0; i<4; ++i)
printf(”%3d “, num[t][i]);
printf(”\n”);
}
return 0;
}
In this example, num[0][0] has the value 1, num[0][1] the value 2, num[0][2] the value 3, and so on. The value of num[2][3] will be 12. You can visualize the num array as shown here:
Two-dimensional arrays are stored in a row-column matrix, where the first index indicates the row and the second indicates the column. This means that the rightmost index changes faster than the leftmost when accessing the elements in the array in the order in which they are actually stored in memory.
In the case of a two-dimensional array, the following formula yields the number of bytes of memory needed to hold it:
bytes = size of 1st index x size of 2nd index x sizeof(base type)
Therefore, assuming 4-byte integers, an integer array with dimensions 10,5 would have 10×5x4 or 200 bytes allocated.
When a two-dimensional array is used as an argument to a function, only a pointer to the first element is actually passed. However, the parameter receiving a two-dimensional array must define at least the size of the rightmost dimension. (You can specify the left dimension if you like, but it is not necessary.) The rightmost dimension is needed because the compiler must know the length of each row if it is to index the array correctly. For example, a function that receives a two-dimensional integer array with dimensions 10,10 is declared like this:
void func1(int x[][10])
{
.
.
.
}
The compiler needs to know the size of the right dimension in order to correctly execute expressions such as
x[2][4]
inside the function. If the length of the rows is not known, the compiler cannot determine where the third row begins.
The following short program uses a two-dimensional array to store the numeric grade for each student in a teacher’s classes. The program assumes that the teacher has three classes and a maximum of 30 students per class. Notice the way the array grade is accessed by each of the functions.
/* A simple student grades database. */
#include
#include
#include
#define CLASSES 3
#define GRADES 30
int grade[CLASSES][GRADES];
void enter_grades(void);
int get_grade(int num);
void disp_grades(int g[][GRADES])
int main(void)
{
char ch, str[80];
for(;;) {
do {
printf(”(E)nter grades\n”);
printf(”(R)eport grades\n”);
printf(”(Q)uit\n”);
gets(str);
ch = toupper(*str);
} while(ch!=’E’ && ch!=’R’ && ch!=’Q');
switch(ch) {
case ‘E’:
enter_grades();
break;
case ‘R’:
disp_grades(grade);
break;
case ‘Q’:
exit(0);
}
}
return 0;
}
/* Enter the student’s grades. */
void enter_grades(void)
{
int t, i;
for(t=0; t
printf(”Class # %d:\n”, t+1);
for(i=0; i
grade[t][i] = get_grade(i);
}
}
/* Read a grade. */
int get_grade(int num)
{
char s[80];
printf(”Enter grade for student # %d:\n”, num+1);
gets(s);
return(atoi(s));
}
/* Display grades. */
void disp_grades(int g[][GRADES])
{
int t, i;
for(t=0; t
printf(”Class # %d:\n”, t+1);
for(i=0; i
printf(”Student #%d is %d\n”, i+1, g[t][i]);
}
}
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