Add problem 42 solution (#1259)

project_euler/problem_42/solution.py

@@ -0,0 +1,50 @@
+"""
+The nth term of the sequence of triangle numbers is given by, tn = ½n(n+1); so
+the first ten triangle numbers are:
+
+1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...
+
+By converting each letter in a word to a number corresponding to its
+alphabetical position and adding these values we form a word value. For example,
+the word value for SKY is 19 + 11 + 25 = 55 = t10. If the word value is a
+triangle number then we shall call the word a triangle word.
+
+Using words.txt (right click and 'Save Link/Target As...'), a 16K text file
+containing nearly two-thousand common English words, how many are triangle
+words?
+"""
+import os
+
+
+# Precomputes a list of the 100 first triangular numbers
+TRIANGULAR_NUMBERS = [int(0.5 * n * (n + 1)) for n in range(1, 101)]
+
+
+def solution():
+    """
+    Finds the amount of triangular words in the words file.
+
+    >>> solution()
+    162
+    """
+    script_dir = os.path.dirname(os.path.realpath(__file__))
+    wordsFilePath = os.path.join(script_dir, 'words.txt')
+
+    words = ''
+    with open(wordsFilePath, 'r') as f:
+        words = f.readline()
+
+    words = list(map(lambda word: word.strip('"'), words.strip('\r\n').split(',')))
+    words = list(
+        filter(
+            lambda word: word in TRIANGULAR_NUMBERS,
+            map(
+                lambda word: sum(map(lambda x: ord(x) - 64, word)),
+                words
+            )
+        )
+    )
+    return len(words)
+
+if __name__ == '__main__':
+    print(solution())