A7: Asmt#7: Polygon area

Write a C program to do the following.

The first number in the data is a show parameter that should be handled as done in previous works.

Write a C program to read in units and the number of points that follow. The points represent a polygon where the last point is omitted and is the same as the first point. Determine the area of the complete polygon.

The first data point is the base of every triangle.

Each vector in a polygon, from point to point, is called a leg of the polygon.

The last leg is inferred as the first point. It is needed for the area of the polygon and the for the perimeter of the polygon (if required for this work).

In this work, we are not interested in whether the any individual or overall triangle area is positive or negative. If the requirements are unclear in any way, use any provided input and output to resolve the discrepancy.

See Guidelines for C programs .

Domain knowledge is the background knowledge that is useful in solving a problem, designing a solution, implementing the solution, etc. The following domain knowledge may be useful for this requirement.

Note that in a previous assignment, A5, the triangle area was actually a parallelogram of two such triangles. A factor of 0.5 has been added to the page for the area of a triangle and the updated formula should be used here.

Note: In addition to specific domain knowledge, you should be familiar with all concepts covered to this point in the course.

The following coding examples and/or notes may be of use for this requirement.

C: Loops for data pairs
C: Loops for data triples (not needed for this work but the ideas are useful in understanding what is being done)

The first point of every triangle is the first point of the data as (x0,y0). Triangles that contribute to the area are then triangles of pairs of adjacent points not counting the first (base) point.

Use the "+=" or "-=" (or similar) operators when they can be used. For example incrementing a counter or adding to a sum. It is best to not use the "++" or "--" operators except within for loops (where for loops are the best choice).

Use a while loop where a while loop is best. There is a major deduction for using a for loop when not appropriate. A for loop is best for going 0 to n-1 or from 1 to n or the other way.

All code should be written in single-entry single-exit style. This means no continue or break statements in loops and all return statements need to be at the end of a function (as in the main function).

For all works, there is a major deduction for submitting a program that does not terminate. For example, a program that contains an infinite loop.

Make all necessary assumptions. Make no unnecessary assumptions.

In addition to specific coding examples, you should be familiar with all concepts covered to this point in the course.

You are provided with the following starter program in file polygons.c.

Do not remove any comment that starts with three slashes.

Here is the C code.

/// File: polygons.c
/// Purpose: Polygon area
/// Author: (your name)
/// Help received:
// (add here)
/// pseudo-code:
// (add here)

#include <stdio.h>
#include <math.h>

int main() {
	///:BEGIN ToDo

	///:END ToDo
	return 0;
	}

You are to fill in the missing parts of the program according to the work requirements (see above).

In the comments at the top, you are to fill in the author (your name), help received (person and type of help), and pseudo-code parts which are not in the solution (usually provided, but you need to add them). Remove the parentheses too.

Begin solution

Here is a possible solution to the above problem. Note: Once a solution is provided, or the day before the next class, further submissions for this work receive no credit.

/// File: polygons.c
/// Purpose: Polygon area
/// Author: (your name)
/// Help received:
// (add here)
/// pseudo-code:
// (add here)

#include <stdio.h>
#include <math.h>

int main() {
	///:BEGIN ToDo
	// declarations
	int show1;
	char units1[256];
	int dataLen1;
	int dataPos1;
	int pairPos1;
	int trianglePos1;
	// triangle points
	double x0;
	double y0;
	double x1;
	double y1;
	double x2;
	double y2;
	// variables for areas
	double dx1;
	double dy1;
	double dx2;
	double dy2;
	double areaOne1;
	double areaAll1;
	// input
	scanf("%d\n",&show1);
	if (show1 > 1) {
		printf("Enter units.\n");
		}
	scanf("%[^\n]s",&units1);
	if (show1 > 1) {
		printf("You entered \"%s\" for units.\n",units1);
		}
	// input, process and output
	areaAll1 = 0.0;
	pairPos1 = 0;
	trianglePos1 = 0;
	dataPos1 = 0;
	while (dataPos1 != dataLen1) {
		dataPos1 += 1;
		if (show1 > 1) {
			printf("\n");
			printf("Enter two numbers for point %d.\n",dataPos1);
			}
		scanf("%lf %lf",&x2,&y2);
		if (show1 > 1) {
			printf("You entered numbers for point %d as %0.3lf and %0.3lf\n",dataPos1,x2,y2);
			}
		if (dataPos1 > 1) {
			pairPos1 += 1;
			if (pairPos1 == 1) {
				x0 = x1;
				y0 = y1;
				if (show1 > 0) {
					printf("\n");
					printf("Point 1 (base point) is (%0.3lf,%0.3lf)\n",x0,y0);
					}
				}
			else {
				trianglePos1 += 1;
				dx1 = x1 - x0;
				dy1 = y1 - y0;
				dx2 = x2 - x0;
				dy2 = y2 - y0;
				areaOne1 = 0.5 * ((dx1 * dy2) - (dx2 * dy1));
				if (show1 > 0) {
					printf("\n");
					printf("Triangle %d from (%0.3lf,%0.3lf) to (%0.3lf,%0.3lf) to (%0.3lf,%0.3lf) has area %0.3lf square %s.\n",trianglePos1,x0,y0,x1,y1,x2,y2,areaOne1,units1);
					}
				areaAll1 += areaOne1;
				}
			}
		x1 = x2;
		y1 = y2;
		}
	if (show1 > 0) {
		printf("\n");
		}
	printf("The area of the polygon with %d triangles is %0.3lf square %s\n",trianglePos1,areaAll1,units1);
	///:END ToDo
	return 0;
	}

End solution

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

Here is an example input from file input1.txt.

2
meters
3
3.000 3.000
4.100 3.000
3.550 3.953

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

Enter units.
You entered "meters" for units.

The area of the polygon with 0 triangles is 0.000 square meters

Here is an example input from file input2.txt.

1
milimeters
4
4.000 4.000
6.200 4.000
6.200 6.200
4.000 6.200

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

The area of the polygon with 0 triangles is 0.000 square milimeters

Here is an example input from file input3.txt.

0
centimeters
5
5.000 5.000
8.300 5.000
9.320 8.138
6.650 10.078
3.980 8.138

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

The area of the polygon with 0 triangles is 0.000 square centimeters

Here is an example input from file input4.txt.

2
yards
6
6.000 6.000
10.400 6.000
12.600 9.811
10.400 13.621
6.000 13.621
3.800 9.811

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

Enter units.
You entered "yards" for units.

The area of the polygon with 0 triangles is 0.000 square yards

Pseudo-code is a "false" code. It is not really code, but a simplified English explanation that is somewhat similar to code. Pseudo-code is useful for understanding what a program does. For this work, the following can be used as the pseudo-code. Adjust if needed.

Get units and number of points
Set total area to 0.0
FOR EACH pair of adjacent points (x1,y1) and (x2,y2)
	IF the pair is the first pair THEN
		set the base (x0,y0) to (x1, y1)
	ELSE
		calculate triangle area of points (x0,y0) , (x1,y1) , (x2,y2)
		add area to total area
		END IF
	END FOR
output total area of the polygon

Note: Only some essential details are included. Some details, such as input, prompts, input echo, and obviously needed output are omitted.

The best way to copy-paste pseudo-code that includes indented code (e.g., with tabs) is as follows (in the code file) using a multi-line comment.

/* (pasted pseudo-code here) */

It is best to separate input, processing and output into different parts of a program.

Since C has limited string handling capabilities (they can get complex), some processing involving string processing is moved to the output section of the code.

CS 101 - A7 : Asmt#7: Polygon area Your grade: _ / 40 [LATE] Late or redo penalty: _ / -40 [SUBMIT] Not submitted properly: _ / -40 [RUN] Does not compile, run-time crash, etc.: _ / -40 [ETHICS] Ethical issues: _ / -40 [HEADER] Header comments not done as required: _ / -20 [STYLE] Inconsistant style or indentation: _ / -20 [CODE] Code guidelines ignored: _ / -40 [XCODE] Code used not yet covered: _ / -20 [REMOVED] Triple slash comments removed: _ / -20 [ADDED] Code added outside of marked areas: _ / -20 [OUTPUT] Functional requirements met: _ / 40 [CREDIT] Extra credit: _ / +8 Comments: