docs(notes): add 2025-04-14-csv-compare-en.md

Author: lzwjava

notes/2025-04-14-csv-compare-en.md

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+---
+title: Comparing Two Single-row CSV
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+Below is a Python script that compares two single-row CSV files containing strings (e.g., "P0001") and identifies:
+- Common items (present in both files)
+- Items unique to the first file
+- Items unique to the second file
+
+The script uses `argparse` to accept two CSV file paths, reads the first row from each, performs a set-based comparison, and writes the results to `comparison.txt`.
+
+```python
+#!/usr/bin/env python3
+import argparse
+import csv
+import os
+
+def main():
+    # Set up argument parser
+    parser = argparse.ArgumentParser(description='Compare two single-row CSV files for common and unique items')
+    parser.add_argument('file1_path', help='Path to the first CSV file')
+    parser.add_argument('file2_path', help='Path to the second CSV file')
+    
+    # Parse arguments
+    args = parser.parse_args()
+    
+    try:
+        # Read the first row of the first CSV file
+        with open(args.file1_path, 'r') as file1:
+            reader1 = csv.reader(file1)
+            for row in reader1:
+                values1 = row
+                break
+        
+        # Read the first row of the second CSV file
+        with open(args.file2_path, 'r') as file2:
+            reader2 = csv.reader(file2)
+            for row in reader2:
+                values2 = row
+                break
+        
+        # Convert to sets for comparison
+        set1 = set(values1)
+        set2 = set(values2)
+        
+        # Find common and unique items
+        common_items = set1.intersection(set2)
+        unique_to_file1 = set1.difference(set2)
+        unique_to_file2 = set2.difference(set1)
+        
+        # Determine output directory (use directory of first file)
+        output_dir = os.path.dirname(args.file1_path) or '.'
+        output_file = os.path.join(output_dir, 'comparison.txt')
+        
+        # Write results to output file
+        with open(output_file, 'w') as file:
+            file.write("Comparison Results:\n\n")
+            
+            file.write("Common Items (in both files):\n")
+            if common_items:
+                for item in sorted(common_items):
+                    file.write(f"{item}\n")
+            else:
+                file.write("None\n")
+                
+            file.write("\nItems only in first file:\n")
+            if unique_to_file1:
+                for item in sorted(unique_to_file1):
+                    file.write(f"{item}\n")
+            else:
+                file.write("None\n")
+                
+            file.write("\nItems only in second file:\n")
+            if unique_to_file2:
+                for item in sorted(unique_to_file2):
+                    file.write(f"{item}\n")
+            else:
+                file.write("None\n")
+        
+        print(f"Comparison complete. Results saved to {output_file}")
+        
+    except FileNotFoundError as e:
+        print(f"Error: File not found - {e}")
+    except Exception as e:
+        print(f"Error: {e}")
+
+if __name__ == "__main__":
+    main()
+```
+
+### How it works:
+1. **Input**: Takes two CSV file paths via command-line arguments.
+2. **Reading**: Reads only the first row from each CSV file.
+3. **Comparison**:
+   - Converts the rows to sets for efficient comparison.
+   - Finds:
+     - **Common items**: Items present in both rows (set intersection).
+     - **Unique to first file**: Items only in the first row (set difference: file1 - file2).
+     - **Unique to second file**: Items only in the second row (set difference: file2 - file1).
+4. **Output**: Writes results to `comparison.txt` in the directory of the first file, listing:
+   - Common items
+   - Items only in the first file
+   - Items only in the second file
+   - Each section is sorted alphabetically; "None" is written if a section is empty.
+5. **Error Handling**: Catches file-not-found errors and other exceptions.
+
+### Example usage:
+Save the script as `compare_csv.py`. Create two CSV files, e.g.:
+- `file1.csv`: `P0001,P0002,P0003`
+- `file2.csv`: `P0001,P0004,P0003`
+
+Run the script:
+```
+python compare_csv.py file1.csv file2.csv
+```
+
+**Output** (`comparison.txt`):
+```
+Comparison Results:
+
+Common Items (in both files):
+P0001
+P0003
+
+Items only in first file:
+P0002
+
+Items only in second file:
+P0004
+```
+
+### Notes:
+- The script ignores the position of items and focuses only on presence/absence.
+- Duplicate items in a row are automatically handled (sets remove duplicates).
+- The output is written to `comparison.txt` in the directory of the first input file.
+- If a file is empty or has no row, the script will treat it as an empty set.
+
+Would you like me to modify the script (e.g., change the output format or add more details)?