A Collection of Code Snippets in as Many Programming Languages as Possible

This project is maintained by TheRenegadeCoder

Welcome to the Selection Sort in Python page! Here, you’ll find the source code for this program as well as a description of how the program works.

```
import sys
def selection_sort(xs, sorted_xs=None):
sorted_xs = sorted_xs or []
if len(xs) <= 0:
return sorted_xs
x = min(xs)
sorted_xs.append(x)
xs.remove(x)
return selection_sort(xs, sorted_xs)
def input_list(list_str):
return [int(x.strip(" "), 10) for x in list_str.split(',')]
def exit_with_error():
print('Usage: please provide a list of at least two integers to sort in the format "1, 2, 3, 4, 5"')
sys.exit(1)
def main(args):
try:
xs = input_list(args[0])
if len(xs) <= 1:
exit_with_error()
print(selection_sort(xs))
except (IndexError, ValueError):
exit_with_error()
if __name__ == "__main__":
main(sys.argv[1:])
```

Selection Sort in Python was written by:

- Jeremy Grifski
- Parker Johansen

If you see anything you’d like to change or update, please consider contributing.

**Note**: The solution shown above is the current solution in the Sample Programs repository as of Oct 15 2020 22:17:17. The solution was first committed on Dec 23 2018 15:10:19. As a result, documentation below may be outdated.

At this point, let’s dig into the code a bit. The following sections break down the Selection Sort in Python functionality.

```
#!/usr/bin/env python
import sys
def selection_sort(xs, sorted_xs=None):
sorted_xs = sorted_xs or []
if len(xs) <= 0:
return sorted_xs
x = min(xs)
sorted_xs.append(x)
xs.remove(x)
return selection_sort(xs, sorted_xs)
def input_list(list_str):
return [int(x.strip(" "), 10) for x in list_str.split(',')]
def exit_with_error():
print('Usage: please provide a list of at least two integers to sort in the format "1, 2, 3, 4, 5"')
sys.exit(1)
def main(args):
try:
xs = input_list(args[0])
if len(xs) <= 1:
exit_with_error()
print(selection_sort(xs))
except (IndexError,ValueError):
exit_with_error()
if __name__ == "__main__":
main(sys.argv[1:])
```

Breaking down this solution bottom up,

```
if __name__ == "__main__":
main(sys.argv[1:])
```

This bit of code checks to see if this is the main module run. If it is it then calls the main
function and passes user input to it. In this case the user input would be a string of numbers
like so `"2, 1, 10, 5, 3"`

(to sort).

```
def main(args):
try:
xs = input_list(args[0])
if len(xs) <= 1:
exit_with_error()
print(selection_sort(xs))
except (IndexError,ValueError):
exit_with_error()
```

This is the main function of this file. It parses the input, then calls our selection sort function (and prints the results). It also deals with any errors raised.

```
def input_list(list_str):
return [int(x.strip(" "), 10) for x in list_str.split(',')]
```

This function takes a string like `"2, 1, 10, 5, 3"`

, and turns into a list of numbers.
It does this using a list comprehension, first we need to convert our string into a
list `list_str.split(',')`

which is a list of strings split by comma (,).
So our original input string becomes `["2", " 1", " 10", " 5", " 3"]`

. Then for each
element in the list `for x in ...`

, we do something to it.

In this example we convert it into a decimal integer, `int(x.strip(" "), 10)`

. Then `x.strip(" ")`

,
removes any whitespace so `" 1"`

becomes `"1"`

. Then `int("1", 10)`

converts the string `"1"`

into a decimal number in this case `1`

. This is done
for every item in the list so our original input of `"2, 1, 10, 5, 3"`

becomes `[2, 1, 10, 5, 3]`

.

```
def exit_with_error():
print('Usage: please provide a list of at least two integers to sort in the format "1, 2, 3, 4, 5"')
sys.exit(1)
```

This function prints a message and then exits the script with an error, `sys.exit(1)`

.
If any non-zero value is returned then the program didn’t complete properly. This function is called
if the user input isn’t correct.

```
def selection_sort(xs, sorted_xs=None):
sorted_xs = sorted_xs or []
if len(xs) <= 0:
return sorted_xs
x = min(xs)
sorted_xs.append(x)
xs.remove(x)
return selection_sort(xs, sorted_xs)
```

Now onto the main part of the program, this is the function that actually sorts our list.
The `selection_sort()`

takes two parameters `xs`

which is the unsorted list and `sorted_xs`

which funnily enough is the current sorted list. When you first call the `selection_sort()`

function you then pass it to your unsorted list as `sorted_xs=None`

by default.

If the `sorted_xs`

value is set (not `None`

) then we make `sorted_xs`

equal itself, else
`sorted_xs`

equals `[]`

(an empty list). You should never make a mutable object a default
argument in Python as you get can get unexpected result. You can get more
information here. Therefore we set
`sorted_xs=None`

instead of `sorted_xs=[]`

.

Then we check if xs is empty (`<=0`

), which would mean we have sorted every element,
then we return the `sorted_xs`

which is the sorted this. We can do this because
as we sort element we move them from `xs`

to `sorted_xs`

( items get removed from the `xs`

list).

If `xs`

still has items then that means we haven’t completely sorted the list.
We found the smallest value in `x = min(xs)`

. We append that value to `sorted_xs`

and then we
remove it from the `xs`

list. Finally, we call the selection sort function with the new `xs`

and
`sorted_xs`

values. This repeats until `xs`

is empty and you are left with a completely sorted
`sorted_xs`

.

Taking a look at a simple example where we want to sort `[5, 1, 3]`

.

1st

- Call
`selection_sort([5, 1, 3])`

`xs = [5, 1, 3]`

,`sorted_xs=[]`

- Minimum value is
`1`

`xs = [5, 3]`

,`sorted_xs = [1]`

`selection_sort([5, 3], [1])`

2nd

`xs = [5, 3]`

,`sorted_xs = [1]`

- Minimum value is
`3`

`xs = [5]`

,`sorted_xs = [1, 3]`

`selection_sort([5], [1, 3])`

3rd

`xs = [5]`

,`sorted_xs = [1, 3]`

- Minimum value is
`5`

`xs = []`

,`sorted_xs = [1, 3, 5]`

`selection_sort([], [1, 3, 5])`

4th

`xs = []`

,`sorted_xs = [1, 3, 5]`

`len(xs) <= 0`

, as we have 0 elements- So we return
`sorted_xs = [1, 3, 5]`

If we want to run this program, we should probably download a copy of Selection Sort in Python. After that, we should make sure we have the latest Python interpreter. From there, we can run the following command in the terminal:

`python selection-sort.py "3, 2, 10, 6, 1, 7"`

Alternatively, we can copy the solution into an online Python interpreter and hit run.