Python Range: For Loop, Create List From Range

This Python page is about range, an immutable sequence type. Range is often useful in for-loops but can be used to create lists.
Range. Sequences are everywhere. Our alphabet and our numbers are sequences. And in computers, sequences of bits take meanings—they form numbers and values.
For looping in Python, we often use an immutable sequence called range. This built-in method returns a sequence based on the numbers we pass it. Range receives a start, end and step.
Consider this program. It shows how to use range() with a for-loop. This style of code may be considered "Pythonic" meaning it is simple and avoids just doing things like other languages.

So: We access the range() immutable sequence. It returns the values 0, 1 and 2, which we print in the loop body.

Iterate: Notice how the program iterates over numbers, but contains no iteration statement like i++. This expression is handled by range.

Python program that uses range, for-loop # This sets "i" to 0, 1 and 2. # ... The step, not specified, is 1. for i in range(0, 3): print(i) Output 0 1 2
One argument. When we use just one argument in range() the start is considered zero. So a range 5 goes from 0 through 4 inclusive.Literal: F
Python program that uses range, one argument # Use a single argument in range to start at zero. for i in range(5): print(f"Range(5): {i}") Output Range(5): 0 Range(5): 1 Range(5): 2 Range(5): 3 Range(5): 4
Adjacent. Often we need to access adjacent list elements. One way to do this is with range(). In this example, we loop through all the indexes in the list.

Then: We access the previous and current element in the list. These are adjacent elements. Please note we start the range at 1, not 0.

Python program that uses range, for-loop names = ["mark", "michelle", "cecil"] # Loop over range starting at index 1. for i in range(1, len(names)): # Adjacent names. print(names[i - 1], names[i]) Output mark michelle michelle cecil
Range, negative. We can use range() to decrement a variable, to move downwards. We specify the "step" value as the optional third parameter of range. We use -1 to decrement by one.

Note: The second value (0) is never reached in the loop body—it is an "exclusive" boundary.

Python program that uses range, decrements # Loop over range with negative step. for i in range(5, 0, -1): print(i) Output 5 4 3 2 1
Create list. We can use range() to create a list based on a sequence of values. This is much simpler than looping and adding numbers—it is shorter, less prone to bugs.

Note: List comprehension can also be used to create lists based on expressions. It is a preferred technique.

Python program that creates list from range # Create a list from a range. values = list(range(0, 4)) print(values) Output [0, 1, 2, 3]
A benchmark. What performance impact does range() have? With range, we provide two or three numbers and loop over them. I found that range loops are faster than while-loops.

So: My test shows that excess worry about the range() call being a slowdown is not warranted.

Preference: For performance, my tests show that a for-loop, with or without a range() call, is faster than while in Python 3.3.

Python program that times, while, for-range loop import time print(time.time()) # Version 1: while-loop. c = 0 i = 0 while i < 10000000: c += i i += 1 print(time.time()) # Version 2: for-range loop. c2 = 0 for y in range(0, 10000000): c2 += y print(time.time()) Output 1406236639.696959 1406236643.343169 while-loop: 3.65 s 1406236645.533295 for-loop, range: 2.19 s
With range(), we keep our code simple. And in the benchmark, we found that range() does not cause any performance problems. It is faster than many alternatives.
An immutable sequence. In the Python documentation, range() is considered an "immutable sequence." This means it cannot be changed after specified.
An optimization. Because range() is immutable, it can be heavily optimized—no allocations in memory are needed for a new range. With simpler, faster code, range is a good choice.
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