Need help with "triangle" program.

[Mohammad Zahed]

Hi. I'm doing my first lab program for my C class, which is due this tuesday:

"Input 3 positive integers from the terminal, determine if they are valid sides of a triangle. If the sides do form a valid triangle, output the type of triangle - equilateral, isosceles, or scalene- and the area of the triangle. If the 3 integers are not the valid sides of a triangle, output an appropriate message and the 3 sides. be sure to comment and error check in your program."

Use: s = (a + b + c)/2.0
Area = sqrt(s*(s-a)*(s-b)*(s-c))
do while loop
char answer

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


int main ( void )
{
    char answer;
    char y = 1;
    char n = 0;
    int a, b, c;


    do {
        printf("Enter 3 positive integers (separated by 1 space) for the triangle: \n");
        scanf("%d %d %d%*c", &a, &b, &c);


        if((a + b) > c || (b + c) > a || (a + c) > b)
        {
            if(a == b && b == c)
            {
                double s = (a + b + c)/2.0;
                double area = sqrt(s*(s - a)*(s - b)*(s - c));
                printf("This is an equilateral triangle.\n");
                printf("The area of this triangle is %d.", area);
            }
            else if((a > b && b == c) || (b > a && a == c) || (c > a && a == b) ||
                    (a < b && b == c) || (b < a && a == c) || (c < a && a == b))
            {
                double s = (a + b + c)/2.0;
                double area = sqrt(s*(s - a)*(s - b)*(s - c));
                printf("This is an isosceles triangle.\n");
                printf("The area of this triangle is %d.", area);
            }
            else
            {
                double s = (a + b + c)/2.0;
                double area = sqrt(s*(s - a)*(s - b)*(s - c));
                printf("This is a scalene triangle.\n");
                printf("The area of this triangle is %d.", area);
            }
        }
        else
        {
            printf("This is an invalid triangle.\n");
            printf("The 3 sides you have entered are: %d, %d and %d.", a, b, c);
        }
        printf("Continue? y/n.\n");
        scanf("%c", &answer);
    }
    while ( answer != 0 );


    return 0;
}

After i build and run it, this is what i get:

Enter 3 positive integers (separated by 1 space) for the triangle:
1 2 3
This is a scalene triangle.
The area of this triangle is 0.Continue? y/n.
y
Enter 3 positive integers (separated by 1 space) for the triangle:
0 0 0
This is an invalid triangle.
The 3 sides you have entered are: 0, 0 and 0.Continue? y/n.
n
Enter 3 positive integers (separated by 1 space) for the triangle:
0 1 1
This is an isosceles triangle.
The area of this triangle is 0.Continue? y/n.
n
Enter 3 positive integers (separated by 1 space) for the triangle:
2 3 4
This is a scalene triangle.
The area of this triangle is 83589923.Continue? y/n.


I'm getting incorrect areas and when i hit 'n', the program doesn't end. The only correct output i'm getting is what kind of triangle it is (when i enter positive integers). What am I doing wrong in my coding? Please help me...

[Matticus]

For starters, heed your compiler warnings. This is what I got when I built your code.

main.c||In function 'main':|
main.c|25|warning: format '%d' expects type 'int', but argument 2 has type 'double'|
main.c|33|warning: format '%d' expects type 'int', but argument 2 has type 'double'|
main.c|40|warning: format '%d' expects type 'int', but argument 2 has type 'double'|
main.c|9|warning: unused variable 'n'|
main.c|8|warning: unused variable 'y'|
||=== Build finished: 0 errors, 5 warnings ===|

The first three tell you that you're using the wrong format specifier in your "printf()" statements.

--------

I haven't done a thorough analysis of your code, but glancing at the first case, it seems your equations are wrong. I'm rusty with trig, but at least I can show you where your program is going wrong for the first case.

Enter 3 positive integers (separated by 1 space) for the triangle:
1 2 3

/*
    a = 1
    b = 2
    c = 3
*/
double s = (a + b + c)/2.0;
/*
    s = (1 + 2 + 3) / 2 = 3
*/
double area = sqrt(s*(s - a)*(s - b)*(s - c));
/*
    area = sqrt(s*(s - a)*(s - b)*(s - c));
         = sqrt(3*(3 - 1)*(3 - 2)*(3 - 3));
         = sqrt(3*(  2  )*(  1  )*(  0  ));
         = sqrt( 0 );
    area = 0

*/
printf("This is a scalene triangle.\n");
printf("The area of this triangle is %d.", area);

--------

...and when i hit 'n', the program doesn't end

printf("Continue? y/n.\n");
scanf("%c", &answer);
while ( answer != 0 );

1. You prompt for input
2. You scan for a character input
3. So why are you checking the user input against zero?

[Click_here]

I can show you where your program is going wrong for the first case.

If you have sides 1, 2 and 3, your area would be 0

Get 3 bits of paper, make the correct ratios and you will see that the 1 and 2 sides make a straight line back across the side 3


Should these be "&&"?

if((a + b) > c || (b + c) > a || (a + c) > b)

Consider the famous triangle 3/4/5

(3+4)>5 && (4+5)>3 && (5+3)>4

[Mohammad Zahed]

The first three tell you that you're using the wrong format specifier in your "printf()" statements.

Ah, thanks thanks.

If you have sides 1, 2 and 3, your area would be 0

Get 3 bits of paper, make the correct ratios and you will see that the 1 and 2 sides make a straight line back across the side 3


Should these be "&&"?

if((a + b) > c || (b + c) > a || (a + c) > b)

Consider the famous triangle 3/4/5

(3+4)>5 && (4+5)>3 && (5+3)>4

"...your area would be 0". OH! I didn't realize that. I thought that really was a mistake.

and you're right, they should be && instead of ||. Thanks!

printf("Continue? y/n.\n");
scanf("%c", &answer);
while ( answer != 0 );

1. You prompt for input
2. You scan for a character input
3. So why are you checking the user input against zero?

thanks. i fixed that. now i don't think i have any more problems with my program except for ending it by typing "n". i decided to use floats instead of double since that's more convenient:

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


int main ( void )
{
    char answer;
    char y = 1;
    char n = 0;
    float a, b, c;


    do {
        printf("Enter 3 positive integers (separated by 1 space) for the triangle: \n");
        scanf("%f %f %f%*c", &a, &b, &c);


        if((a + b) > c && (b + c) > a && (a + c) > b)
        {
            if(a == b && b == c)
            {
                float s = (a+b+c)/2;
                float area = sqrt(s*(s-a)*(s-b)*(s-c));
                printf("This is an equilateral triangle.\n");
                printf("The area of this triangle is %.3f. ", area);
            }
            else if((a > b && b == c) || (b > a && a == c) || (c > a && a == b) ||
                    (a < b && b == c) || (b < a && a == c) || (c < a && a == b))
            {
                float s = (a+b+c)/2;
                float area = sqrt(s*(s-a)*(s-b)*(s-c));
                printf("This is an isosceles triangle.\n");
                printf("The area of this triangle is %.3f. ", area);
            }
            else
            {
                float s = (a+b+c)/2;
                float area = sqrt(s*(s-a)*(s-b)*(s-c));
                printf("This is a scalene triangle.\n");
                printf("The area of this triangle is %.3f. ", area);
            }
        }
        else
        {
            printf("This is an invalid triangle.\n");
            printf("The 3 sides you have entered are: %f, %f and %f. ", a, b, c);
        }
        printf("Continue? y/n.\n");
        scanf("%c", &answer);
    }
    while ( answer != n );


    return 0;
}

so now, even when i type "n", the program doesn't end. Can anyone help me with this last problem with my program?

[Tclausex]

'n' not n. Literal characters must have the single quotes around them.

[Mohammad Zahed]

'n' not n. Literal characters must have the single quotes around them.

Thanks! It works perfectly now.

[Click_here]

OH! I didn't realize that. I thought that really was a mistake.

I think that a better approach would be to have pre-calculated inputs, not just random numbers - That way you can actually say whether your code is behaving correctly or not.

[Mohammad Zahed]

I think that a better approach would be to have pre-calculated inputs, not just random numbers - That way you can actually say whether your code is behaving correctly or not.

Yeah, i'll keep that in mind. thanks again.

[iMalc]

Don't duplicate code unnecessarily. Lines 22 & 23 can be move up to where lines 20 & 21 are, then you wont need lines 30, 31, 37, & 38.
Similarly, line 25 can be move to after the if-statements to about where line 41 is now.

[Mohammad Zahed]

Don't duplicate code unnecessarily. Lines 22 & 23 can be move up to where lines 20 & 21 are, then you wont need lines 30, 31, 37, & 38.
Similarly, line 25 can be move to after the if-statements to about where line 41 is now.

alright. thanks!