Sequence of if-else if in C

Question

I' using the following logic for testing whether the triangle is isosceles, equilateral, scalene or right angled.

if (side1 == side2 || side2 == side3 || side1 == side3)
    printf("Isosceles triangle.");
else if (side1 == side2 && side2 == side3 && side3 == side1)
    printf("equilateral triangle");

I get the output for sides 3 3 3 as isosceles but not equilateral but when I interchange the logic that is write the logic of equilateral first I get equilateral. I can't understand what's happening?

'else' is executed only if 'if' is not executed. Just remove 'else'. — sashkello, Jul 11 '13 at 13:14
Note that your list is not mutually exclusive; a right angled triangle can be isosceles. And beware comparing floating point ==. — Bathsheba, Jul 11 '13 at 13:16

score 5 · Answer 1 · answered Jul 11 '13 at 13:14

5

You shouldn't use else in this case.

Code:

if (condition)
     code
else if (condition2)
     code2

Checks if condition is true. If so it executes code. Only if condition is false, condition2 is checked and code2 could be executed.

answered Jul 11 '13 at 13:14

Ari

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score 3 · Answer 2 · answered Jul 11 '13 at 13:22

3

Your code "checks" the second if only when the first if is false.
logically the second if can be true only if the first if is true...

I would change the code to:

if (side1 == side2 || side2 == side3 || side1 == side3)
{
    printf("Isosceles triangle.");
    if (side1 == side2 && side2 == side3 && side3 == side1)
        printf("equilateral triangle");
}

answered Jul 11 '13 at 13:22

Roee Gavirel

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1

+1 for the nested `if` (instead of just remove the `else`). It implied a equilateral triangle must be a isosceles triangle. – johnchen902 Jul 11 '13 at 13:29

score 2 · Answer 3 · answered Jul 11 '13 at 13:15

2

else is executed only if if is not executed. Just remove else and it will be able to print both statements in such case.

answered Jul 11 '13 at 13:15

sashkello

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score 2 · Answer 4 · answered Jul 11 '13 at 13:17

2

Side1 = 3, side2 = 3, side = 3, which means that side1 == side2 is true. That's why your program print out "Iosceles". Beacuse first if is true, second won't be checked. It would be only if the first one was false.

answered Jul 11 '13 at 13:17

Qiu

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zztops · Answer 5 · 2013-07-11T13:48:26.633

Just switch the order of the if statements. Since every equilateral triangle is isosceles, you never make it into the elseif. Make your code read like this:

if (side1 == side2 && side2 == side3 && side3 == side1)
    printf("Equilateral triangle");
else if (side1 == side2 || side2 == side3 || side1 == side3)
    printf("Isosceles triangle.");

Alternatively, you can nest the equilateral if block inside the isosceles if block if you want both results printed:

if (side1 == side2 || side2 == side3 || side1 == side3){
    if (side1 == side2 && side2 == side3 && side3 == side1){
        printf("Equilateral triangle");
    }
    printf("Isosceles triangle.");
}

Another optimization to consider is that your equilateral check only needs two check to equalities. ie:

(side1 == side2 && side2 == side3) => (side1 == side3)

So, the if statement can read like this:

if (side1 == side2 && side2 == side3)

Sequence of if-else if in C

5 Answers5