C++ program on determining triangle

As your professor suggested, you should look at:

http://en.wikipedia.org/wiki/Triangle#Types_of_triangles

You should also look at:

http://www.teacherschoice.com.au/maths_library/trigonometry/solve_trig_sss.htm

Algorithm:

Solve for all angles, a1, a2, a3 (see the article above) If you can't find a solution: Output "Error: Not a valid triangle" Else: If (a1 == a2) && (a2 == a3): Output "EQUILATERAL" and stop If (a1 == a2) || (a2 == a3) || (a1 == a3): Output "ISOSCELES" and stop Output "SCALENE" and stop 

Also note: Be careful about "equality" with floating point (float/double) values (such as angles). If you are doing such a comparison, you should usually use this instead:

abs(x - y) < epsilon 

Where epsilon is a "sufficiently small value".

The logic falls out neatly from the definition of these different types of triangles, which as the professor notes, is information readily obtained from Wikipedia. It just involves a simple comparison of side lengths; you don't have to go as far as angles. But I'll give you some help with the "not a triangle" condition. Don't be afraid to put on your math hat here and go wandering in, a little logic isn't a bad thing for a poli sci student to endure every now and then. :-)

For the sides to make a proper triangle, for each pair of sides (I'll call them f and g), they must add up to greater than the third side's length (I'll call it h). If you're dealing with equilateral triangles, you automatically know this condition is met (why?). If you're dealing with isosceles or scalene triangles, you technically only need to check the smaller two sides against the largest side, and if it's true for them, it's true for the other two cases as well (why?). However, it may be just as convenient for you to check all three cases.

Looking at why this inequality has to hold: if the sum of two sides was exactly equal to the third side's length, you'd have a "degenerate" triangle where sides f and g could only lay on top of h! If they added up to less, the two sides could connect to the endpoints of h but then would never meet at a third point! You can test this yourself by cutting lengths of string or strips of paper and trying it out.

Three other things to think about:

1. In C++, double and float are not the same thing. One has less precision than the other. Make sure you use the one the professor asks for.
2. Checking to make sure the sides are non-negative is a great idea. You could probably reasonably rule out lengths of 0 as well, to eliminate the possibility of degenerate triangles that just look like line segments or points.
3. When comparing floating-point numbers, you should always be careful to consider whether a strict equality is going to get you what you want. For checking the equilateral/isosceles/scalene conditions, you're fine because the user is directly entering in the floating-point numbers and you're not manipulating them, so there's no chance for you to introduce error. But when checking the "not a triangle" condition, it's relatively easy to set up a situation where adding the two sides rounds off (because of the vicissitudes of floating-point arithmetic in the CPU) to something that's very close to, but not quite exactly, the third side. In those cases, if you want to catch degenerate triangles, what you usually do is pick an "epsilon" value (some very small value relative to the numbers you're dealing with) that represents the maximum amount of roundoff you're willing to tolerate. You then check whether the sum of f and g is somewhere between h - epsilon and h + epsilon – or put another way, whether the absolute value of f + g - h is less than or equal to epsilon. If it is, you claim that f + g = h (as best as you can tell) and deal with the degenerate case.

To complete this program you will need the following:

Make sure input is valid. In this case, input must be greater than 0. You could catch your input using a loop like

 while (invar <= 0) { cout<<"Enter length"<<endl; cin>>invar; if (invar <= 0) { cout<<"invalid input"<<endl; } } 

I am not sure if this is proper c++ syntax, I haven't used it in about 8 years. you can do this for all 3 inputs. I would probably make a function to determine the triangle using 3 input variables and 1 return variable. The following is pseudo-code

if (a + b <= c) or (a + c <= b) or (b + c <= a) { return "you don't have a triangle." } else { if (a == b) or (a == c) or (b == c) { if (a == b and b == c) { return "equilateral" } return "isosceles" } return "scalene" } return -1 

#include<stdio.h> #include<ctype.h> #include<conio.h> #include<math.h> int main() { float Side1,Side2,Side3; float Flag1,Flag2,Sum_of_sq1,Sum_of_sq2,Sum_of_sq3; clrscr(); printf("Enter Three Sides Side1 Side2 Side3 :"); scanf("%f %f %f", &Side1 , &Side2 , &Side3); Flag1=(Side1==Side2)?(Side2==Side3?1:0):((Side2==Side3)?0:-1); if(Flag1==0) { printf("Triangle is Isoceles\n"); } if (Flag1==1) { printf("Equilateral Triangle"); } Sum_of_sq1=pow(Side1,2)+pow(Side2,2); Sum_of_sq2=pow(Side1,2)+pow(Side3,2); Sum_of_sq3=pow(Side2,2)+pow(Side3,2); if (sqrt(Sum_of_sq1)==Side3 ||sqrt(Sum_of_sq2)==Side2 || sqrt(Sum_of_sq3)==Side1) printf("The Triangle is Right Angled Triangle"); getch(); return(0); } 

#include<iostream> using namespace std; //create a class class Triangle { //declare three sides for the triangle double side1; double side2; double side3; public: //constructor to initialize the data members Triangle(double s1, double s2, double s3) { side1 = s1; side2 = s2; side3 = s3; } void triangleType() { //all sides equal if((side1 == side2)&&(side2 == side3)) cout << "It is an Equilateral Triangle" << endl; //at least two sides are equal else if((side1 == side2) || (side2 == side3) || (side1 == side3)) cout << "It is an Isosceles Triangle" << endl; //all are different else cout << "It is a Scalene Triangle" << endl; } }; int main() { //local variable double a_side, b_side, c_side; //taking the user inputs cout << "Enter the three sides of a triangle: " << endl; cin >> a_side >> b_side >> c_side; Triangle t1(a_side, b_side, c_side); //create an object of Triangle t1.triangleType(); //call the function return 0; }