feat(04): add an exercise, "Swap Index" (#6)

Co-authored-by: HyoJeong Yun <[email protected]>

Author: sohnryang

04-functions-modules/guide.md

@@ -1,12 +1,20 @@
 # functions and modules
 
 ## MinMax
+
 - difficulty: ★☆☆
 - time: ~ 15m for beginners
 - background: list, iteration
 
 Implement `min`, `max` functions on your own.
 
+## Swap Index
+
+- difficulty: ★☆☆
+- time: ~ 15m for beginners
+- background: list
+
+Implement a function which swaps indices of list.
 
 ## Hanoi
 - difficulty: ★☆☆
@@ -22,4 +30,3 @@ Implement `hanoi` function on your own. `hanoi` function is a guide that tells h
 - background: function, recursion
 
 Implement a function which calculates binomial coefficient, $\prescript{}{n}C_k$.
-

04-functions-modules/swap_index-answer.py

@@ -0,0 +1,19 @@
+def swap_indices(ls, i, j):
+    """
+    Swap ls[i] and ls[j].
+
+    >>> ls = [0, 1, 2, 3]
+    >>> swap_indices(ls, 0, 2)
+    >>> ls
+    [2, 1, 0, 3]
+    >>> swap_indices(ls, 1, 2)
+    >>> ls
+    [2, 0, 1, 3]
+    """
+    ls[i], ls[j] = ls[j], ls[i]
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod(verbose=True)

04-functions-modules/swap_index.py

@@ -0,0 +1,19 @@
+def swap_indices(ls, i, j):
+    """
+    Swap ls[i] and ls[j].
+
+    >>> ls = [0, 1, 2, 3]
+    >>> swap_indices(ls, 0, 2)
+    >>> ls
+    [2, 1, 0, 3]
+    >>> swap_indices(ls, 1, 2)
+    >>> ls
+    [2, 0, 1, 3]
+    """
+    pass
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod(verbose=True)