A 2D array example

A 1D, one dimensional, array is an array with simple elements such as integers, characters, etc., or a simple record structure.

Lists are usually represented with 1D arrays.

A 2D, two dimensional, array, is an array with elements that are arrays.

Tables of rows and columns of cells are usually represented using 2D arrays.

A 0D (zero dimensional) variable is the plane primitive variable.

The program requirement is as follows.

Read in a two dimensional table of rows and columns, square each value, and output the results.

Following good programming design, separate input, processing and output.

In most programs, the input and output are straight-forward (often trivial). The pseudo-code algorithm is often just the processing part - which is often not trivial. Here is the pseudo-code for the processing part.

FOR EACH cell in the table DO

Here is a refined pseudo-code - more detailed than would normally be done. Programmers are expected to be able to fill in straight-forward details of pseudo-code.

FOR EACH row in the table DO

Here is a 2D array declaration of array a that has 4 rows and 4 columns.

const int RMAX1 = 3;
const int CMAX1 = 3;
int a[RMAX1+1][CMAX1+1];
int rlen1;
int clen1;

The variables rlen1 and clen1 are to keep track of exactly how many rows and columns of the maximum total are actually being used.

Rows are indexed from 1 to 3 and columns are indexed, within the rows, from 1 to 3. Why? It was decided to start at 1 and not at 0. If starting at 0, the code needs to be adjusted appropriately.

The program is divided into input, process, and output parts.

scanf("%d %d",&rlen1,&clen1);
printf("Header loop: expecting %d rows of %d cols\n",rlen1,clen1);
printf("\n");
printf("Input:\n");
printf("\n");
for (int rpos1=1; rpos1 <= rlen1; rpos1++) {
	printf("\tr%d:\n",rpos1);
	for (int cpos1=1; cpos1 <= clen1; cpos1++) {
		scanf("%d",&a[rpos1][cpos1]);
		printf("\t\tc%d is %d\n",cpos1,a[rpos1][cpos1]);
		}
	}

printf("Process:\n");
printf("\n");
for (int rpos1=1; rpos1 <= rlen1; rpos1++) {
	printf("\tr%d:\n",rpos1);
	for (int cpos1=1; cpos1 <= clen1; cpos1++) {
		value1 = a[rpos1][cpos1];
		value2 = value1*value1;
		a[rpos1][cpos1] = value2;
		printf("\t\tc%d : old %d to new %d\n",cpos1,value1,value2);
		}
	}

printf("Output:\n");
printf("\n");
for (int rpos1=1; rpos1 <= rlen1; rpos1++) {
	printf("\tr%d:\n",rpos1);
	for (int cpos1=1; cpos1 <= clen1; cpos1++) {
		printf("\t\tc%d : %d \n",cpos1,a[rpos1][cpos1]);
		}
	}

Here is the C code.

#include <stdio.h>

int main() {
	int value1;
	int value2;
	const int RMAX1 = 3;
	const int CMAX1 = 3;
	int a[RMAX1+1][CMAX1+1];
	int rlen1;
	int clen1;
	// input 2d array using header loop
	scanf("%d %d",&rlen1,&clen1);
	printf("Header loop: expecting %d rows of %d cols\n",rlen1,clen1);
	printf("\n");
	printf("Input:\n");
	printf("\n");
	for (int rpos1=1; rpos1 <= rlen1; rpos1++) {
		printf("\tr%d:\n",rpos1);
		for (int cpos1=1; cpos1 <= clen1; cpos1++) {
			scanf("%d",&a[rpos1][cpos1]);
			printf("\t\tc%d is %d\n",cpos1,a[rpos1][cpos1]);
			}
		}
	printf("Process:\n");
	printf("\n");
	for (int rpos1=1; rpos1 <= rlen1; rpos1++) {
		printf("\tr%d:\n",rpos1);
		for (int cpos1=1; cpos1 <= clen1; cpos1++) {
			value1 = a[rpos1][cpos1];
			value2 = value1*value1;
			a[rpos1][cpos1] = value2;
			printf("\t\tc%d : old %d to new %d\n",cpos1,value1,value2);
			}
		}
	printf("Output:\n");
	printf("\n");
	for (int rpos1=1; rpos1 <= rlen1; rpos1++) {
		printf("\tr%d:\n",rpos1);
		for (int cpos1=1; cpos1 <= clen1; cpos1++) {
			printf("\t\tc%d : %d \n",cpos1,a[rpos1][cpos1]);
			}
		}
	// process 2d array
	printf("\n");
	// output 2d array
	printf("\n");
	return 0;
	}

Here are some examples of input and output for the above program code.

Here is an example input.

For the above example input, here is the expected output.

Header loop: expecting 3 rows of 3 cols

Input:

	r1:
		c1 is 2
		c2 is 3
		c3 is 5
	r2:
		c1 is 7
		c2 is 11
		c3 is 13
	r3:
		c1 is 17
		c2 is 19
		c3 is 23
Process:

	r1:
		c1 : old 2 to new 4
		c2 : old 3 to new 9
		c3 : old 5 to new 25
	r2:
		c1 : old 7 to new 49
		c2 : old 11 to new 121
		c3 : old 13 to new 169
	r3:
		c1 : old 17 to new 289
		c2 : old 19 to new 361
		c3 : old 23 to new 529
Output:

	r1:
		c1 : 4 
		c2 : 9 
		c3 : 25 
	r2:
		c1 : 49 
		c2 : 121 
		c3 : 169 
	r3:
		c1 : 289 
		c2 : 361 
		c3 : 529

Here is an example input.

2 2
2 3
5 7

For the above example input, here is the expected output.

Header loop: expecting 2 rows of 2 cols

Input:

	r1:
		c1 is 2
		c2 is 3
	r2:
		c1 is 5
		c2 is 7
Process:

	r1:
		c1 : old 2 to new 4
		c2 : old 3 to new 9
	r2:
		c1 : old 5 to new 25
		c2 : old 7 to new 49
Output:

	r1:
		c1 : 4 
		c2 : 9 
	r2:
		c1 : 25 
		c2 : 49

Here is an example input.

1 1
2

For the above example input, here is the expected output.

Header loop: expecting 1 rows of 1 cols

Input:

	r1:
		c1 is 2
Process:

	r1:
		c1 : old 2 to new 4
Output:

	r1:
		c1 : 4

Here is an example input.

0 0

For the above example input, here is the expected output.

Header loop: expecting 0 rows of 0 cols

Input:

Process:

Output:

The see the pattern of 1D and 2D arrays, going the other direction, a zero dimension array can be thought of as a variable.

Here is the C code.

#include <stdio.h>

int main() {
	int a0;
	int a[1];
	a0 = 2;
	a[0] = 3;
	printf("%d\n",a0);
	printf("%d\n",a[0]);
	return 0;
	}

Here is the output of the C code.

2
3