How to know whether the triangle is valid or not by angles?

Question

I want to write a c++ program that finds whether the triangle is valid or not
here's my code:

#include <iostream>
using namespace std;
int main () {
    int a,b,c;
    cin>>a>>b>>c;
    if (a+b+c==180)
    cout <<"Yes";
    else
    cout <<"No";
    return 0;
}

The user will enter 3 angles e.g 50 30 100 it will print valid Yes, but what if he enters
0 0 180 it will print yes by entering only one angle. that isn't triangle and my program will print yes. What should I do to fix this?

considering `0 0 180` as a triangle or not is rather arbitrary, so you have to catch that case with an extra branch — 463035818_is_not_an_ai, Dec 04 '17 at 12:21
btw I would be more worried about a user typing something like `-20 -10 210`.... — 463035818_is_not_an_ai, Dec 04 '17 at 12:22
it is up to you what to do in that cases. If you dont want to have negative angles, using an unsigned type would be a start — 463035818_is_not_an_ai, Dec 04 '17 at 12:25
Next to checking the sum, you should also do some valid range checking per angle (e.g. 1 <= angle <= 179) — stefaanv, Dec 04 '17 at 12:30
@AdamEve _"should I use while loop?"_ That's up to you, respectively your requirements. — user0042, Dec 04 '17 at 12:34
An other approach would be to ask for 2 angles, and to compute the third. — Oliv, Dec 04 '17 at 16:02

Nasser Al-Shawwa · Answer 1 · 2017-12-04T15:23:01.963

1

In a sense, you are right; there is no point in continuing the program if you have enough information to determine that the triangle is invalid. So, in the code snippet I propose below, you can see that once the user enters a value < 0 or > 180, the program terminates with a suitable error message. Similarly, once the running sum exceeds 180, the program also terminates early.

Finally once all three values are input for the angles, all of which independently are valid angles of a triangle, their sum is checked.

int main() 
{
    // assuming angles must be integers:
    int angle, sum = 0;
    for (int i = 0; i < 3; i++) {
        cin >> angle;
        if (angle <= 0 || angle >= 180) {
            cout << "Error: Angle must be between 0 and 180\n";
            return 0;
        }

        sum += angle;

        if (sum > 180) {
            cout << "Error: angles too large\n";
            return 0;
        }
    }

    if (sum == 180) {
        cout << "Valid Triangle!\n";
    } else {
        cout << "Error: angles must add up to 180\n";
    }
    return 0;
}

edited Dec 04 '17 at 15:23

answered Dec 04 '17 at 12:33

Nasser Al-Shawwa

3,573
17
27

Downvoter, please explain? This answer is correct, and addresses all the cases presented in the question – Nasser Al-Shawwa Dec 04 '17 at 12:35
code only answers are rarely useful in the long term (ie anything beyond OP copy and pasting this code) – 463035818_is_not_an_ai Dec 04 '17 at 12:37
@tobi303 Fair enough. Added explanation – Nasser Al-Shawwa Dec 04 '17 at 12:39
@Melebius why? the program ran to completion successfully – Caleth Dec 04 '17 at 12:43
@Caleth that was my rationale as well – Nasser Al-Shawwa Dec 04 '17 at 12:47
[Zero return is success](https://stackoverflow.com/questions/4818888/main-functions-return-value) – crashmstr Dec 04 '17 at 12:52
1

@Caleth It allows simpler usage of the program in scripts like this: `echo "10 20 50" | is_triangle && do_something_with_triangles` – Melebius Dec 04 '17 at 13:01
Or, even better, `EXIT_FAILURE` or `EXIT_SUCCESS`, see http://en.cppreference.com/w/c/program/EXIT_status – Mark Setchell Dec 04 '17 at 14:51
1

First `if` statement should be `if (angle <= 0 || angle >= 180)` - you are already toast if any angle is 0 or 180. – Mark Setchell Dec 04 '17 at 14:55
@MarkSetchell Well Spotted! – Nasser Al-Shawwa Dec 04 '17 at 15:10

score 0 · Answer 2 · answered Dec 04 '17 at 13:50

0

There are two issues that you have to think about in this code. First: are the angles that the user typed in valid (i.e., not negative, not more than 180 degrees, and maybe not 0). Second, once you have decided that the angles are valid, determine whether the three angles can be part of a triangle.

So write two functions: one that gets an angle by prompting for input, checking that the angle is valid, and looping back if it isn't; and one that decides whether three valid angles are a triangle. Then fasten them together with some appropriate glue.

answered Dec 04 '17 at 13:50

Pete Becker

74,985
8
76
165

I got it, I wrote if (a+b+c == 180 && a>0 && b>0 &&c>0) else cout <<"No"; – Adam Eve Dec 07 '17 at 13:17
@AdamEve — cool! Just a personal style tip: I’d write the tests the other way around, testing the requirements first and then the “real” test: `if (a>0 && b>0 && c>0 && a+b+c==180)`. Yours is correct, but in other contexts the order matters. For example: `int *ip = 0; if (ip && *ip)...` — here, testing the validity of the pointer first prevents dereferencing an invalid pointer. – Pete Becker Dec 07 '17 at 14:19

How to know whether the triangle is valid or not by angles?

2 Answers2