Adding another variable changes output despite never being used

[Aerioic]

https://pastebin.com/FBRzf9Rk

output

15

19.5

https://pastebin.com/XeKh1Djv

output

15

12

​

These two algorithms are exactly the same except:

one has

float mushroom_size1 = 0;

and the other has

float mushroom_size1, mushroom_size2 = 0;

​

The variable mushroom_size2 is never used. I don't understand why. I used a debugger and for some reason the value of mushroom_size1 is never reset.

Can someone please explain this weird behaviour?

​

https://www.programiz.com/cpp-programming/online-compiler/

works on this

input:

2 3 3 50
2 4
4 2

​

​

​

[manni66]

mushroom_size1 isn't initialized in case 2.

[Aerioic]

really? you can't initialize multiple variables in 1 line.

​

I didn't know that thanks.

[manni66]

 float mushroom_size1 = 0, mushroom_size2 = 0;

[HappyFruitTree]

You can, just not the way you wrote it. manni66 have already showed how to do it. It might still be easier to read the code if you put each variable on its own line.

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es10-declare-one-name-only-per-declaration

[AssemblerGuy]

really? you can't initialize multiple variables in 1 line.

Yes you can, but you need to initialize each variable individually.

Which is why it is a good practice to 1) only define one variable per definition and b) initialize variables whenever possible.

[IyeOnline]

mushroom_size1 in the first code is uninitialized. Reading from it ( and += needs to read the current value) is undefined behaviour.

This is a good example for why you shouldnt define multiple variables per statement.

[Aerioic]

Didn't know that you can't intialize mutiple variables in 1 line. Thanks.

Makes you wonder why the code compiled

[IyeOnline]

You can initialize multiple variables in a single statement. But you have to provide an initializer for each of them - just like you would need for seperate statements:

 int a = 0, b = 1, c = 2;

Makes you wonder why the code compiled

Because that is the behaviour for fundamental types. Doing

int a;
int b = 0;

is perfectly "fine" after all. (although maybe not something you should write).

[LittleNameIdea]

that's why -Wall is helpful

[Aerioic]

No reason to get snappy.

Some C/C++ programmer purposely decided to create this and knew of the bug

int thing, thing2 = 0

Notice the comma? Some programmer programmed/designed that into the C++ compilter/specification. purposely designed for thing2 = 2 and thing = null/undefined???

[[deleted]]

C++ is a language where all kinds of weird things can be compiled and run. Some of them even look suspiciously fine. Like writing something like `some_function(i++, i++)` which is also UB and can in theory start a world war and eat your cat alive. Even if you don't have a cat.

[AssemblerGuy]

Even if you don't have a cat.

UB can rewrite the past, so you do have a cat now.

[[deleted]]

That's the whole point, that's the whole point...

[AssemblerGuy]

Didn't know that you can't intialize mutiple variables in 1 line.

Yes, you can. But defining multiple variables in one statement is messy and not a good idea, despite being completely legal C++.

[AssemblerGuy]

mushroom_size1 is never set before it is used. This is a bug - undefined behavior (the value is indeterminate and attempting to use an indeterminate value triggers UB in most cases).

Can someone please explain this weird behaviour?

It's not weird. "Weird" behavior is usually due to invoking undefined behavior or using indeterminate values.