# W2202 Insertion Sort

## Algorithm[edit]

Unlike Bubble Sort which only compares two elements at a time, insertion sort works by looking at each element and finding where it belongs in the sorted array. At its base, insertion sort is more complicated than bubble sort, but not by much:

- Iterate over the array
- For each element at index
**i**, loop back through the array starting at**i**to**0**- Similar to bubble sort, if the element on the left is larger than the element on the right, swap them
- Because this starts by comparing
**i**and**i - 1**, if the current element is not in place it will get swapped to**i - 1**. If it is still not in place in the next iteration, it will be swapped to**i - 2**, and so on until it is in the correct place

- Because this starts by comparing

- Similar to bubble sort, if the element on the left is larger than the element on the right, swap them
- After each iteration, all of the elements up to
**i + 1**will be in the correct order although not necessarily*sorted*in the context of the full array - Unlike bubble sort, you only need to iterate through the array once. Once you reach the end, the array is sorted

### Swap[edit]

You can use the same swap function as in bubble sort because you only need to be able to swap two adjacent elements at a time.

### Performance Considerations[edit]

Similar to bubble sort, consider passing the array **by reference** into the swap function (if you have one) instead of **by value** to avoid copying the entire array for each swap.

Additionally, in insertion sort, you can stop/break the inner loop once you reach two sorted elements. In other words, you only need to swap until the element on the left is smaller than the element on the right, at which point you can exit the loop.

### Example[edit]

Here is an example with an array of 5 elements: Note: The Array contents are shown, directly after each pass of the insertion sort

Array: [95, 47, 62, 33, 89] | Initial | // The algorithm first looks at the element at index 1: 47. Since 47 < 95, the two elements are swapped. Array: [47, 95, 62, 33, 89] | Swaps: 1, Pass: 1 <- Result immediately after the first pass | // The algorithm then looks at the element at index 2: 62. Since 62 < 95, the two elements are swapped // The algorithm then checks if 62 < 47, this is not true so the pass is completed. Array: [47, 62, 95, 33, 89] | Swaps: 1, Pass: 2 | // The algorithm then looks at the element at index 3: 33. Since 33 < 95, the two elements are swapped. // This is repeated until 33 is at the beginning of the list Array: [33, 47, 62, 95, 89] | Swaps: 3, Pass: 3 | // The algorithm now looks at the element at index 4: 89. Since 89 < 95, the two elements are swapped. // 89 is only swapped once because, after the first swap, 89 !< 62. Array: [33, 47, 62, 89, 95] | Swaps: 1, Pass: 4 <- Result after 4 passes

## Quiz[edit]

## References[edit]

Insertion Sort (Wikipedia)