The behavior of `numpy.arange()` when dealing with floating-point numbers and integer numbers stems from how the function operates internally, particularly with respect to how floating-point arithmetic is handled.
### Floating-Point Arithmetic in `np.arange`:
Floating-point arithmetic can introduce small precision errors due to the way numbers are represented in memory. This can affect whether or not the upper bound is included in the output.
import numpy as np
print(np.arange(9, 6.6, -0.3))
- **Start**: `9`
- **Stop**: `6.6`
- **Step**: `-0.3`
Here’s what happens:
1. **Initial Value**: The sequence starts at `9.0`.
2. **Subsequent Values**: Each subsequent value is obtained by subtracting `0.3` from the previous value:
- 9.0
- 8.7
- 8.4
- 8.1
- 7.8
- 7.5
- 7.2
- 6.9
- 6.6
3. **Stop Condition**: The function stops generating values when the next value would be less than `6.6`.
**Output**:
[9. 8.7 8.4 8.1 7.8 7.5 7.2 6.9 6.6]
The output includes `6.6` because, after subtracting `0.3` multiple times from `9.0`, the final value exactly matches `6.6` due to the nature of floating-point arithmetic.
### Why Integer Values Exclude Upper Bound:
For integer values, `np.arange(start, stop, step)` does not include the `stop` value by default. This is a design choice consistent with Python's `range()` function, where the range is half-open, meaning it includes the start value but excludes the stop value.
Example with integers:
print(np.arange(0, 10, 2))
**Output**:
[0 2 4 6 8]
Here, `10` is excluded because the sequence would go `0, 2, 4, 6, 8, 10`, but since `10` is the stop value, it is not included.
### Summary:
- **Floating-Point Arithmetic**: Precision issues can cause `np.arange()` to sometimes include the stop value if it matches exactly due to how the steps add up.
- **Integer Sequences**: By design, `np.arange()` excludes the stop value, similar to Python’s `range()` function, providing a half-open interval `[start, stop)`.
### Important Note:
For better control over floating-point sequences, especially when precision matters, it's often recommended to use `np.linspace` instead of `np.arange`, as `np.linspace` allows you to specify the number of points and guarantees the inclusion of both the start and stop values.
If you want to generate values from `9` to `6.6` without including `6.6`, you'd need to either:
**Use `endpoint=False`** in `np.linspace()` to exclude the stop value:
import numpy as np
print(np.linspace(9, 6.6, num=9, endpoint=False))
### Output with `endpoint=False`:
[9. 8.73 8.46 8.19 7.92 7.65 7.38 7.11 6.84]
In this case, `6.6` is not included because the sequence stops before reaching the exact stop value.
