7 ms·

> Relying on the difference between the pre and post variations of these operators is a classic area of C programmer ego showmanship. Ugh. He's talking about

> Relying on the difference between the pre and post variations of these operators is a classic area of C programmer ego showmanship. Ugh. He's talking about inline use of post-increment and pre-increment (i.e. x++ and ++x) here. This is perfectly readable to a C programmer, and sidestepping them actually makes the code harder to understand.

miketuritzin 6y ago
Can you give an example where not using them 'inline' makes code harder to understand?
- Koshkin 6y ago
  You can't beat return x++; :)
  - sigjuice 6y ago
    Why? This should be straightforward.
- a1369209993 6y ago
  Consider the idiomatic way of interating backward through a array: for(i=n; i-- > 0 ;) { /* operate on a[i] */ } converting i--; to a statement at the start of block makes it less clear that it's part of the iteration idiom rather than a ad hoc adjustment that's specific to this particular logic. There are other examples, but they're either more involved or statementification is less obviously wrong.
  - ramshorns 6y ago
    That's the idiomatic way? Cool. The more straightforward-looking way, for(i = n-1; i >= 0; i--) { /* operate on a[i] */ } breaks if i is unsigned, like a size_t.
    - a1369209993 6y ago
      Yep. That why it's a idiom, rather than a obvious-way-of-doing-it-that-anyone-competent-would-use.
  - nikki93 6y ago
    Hmm, I think `for (i = n - 1; i >= 0; --i)` is way clearer and maybe more common? edit: Ah unsigned underflow. :O
    - ramshorns 6y ago
      Yeah, so then you write for (size_t i = n-1; i < n; --i) { /* operate on a[i] */ } It works fine (unsigned overflow is well defined) but it's even less clear.
      - nikki93 6y ago
        It seems sensible to always just use signed values for indices. Indices are difference types, which should include negative values so that you can subtract two indices and get a sane delta. The range of signed values seems 'big enough.'
        a1369209993 6y ago
        > Indices are difference types Umm, no? Indices are ordinals[0], forming the canonical/nominal well-ordering of a collection such as a array. > an ordinal number, or ordinal, is one generalization of the concept of a natural number that is used to describe a way to arrange a (possibly infinite) collection of objects in order, one after another. [...] Ordinal numbers are thus the "labels" needed to arrange collections of objects in order. 0: https://en.wikipedia.org/wiki/Ordinal_number https://en.wikipedia.org/wiki/Ordinal_number
        nikki93 6y ago
        In C an index is a difference that you add to a pointer to get a pointer. `a[i]` is `*(a + i)`. Given two indices `i` and `j`, you want `i - j` to be such that `a[j + (i - j)]` is `a[i]`, and it then makes sense to me that `i - j` is signed. The expression works out whether they are signed or unsigned, but just in terms of their interpretation on the part of a user (eg. "oh this is 2 elements before bc. it says -2") or so that comparisons like `i < j` are equivalent to `i - j < 0` and so on. That's why it's always made sense to me to use `ptrdiff_t` (or just `int`) for an index, vs. using `size_t`.
- moonchild 6y ago
  Imagine operating on something like a stack. x = *stack--; // pop 'x' off of the stack *++stack = y; // push 'y' onto the stack This way is simple, direct, and it avoids inconsistent state.
  - harry8 6y ago
    int pop_int () { int x = *stack; --stack; return x; } void push_int(int x) { ++stack; *stack = x; } Genunine questions: - Is this worse? - How does the state get inconsistent?
    - saagarjha 6y ago
      For one it’s three and two lines for what is two logical operations. I assume the “inconsistent state” is the time between the lines where the stack is not truly in the right state-many people prefer to preserve their invariants as much as possible.
      - harry8 6y ago
        it will produce indistinguishable assembly language, no?
        saagarjha 6y ago
        The use of that construct is mainly a stylistic choice. On any compiler from this millennium there should be no difference in the code that it produces.
        harry8 6y ago
        Yep so if we're going with style I'm very happy with the functions dashed off there. Nobody will confuse those even when very, very tired (similar effect on the brain to being drunk). There is zero difference in the generated output. Calling those functions tells you exactly what they are and what they do. Vertical space is not an issue at all with 3 line functions. Relying on post-increment? Make sure it's a one line block that is totally unbraced with only single letter variable names if you do it because otherwise it's just faux-macho C and that's /weak/.

miketuritzin 6y ago

Can you give an example where not using them 'inline' makes code harder to understand?

Koshkin 6y ago
You can't beat return x++; :)
- sigjuice 6y ago
  Why? This should be straightforward.
a1369209993 6y ago
Consider the idiomatic way of interating backward through a array: for(i=n; i-- > 0 ;) { /* operate on a[i] */ } converting i--; to a statement at the start of block makes it less clear that it's part of the iteration idiom rather than a ad hoc adjustment that's specific to this particular logic. There are other examples, but they're either more involved or statementification is less obviously wrong.
- ramshorns 6y ago
  That's the idiomatic way? Cool. The more straightforward-looking way, for(i = n-1; i >= 0; i--) { /* operate on a[i] */ } breaks if i is unsigned, like a size_t.
  - a1369209993 6y ago
    Yep. That why it's a idiom, rather than a obvious-way-of-doing-it-that-anyone-competent-would-use.
- nikki93 6y ago
  Hmm, I think `for (i = n - 1; i >= 0; --i)` is way clearer and maybe more common? edit: Ah unsigned underflow. :O
  - ramshorns 6y ago
    Yeah, so then you write for (size_t i = n-1; i < n; --i) { /* operate on a[i] */ } It works fine (unsigned overflow is well defined) but it's even less clear.
    - nikki93 6y ago
      It seems sensible to always just use signed values for indices. Indices are difference types, which should include negative values so that you can subtract two indices and get a sane delta. The range of signed values seems 'big enough.'
      - a1369209993 6y ago
        > Indices are difference types Umm, no? Indices are ordinals[0], forming the canonical/nominal well-ordering of a collection such as a array. > an ordinal number, or ordinal, is one generalization of the concept of a natural number that is used to describe a way to arrange a (possibly infinite) collection of objects in order, one after another. [...] Ordinal numbers are thus the "labels" needed to arrange collections of objects in order. 0: https://en.wikipedia.org/wiki/Ordinal_number https://en.wikipedia.org/wiki/Ordinal_number
        nikki93 6y ago
        In C an index is a difference that you add to a pointer to get a pointer. `a[i]` is `*(a + i)`. Given two indices `i` and `j`, you want `i - j` to be such that `a[j + (i - j)]` is `a[i]`, and it then makes sense to me that `i - j` is signed. The expression works out whether they are signed or unsigned, but just in terms of their interpretation on the part of a user (eg. "oh this is 2 elements before bc. it says -2") or so that comparisons like `i < j` are equivalent to `i - j < 0` and so on. That's why it's always made sense to me to use `ptrdiff_t` (or just `int`) for an index, vs. using `size_t`.
moonchild 6y ago
Imagine operating on something like a stack. x = *stack--; // pop 'x' off of the stack *++stack = y; // push 'y' onto the stack This way is simple, direct, and it avoids inconsistent state.
- harry8 6y ago
  int pop_int () { int x = *stack; --stack; return x; } void push_int(int x) { ++stack; *stack = x; } Genunine questions: - Is this worse? - How does the state get inconsistent?
  - saagarjha 6y ago
    For one it’s three and two lines for what is two logical operations. I assume the “inconsistent state” is the time between the lines where the stack is not truly in the right state-many people prefer to preserve their invariants as much as possible.
    - harry8 6y ago
      it will produce indistinguishable assembly language, no?
      - saagarjha 6y ago
        The use of that construct is mainly a stylistic choice. On any compiler from this millennium there should be no difference in the code that it produces.
        harry8 6y ago
        Yep so if we're going with style I'm very happy with the functions dashed off there. Nobody will confuse those even when very, very tired (similar effect on the brain to being drunk). There is zero difference in the generated output. Calling those functions tells you exactly what they are and what they do. Vertical space is not an issue at all with 3 line functions. Relying on post-increment? Make sure it's a one line block that is totally unbraced with only single letter variable names if you do it because otherwise it's just faux-macho C and that's /weak/.