## 2.5.1. Insertion Sort¶

What would you do if you have a stack of phone bills from the pasttwo years and you want to order by date?A fairly natural way to handle this is to look at the first twobills and put them in order.Then take the third bill and put it into the right position withrespect to the first two, and so on.As you take each bill, you would add it to the sorted pile that youhave already made.This simple approach is the inspiration forour first sorting algorithm, called Insertion Sort.

Insertion Sort iterates through a list of records.For each iteration, the current record is inserted in turn at thecorrect position within a sorted list composed of those recordsalready processed.Here is an implementation.The input is an array named `A`

that stores \(n\) records.

- Java
- Python

public static <T extends Comparable<T>> void insertionSort(T[] A) { for (int i = 1; i < A.length; i++) { // Insert i'th record. int j = i; while (j > 0 && A[j].compareTo(A[j-1]) < 0) { Util.swap(A, j, j-1); j--; } }}

def insertionSort(A): for i in range(len(A)): # Insert i'th record j = i while j > 0 and A[j] < A[j-1]: swap(A, j, j-1) j -= 1

(Note that to make the explanation for these sorting algorithms assimple as possible, our visualizations will show the array as thoughit stored simple integers rather than more complex records.But you should realize that in practice, there is rarely any pointto sorting an array of simple integers.Nearly always we want to sort more complex records that each have akey value.In such cases we must have a way toassociate a key value with a record.The sorting algorithms will simply assume that the records arecomparable.)

Here we see the first few iterations of Insertion Sort.

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This continues on with each record in turn.Call the current record \(x\).Insertion Sort will move it to the left solong as its value is less than that of the record immediatelypreceding it.As soon as a key value less than or equal to \(x\) isencountered, `insertionSort`

is done with that record because allrecords to its left in the array must have smaller keys.

## 2.5.2. Insertion Sort Analysis¶

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While the best case is significantly faster than the average and worstcases, the average and worst cases are usually more reliableindicators of the “typical” running time.However, there are situations where we can expect the input to be insorted or nearly sorted order.One example is when an already sorted list is slightly disordered by asmall number of additions to the list;restoring sorted order using Insertion Sort might be a good idea if weknow that the disordering is slight.And even when the input is not perfectly sorted, Insertion Sort’s costgoes up in proportion to the number of inversions.So a “nearly sorted” list will always be cheap to sort with InsertionSort.Examples of algorithms that take advantage of Insertion Sort’snear-best-case running time areShellsortand Quicksort.

Counting comparisons or swaps yields similar results.Each time through the inner `for`

loop yields both acomparison and a swap, except the last (i.e., the comparison thatfails the inner `for`

loop’s test), which has no swap.Thus, the number of swaps for the entire sort operation is\(n-1\) less than the number of comparisons.This is 0 in the best case, and \(\Theta(n^2)\) in theaverage and worst cases.

Later we will see algorithms whose growth rate is muchbetter than \(\Theta(n^2)\).Thus for larger arrays, Insertion Sort will not be so good aperformer as other algorithms.So Insertion Sort is not the best sorting algorithm to use in mostsituations.But there are special situations where it is ideal.We already know that Insertion Sort works great when the input issorted or nearly so.Another good time to use Insertion Sort is when the array is verysmall, since Insertion Sort is so simple.The algorithms that have better asymptotic growth rates tend to bemore complicated, which leads to larger constant factors in theirrunning time.That means they typically need fewer comparisons for larger arrays,but they cost more per comparison.This observation might not seem that helpful, since even an algorithmwith high cost per comparison will be fast on small input sizes.But there are times when we might need to do many, many sorts on verysmall arrays.You should spend some time right now trying to think of a situationwhere you will need to sort many small arrays.Actually, it happens a lot.

See Computational Fairy Tales: Why Tailors Use Insertion Sort for a discussion on how the relative costs ofsearch and insert can affect what is the best sort algorithm to use.