Weka Data Processing

Objectives

Process Weka ARFF files for use in Python machine learning projects, particularly converting to CSV format with proper column headers.

Instructions

1. Understanding ARFF Format

ARFF (Attribute-Relation File Format) is Weka's native data format:

@relation diabetes

@attribute preg numeric
@attribute plas numeric
@attribute pres numeric
@attribute skin numeric
@attribute insu numeric
@attribute mass numeric
@attribute pedi numeric
@attribute age numeric
@attribute class {tested_negative,tested_positive}

@data
6,148,72,35,0,33.6,0.627,50,tested_positive
1,85,66,29,0,26.6,0.351,31,tested_negative
...

2. Converting ARFF to CSV

Method 1: Using Python (Recommended)

import pandas as pd
from scipy.io import arff

# Load ARFF file
data, meta = arff.loadarff('diabetes.arff')
df = pd.DataFrame(data)

# Convert byte strings to regular strings (for categorical columns)
for col in df.columns:
    if df[col].dtype == 'object':
        df[col] = df[col].str.decode('utf-8')

# Save to CSV
df.to_csv('diabetes.csv', index=False)

Method 2: Manual Extraction

Read ARFF file and extract:

1. •Column names from @attribute lines
2. •Data from @data section
3. •Write to CSV with headers

3. Common Weka Datasets

Diabetes Dataset:

• •Location: weka/data/diabetes.arff
• •Features: 8 numeric attributes
• •Target: binary class (tested_negative, tested_positive)
• •Instances: 768

Column Names:

• •preg (pregnancies)
• •plas (plasma glucose)
• •pres (blood pressure)
• •skin (skin thickness)
• •insu (insulin)
• •mass (BMI)
• •pedi (diabetes pedigree function)
• •age
• •class (target)

4. Handling Class Labels

Weka uses string labels, Python ML libraries prefer numeric:

# Convert class labels to numeric
df['class'] = df['class'].map({
    'tested_negative': 0,
    'tested_positive': 1
})

5. Required Packages

uv add scipy pandas

Validation

After conversion, verify:

• • CSV file has correct number of columns
• • Column names match ARFF attributes
• • Number of rows matches ARFF data section
• • Class labels properly converted
• • No missing values introduced

Common Issues

Issue: Byte strings in DataFrame

• •Solution: Use .str.decode('utf-8') for object columns

Issue: Missing column names

• •Solution: Extract from @attribute lines in ARFF

Issue: Class labels as bytes

• •Solution: Decode and optionally map to numeric

Example Workflow

1. •Locate Weka data file (usually in weka/data/)
2. •Load ARFF using scipy.io.arff
3. •Convert to pandas DataFrame
4. •Decode string columns
5. •Save to CSV with headers
6. •Verify column names and data integrity

Anti-Patterns

• •❌ Using CSV without column headers
• •❌ Not decoding byte strings
• •❌ Losing class label information
• •❌ Not verifying data integrity after conversion