Sum. A list contains the numbers 10, 20 and 5: its sum is 35. We can a for-loop to iterate and add up the element values. Or we can use a simpler built-in method.
Two arguments. The sum() built-in receives 2 arguments. The first is an iterable collection like list—these elements are summed. The second argument is added to the sum.
An example. Here we introduce a list with 3 integers in it. We sum those integers with the sum() built-in method. And then we use sum with 2 arguments.
Argument 1 An iterable like a list we want to sum. The list must have summable elements like integers, not sub lists.
Argument 2 A value that is added to the sum of the iterable's elements. We can use this argument to sum previous sums.
values = [1, 1, 2]
# The sum of the 3 values is this number.
result = sum(values)
print(result)
# Use sum with 2 arguments.# ... The second argument is added to the element sum.
result = sum(values, 1000)
print(result)4
1004
Error in summing. We cannot sum nested iterables. Instead sum() only counts up top-level elements in our list. A nested list leads to a TypeError.
# This list has a nested list element.# ... It cannot be summed with sum.
values = [10, [20, 30]]
# This will cause a TypeError.
result = sum(values)
print(result)Traceback (most recent call last):
File "C:\programs\file.py", line 9, in <module>
result = sum(values)
TypeError: unsupported operand type(s) for +: 'int' and 'list'
Fsum example. The sum() built-in will work with float numbers like 1.5. But it may return a less-accurate result than the math.fsum specialty method.
A review. Why use sum? Why not just loop over elements and add them up? In some programs, sum() leads to simpler, more maintainable code, and this causes fewer bugs.
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