74 — Merge: joining two lists

Real data often lives in more than one file. A school keeps students in one list and their scores in another. Joining them means looking a name up in the second list — exactly what a dict is for.

#Steps

1. Create two data files. students.json has one dict per student:

   [
     {"name": "Mina", "class": "A"},
     {"name": "Jun", "class": "A"},
     {"name": "Sora", "class": "B"},
     {"name": "Tom", "class": "B"}
   ]
   

   copyrun it →

2. scores.json has the same names with a score:

   [
     {"name": "Mina", "score": 92},
     {"name": "Jun", "score": 88},
     {"name": "Tom", "score": 75}
   ]
   

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3. Build a lookup dict from the scores: each name maps to its score, so finding a student's score is one fast [] lookup instead of a scan:

   scores_by_name = {}
   for record in scores:
       scores_by_name[record["name"]] = record["score"]
   

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4. The full program loads both lists, joins them, and prints the combined report. Save merge.nme:

   # merge.nme — join students with scores by name.
   # Run: nme r merge
   # students.json and scores.json must be in the same folder.
   
   use file latest
   
   students = json_load("students.json")
   scores = json_load("scores.json")
   
   scores_by_name = {}
   for record in scores:
       scores_by_name[record["name"]] = record["score"]
   
   show f"class report ({len(students)} students):"
   for student in students:
       name = student["name"]
       score = scores_by_name.get(name, 0)
       show f"  {name} in class {student['class']}: {score} points"
   

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   scores_by_name.get(name, 0) returns the score, or 0 when a student has no score yet — so Sora still appears in the report.

5. Run it:

   nme r merge
   

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   class report (4 students):
     Mina in class A: 92 points
     Jun in class A: 88 points
     Sora in class B: 0 points
     Tom in class B: 75 points
   

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#Try it yourself

Add a second score for Mina in scores.json and change the lookup to keep the highest score, or add a grade field derived from the score in the loop.

#What you learned

• A dict turns a name into a value for fast lookup.
• Joining two lists means building a lookup dict from one and looping the other.
• dict.get(key, default) handles missing keys without crashing.
• Merging is how programs combine data that lives in separate files.