Insertion sort algorithm gives overflow error - rust

Question

When trying to run the insertion sort algorithm as shown below in Rust 1.15.
fn main() {
println!("The sorted set is now: {:?}", insertion_sort(vec![5,2,4,6,1,3]));
}
fn insertion_sort(set: Vec<i32>) -> Vec<i32> {
let mut A = set.to_vec();
for j in 1..set.len() {
let key = A[j];
let mut i = j - 1;
while (i >= 0) && (A[i] > key) {
A[i + 1] = A[i];
i = i - 1;
}
A[i + 1] = key;
}
A
}
I get the error:
thread 'main' panicked at 'attempt to subtract with overflow', insertion_sort.rs:12
note: Run with `RUST_BACKTRACE=1` for a backtrace
Why does an overflow happen here and how is the problem alleviated?

Answer 1

The reason is you tried to calculate 0 - 1 in usize type, which is unsigned (nonnegative). This may lead to an error in Rust.
Why usize? Because Rust expects usize for lengths and indices. You can explicitly convert them into/from signed ones e.g. isize.
fn main() {
println!("The sorted set is now: {:?}", insertion_sort(vec![5,2,4,6,1,3]));
}
fn insertion_sort(set: Vec<i32>) -> Vec<i32> {
let mut A = set.to_vec();
for j in 1..set.len() as isize {
let key = A[j as usize];
let mut i = j - 1;
while (i >= 0) && (A[i as usize] > key) {
A[(i + 1) as usize] = A[i as usize];
i = i - 1;
}
A[(i + 1) as usize] = key;
}
A
}
Another solution, which I recommend, is to avoid negative indices at all. In this case you can use i + 1 instead of i like this:
fn main() {
println!("The sorted set is now: {:?}", insertion_sort(vec![5,2,4,6,1,3]));
}
fn insertion_sort(set: Vec<i32>) -> Vec<i32> {
let mut A = set.to_vec();
for j in 1..set.len() {
let key = A[j];
let mut i = j;
while (i > 0) && (A[i - 1] > key) {
A[i] = A[i - 1];
i = i - 1;
}
A[i] = key;
}
A
}