How to increment pointer in rust like C code below

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3

Hm. If I'm reading that code correctly, it looks like bubble sort, but written without using C array indexing (p[i]) and instead using explicit pointer arithmetic (*(p + i)). I'm not sure why you'd want to do it that way, but let's talk about it. Specifically, the sort function.

Formatted, the sort function reads:

void sort(int *p)
{
  int round,t,i;
  for(round=1;round<=4;round++)
  {
    for(i=0;i<=4-round;i++)
      if(*(p+i) > *(p+i+1))
      {
        t=*(p+i); // a[i] *(p+i)
        *(p+i)=*(p+i+1);
        *(p+i+1)= t;
      }
  }
}

A mostly-literal translation into Rust, which is not how I would do it, would go something like this:

unsafe fn sort(p: *mut u32) {
    for round in 1..=4 {
        for i in 0..=4-round {
            if *(p.add(i)) > *(p.add(i+1)) {
                let t = *(p.add(i)); // a[i] *(p+i)
                *(p.add(i)) = *(p.add(i+1));
                *(p.add(i+1)) = t;
            }
        }
    }
}

That function is marked unsafe because it's dealing in raw pointers and performing arbitrary pointer arithmetic, which I think is what you were asking about. (Rust doesn't overload operators like + for raw pointers, so you use the add method instead.)

However, it doesn't need to do any of that unsafe stuff. An idiomatic safe Rust implementation would look like this:

fn sort(p: &mut [u32]) {  // <-- slice reference instead of raw pointer
    for round in 1..p.len() {  // <-- adapts to any length of array
        for i in 0..p.len() - round {
            if p[i] > p[i + 1] {  // <-- using indexing instead of pointer arithmetic
                p.swap(i, i+1);  // <-- slice::swap exchanges two elements
            }
        }
    }
}

Finally, just a warning that your input routine will process uninitialized memory if fewer than five numbers are provided, which is undefined behavior in both C and Rust. You could avoid this by checking the return code from scanf and/or initializing your a array. (Local variables, including arrays, in C are uninitialized unless you explicitly initialize them.)

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