## Issue

I have a program that finds, for all integers less than or equal to the input, numbers that can be represented as the sum of two cubes, twice, aka the Ramanujan’s number problem.

I have written this in Java and Rust, however, it runs more than twice as slow in Rust as compared to Java.

Is there anything I can do to make it perform better, or otherwise improve it?

Rust code:

```
use num_integer::Roots;
fn main() {
let v = 984067;
// let v = 87539319;
for i in 1..=v {
ramanujan(i)
}
}
fn ramanujan(m: i32) {
let maxcube = m.cbrt();
let mut res1 = 0;
let mut res2 = 0;
let mut _res3 = 0;
let mut _res4 = 0;
for i in 1..=maxcube {
for j in 1..=maxcube {
if i * i * i + j * j * j == m {
res1 = i;
res2 = j;
break;
}
}
}
for k in 1..=maxcube {
for l in 1..=maxcube {
if k == res1 || k == res2 || l == res1 || l == res2 {
continue;
}
if k * k * k + l * l * l == m {
_res3 = k;
_res4 = l;
break;
}
}
}
// if ((res1 * res1 * res1) + (res2 * res2 * res2) == m) && ((res3 * res3 * res3) + (res4 * res4 * res4) == m) {
// println!("{} is representable as the sums of two different sets of two cubes!\nThese values are {}, {}, and {}, {}.", m, res1, res2, res3, res4);
// }
}
```

Java code:

```
public class Ramun {
public static void main(String[] args) {
int v = 984067;
// int v = 87539319;
for (int i = 1; i <= v; i++) {
ramanujan(i);
}
}
public static void ramanujan(int m) {
int maxcube = (int) Math.round(Math.cbrt(m));
int res1 = 0, res2 = 0, res3 = 0, res4 = 0;
for (int i = 1; i <= maxcube; i++) {
for (int j = 1; j <= maxcube; j++) {
if (((i * i * i) + (j * j * j)) == m) {
res1 = i;
res2 = j;
break;
}
}
}
for (int k = 1; k <= maxcube; k++) {
for (int l = 1; l <= maxcube; l++) {
if (k == res1 || k == res2 || l == res1 || l == res2)
continue;
if (((k * k * k) + (l * l * l)) == m) {
res3 = k;
res4 = l;
break;
}
}
}
// if (((res1 * res1 * res1) + (res2 * res2 * res2) == m) && ((res3 * res3 * res3) + (res4 * res4 * res4) == m)) {
// System.out.printf("%d is representable as the sums of two different sets of two cubes!%nThese values are %d, %d, and %d, %d.%n", m, res1, res2, res3, res4);
// }
}
}
```

## Solution

The problem lies in `RangeInclusive`

which can be expensive.

Here’s a version avoiding it:

```
fn ramanujan(m: i32) {
let maxcube = m.cbrt() + 1; // we know it can't overflow
let mut res1 = 0;
let mut res2 = 0;
let mut res3 = 0;
let mut res4 = 0;
for i in 1..maxcube {
for j in 1..maxcube {
if i * i * i + j * j * j == m {
res1 = i;
res2 = j;
break;
}
}
}
for k in 1..maxcube {
for l in 1..maxcube {
if k == res1 || k == res2 || l == res1 || l == res2 {
continue;
}
if k * k * k + l * l * l == m {
res3 = k;
res4 = l;
break;
}
}
}
}
```

Result:

```
From: 0.01s user 0.00s system 0% cpu 17.993 total
To: 0.00s user 0.01s system 0% cpu 3.494 total
```

I added a comment to #45222 to draw attention to this issue.

Looks like `for_each()`

allows better performance too (as `for`

loops are more natural and should have the same performance, it should be considered as a bug):

```
fn ramanujan(m: i32) {
let maxcube = m.cbrt();
let mut res1 = 0;
let mut res2 = 0;
let mut res3 = 0;
let mut res4 = 0;
(1..=maxcube).for_each(|i| {
(1..=maxcube).try_for_each(|j| {
if i * i * i + j * j * j == m {
res1 = i;
res2 = j;
ControlFlow::Break(())
} else {
ControlFlow::Continue(())
}
});
});
(1..=maxcube).for_each(|k| {
(1..=maxcube).try_for_each(|l| {
if k != res1 && k != res2 && l != res1 && l != res2 && k * k * k + l * l * l == m {
res3 = k;
res4 = l;
ControlFlow::Break(())
} else {
ControlFlow::Continue(())
}
});
});
}
```

```
0.00s user 0.01s system 0% cpu 4.029 total
```

Answered By – Stargateur

Answer Checked By – Katrina (BugsFixing Volunteer)