# 21.2: Analysis of basic Python operations

In Python, most arithmetic operations are constant time; multiplication usually takes longer than addition and subtraction, and division takes even longer, but these run times don’t depend on the magnitude of the operands. Very large integers are an exception; in that case the run time increases with the number of digits.

Indexing operations—reading or writing elements in a sequence or dictionary—are also constant time, regardless of the size of the data structure.

A for loop that traverses a sequence or dictionary is usually linear, as long as all of the operations in the body of the loop are constant time. For example, adding up the elements of a list is linear:

    total = 0
for x in t:
total += x


The built-in function sum is also linear because it does the same thing, but it tends to be faster because it is a more efficient implementation; in the language of algorithmic analysis, it has a smaller leading coefficient.

As a rule of thumb, if the body of a loop is in $$O(n^a)$$ then the whole loop is in $$O(n^{a+1})$$. The exception is if you can show that the loop exits after a constant number of iterations. If a loop runs $$k$$ times regardless of $$n$$, then the loop is in $$O(n^a)$$, even for large $$k$$.

Multiplying by $$k$$ doesn’t change the order of growth, but neither does dividing. So if the body of a loop is in $$O(n^a)$$ and it runs $$n/k$$ times, the loop is in $$O(n^{a+1})$$, even for large $$k$$.

Most string and tuple operations are linear, except indexing and len, which are constant time. The built-in functions min and max are linear. The run-time of a slice operation is proportional to the length of the output, but independent of the size of the input.

String concatenation is linear; the run time depends on the sum of the lengths of the operands.

All string methods are linear, but if the lengths of the strings are bounded by a constant—for example, operations on single characters—they are considered constant time. The string method join is linear; the run time depends on the total length of the strings.

Most list methods are linear, but there are some exceptions:

• Adding an element to the end of a list is constant time on average; when it runs out of room it occasionally gets copied to a bigger location, but the total time for $$n$$ operations is $$O(n)$$, so the average time for each operation is $$O(1)$$.
• Removing an element from the end of a list is constant time.
• Sorting is $$O(n\log{n})$$.

Most dictionary operations and methods are constant time, but there are some exceptions:

• The run time of update is proportional to the size of the dictionary passed as a parameter, not the dictionary being updated.
• keys, values and items are constant time because they return iterators. But if you loop through the iterators, the loop will be linear.

The performance of dictionaries is one of the minor miracles of computer science. We will see how they work in Section B.4.

Exercise $$\PageIndex{1}$$

7. Many of the non-comparison sorts are linear, so why does does Python use an $$O(n\log{n})$$ comparison sort?