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digitalmars.D.learn - Floating point values in structs.

reply Dave P. <dave287091 gmail.com> writes:
I got burned by behavior of struct initialization I didn’t 
anticipate last night.

Consider a struct:

struct Foo {
     float x, y, z;
     int a, b, c;
}

My source C code was initializing it by doing something like:

Foo f = {.y = 3}

Which uses the C behavior that all the other fields will be set 
to 0.

I thought that the D translation would be:

Foo f = {y: 3}

But to my surprise, that leaves x and z “default initialized”, 
which means nan!

I have some large structs that are essentially configuration 
settings for certain subsystems. Is the proper solution to change 
the struct definition to:

struct Foo {
     float x=0, y=0, z=0;
     int a, b, c;
}

to get the behavior I want?

I find the setting floats to nan pretty bizarre. If the compiler 
is going to introduce the overhead of initializing all the 
variables anyway, why set it to nan when integer types get set to 
the useful default of 0? And if it knows that I am not 
initializing my floats, why not tell me?
Dec 18 2020
next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 18 December 2020 at 16:18:12 UTC, Dave P. wrote:

 Is the proper solution to change the struct definition to:


yeah that's the best option right now


 I find the setting floats to nan pretty bizarre.


yeah i wrote about it here not long ago 
http://dpldocs.info/this-week-in-d/Blog.Posted_2020_11_23.html#looking-back


excerpt:

"See, the idea behind D's initialization is it catches 
uninitialized values at runtime. Null pointers crash. NaN floats 
give NaN results. \ff chars give invalid strings.

The problem is it sets the initializer for int (and friends) to 
be 0. Which is actually pretty useful and convenient, so people 
rely on it. Now making uninitialized things an error is 
infeasible and the fact is it sets a habit to expect 0."

so i think it should be changed. but floats and chars are all 
weird initialized so be aware of it
Dec 18 2020
parent Dave P. <dave287091 gmail.com> writes:
On Friday, 18 December 2020 at 16:23:20 UTC, Adam D. Ruppe wrote:

 On Friday, 18 December 2020 at 16:18:12 UTC, Dave P. wrote:

 Is the proper solution to change the struct definition to:


 yeah that's the best option right now


A bit of a pain when porting, but alright.


 I find the setting floats to nan pretty bizarre.


 yeah i wrote about it here not long ago 
 http://dpldocs.info/this-week-in-d/Blog.Posted_2020_11_23.html#looking-back

 [...]


I agree.

Thanks!
Dec 18 2020
prev sibling parent reply Dukc <ajieskola gmail.com> writes:
On Friday, 18 December 2020 at 16:18:12 UTC, Dave P. wrote:

 If the compiler is going to introduce the overhead of 
 initializing all the variables anyway, why set it to nan when 
 integer types get set to the useful default of 0?


Consider default value of `int*`. It is `null`, not a pointer to 
a newly-allocated 0. Most would probably agree that `null` works 
better, because it stands for "empty". 0 may or may not, 
depending on context.

It's this philosophy with floating-point values also. `0.0` might 
seem like a value when no value is intended, with `nan` there are 
no ambiguities.

More about this:
https://digitalmars.com/articles/b81.html
https://dlang.org/blog/2018/10/17/interfacing-d-with-c-arrays-part-1/
Dec 22 2020
parent Dave P. <dave287091 gmail.com> writes:
On Tuesday, 22 December 2020 at 18:10:22 UTC, Dukc wrote:

 On Friday, 18 December 2020 at 16:18:12 UTC, Dave P. wrote:

 [...]




Honestly, I just want all bits zero. I don’t work with any 
platforms where null is not 0 and all-zero-bits aggregates can be 
efficiently represented in static storage. I don’t want the 
compiler inserting a ton of stores of nan to my float members. If 
I wanted something other than all bits zero for unmentioned 
members I would write constructors instead of using the struct 
initializer syntax. I generally design my data types such that 
all bits zero is a fully valid struct.

But it is what it is. I just have to remember to write `=0` 
whenever I have a float member or a char member.
Dec 22 2020