*Below the first code snippet prints 1*

```
for i in stride(from: 1, through: 1, by: -1) {
print(i)
}
```

*but why the below snippet prints nothing?*

```
for i in stride(from: 1, through: 2, by: -1) {
print(i)
}
```

4 2021-03-14 20:08:11

*For a positive stride amount, stride(from:through:by:) returns a sequence of all values x satisfying*

```
start <= x and x <= end
```

*which can be reached from the start value by adding the stride zero or more times.* *For a negative stride, the condition is the other way around:*

```
start >= x and x >= end
```

*That behavior is symmetric with respect to positive and negative strides:*

```
for i in stride(from: 0, through: 0, by: 1) { print(i) } // prints 0
for i in stride(from: 0, through: -1, by: 1) { print(i) } // prints nothing
for i in stride(from: 0, through: 0, by: -1) { print(i) } // prints 0
for i in stride(from: 0, through: 1, by: -1) { print(i) } // prints nothing
```

*The implementation can be found in Stride.swift in the Swift source code repository:*

```
public mutating func next() -> Element? {
let result = _current.value
if _stride > 0 ? result >= _end : result <= _end {
// Note the `>=` and `<=` operators above. When `result == _end`, the
// following check is needed to prevent advancing `_current` past the
// representable bounds of the `Strideable` type unnecessarily.
//
// If the `Strideable` type is a fixed-width integer, overflowed results
// are represented using a sentinel value for `_current.index`, `Int.min`.
if result == _end && !_didReturnEnd && _current.index != .min {
_didReturnEnd = true
return result
}
return nil
}
_current = Element._step(after: _current, from: _start, by: _stride)
return result
}
```

*Your first example*

```
for i in stride(from: 1, through: 1, by: -1) { print(i) }
```

*prints 1 because that value satisfies 1 >= 1 and 1 >= 1 .*

```
for i in stride(from: 1, through: 2, by: -1) { print(i) }
```

*prints nothing because no value satisfies both conditions 1 >= x and x >= 2 .*

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