simplify the function to create particles

goal is to leave lots of room for student improvements
in class.

also, introduce missingness in the data

README.md

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+# Particle Cloud
+
+A sample library created for students of [Diller Digital's](https://dillerdigital.com) _Software Engineering in the Age of AI_ class.
+
+**Class Date**
+
+15 June 2026
+
+**Students in Attendance**
+-

particlecloud/util.py

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+import random
+import sys
+
+import numpy as np
+
+
+FILE_COLUMN_NAMES = [
+    "n", "m", "v1", "v2", "v3", "p1", "p2", "p3",
+]
+
+
+def write_data_file(filename, n=20, missing_values=False):
+    """Write a csv file with data
+    """
+
+    with open(filename, "w") as fp:
+        fp.write(", ".join(FILE_COLUMN_NAMES) + ",\n")
+        for i in range(n):
+            vals = []
+            vals.append(random.gauss(3., 0.5))
+            vals.extend(
+                (
+                    random.gauss(0., 25),
+                    random.gauss(0., 25),
+                    random.gauss(0., 25),
+                )
+            )
+            vals.extend(
+                (
+                    random.gauss(0., 1.5),
+                    random.gauss(0., 1.5),
+                    random.gauss(0., 1.5),
+                )
+            )
+            if missing_values:
+                if random.randint(0,10) < 1:
+                    idx = random.randint(1, len(vals))
+                    vals[idx] = np.nan
+            line = f"{i}, " + ", ".join(f"{v:7.5f}" for v in vals) + ",\n"
+            fp.write(line)
+
+
+if __name__ == '__main__':
+    missing_values = sys.argv[-1] == '-m'
+
+    write_data_file("sample_data.csv", n=20, missing_values=True)