85 lines
2.9 KiB
Markdown
85 lines
2.9 KiB
Markdown
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---
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title: The Python Range
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---
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## Python Ranges
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Rather than being a function, a range is actually an <a href='https://docs.python.org/3/library/stdtypes.html#immutable-sequence-types' target='_blank' rel='nofollow'>immutable sequence type</a> and is commonly used for looping a specific number of times in for loops.
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**Creation:**
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`ranges` are created using the `range` constructor. The parameters for the constructor are:
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* `start`: Inclusive first value of the range (optional integer, defaults to 0).
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* `stop` : Exclusive stop value, range stops when this value or greater would be provided (required integer).
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* `step` : The amount added to the current value to get the next value (optional integer, defaults to 1).
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```python
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>>> range(10) # Only the stop parameter is required.
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range(0, 10)
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>>> range(0, 10) # Default for start parameter is 0.
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range(0, 10)
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>>> range(0, 10, 1) # Default for step is 1\. Start parameter is required if
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step is needed.
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range(0, 10)
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```
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**Examples:**
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Since `ranges` are iterables they can be passed into the `list` and `tuple` constructors to create those types of sequences. Using this fact, we can visualize some examples:
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```python
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>>> list(range(10)) # range as argument for list constructor.
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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>>> tuple(range(10)) # range as argument for tuple constructor.
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(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
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```
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Zero length `ranges`:
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```python
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>>> list(range(10, 0)) # start greater than stop with postive step.
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[]
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>>> list(range(10, 10)) # start equal to stop with postive step.
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[]
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>>> list(range(10, 10, -1)) # start equal to stop with negative step.
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[]
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>>> list(range(0, 10, -1)) # start less than stop with negative step.
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[]
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```
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`ranges` with step arguments:
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```python
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>>> list(range(0, 10, 2)) # next value would be 10, stops at 8.
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[0, 2, 4, 6, 8]
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>>> list(range(0, 10, 3)) # next value would be 12, stops at 9.
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[0, 3, 6, 9]
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>>> list(range(0, 10, 4)) # next value would be 12, stops at 8.
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[0, 4, 8]
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>>> list(range(10, 0, -1)) # negative step makes decreasing ranges.
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[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
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>>> list(range(-5, -30, -3)) # negative integers are valid arguments.
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[-5, -8, -11, -14, -17, -20, -23, -26, -29]
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```
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**Benefits:**
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The benefit of using `range` is that regardless of how large of a range specified, only a small amount of memory is needed to store the `range`, the values for start, stop, and step. The individual values of the `ranges` are calculated upon iteration.
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```python
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>>> import sys
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>>> a_range = range(1000000)
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>>> a_list = list(a_range)
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>>> a_tuple = tuple(a_range)
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>>> sys.getsizeof(a_range)
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48
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>>> sys.getsizeof(a_list)
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9000112
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>>> sys.getsizeof(a_tuple)
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8000048
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```
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### More Inforamtion:
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<a href='https://docs.python.org/3/library/stdtypes.html#ranges' target='_blank' rel='nofollow'>Python Doc - Ranges</a>
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**TODO: Methods `ranges` do and do not implement**
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