---
id: wiki-2026-0508-one-hot-encoding
title: One-Hot Encoding
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [One-Hot, OHE, Indicator-Encoding, Dummy-Encoding]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [feature-engineering, categorical, preprocessing, sklearn, pandas]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: python
  framework: sklearn-pandas
---

# One-Hot Encoding

## 매 한 줄
> **"매 categorical value → orthogonal binary vector"**. One-hot encoding 은 K 개 category 를 K 개 0/1 column 으로 펼치는 매 가장 단순한 categorical → numeric 변환. 매 linear model / tree-based model 의 default, 그러나 high-cardinality 에서는 target / hash encoding 으로 교체.

## 매 핵심

### 매 정의
- category set `{A, B, C}` → vectors `(1,0,0), (0,1,0), (0,0,1)`.
- ordinal encoding (0,1,2) 와 달리 **순서 가정 없음**.
- linear / kernel model 의 가정 (numeric distance) 을 깨지 않음.

### 매 dummy variable trap
- K columns → 1 redundant (sum=1 의 collinearity).
- linear regression 의 unregularized 경우 → drop_first=True.
- tree / regularized model (Lasso, Ridge) → 매 전체 K 유지 가능.

### 매 cardinality 의 문제
- high-cardinality (>50): sparse matrix 폭발, leak 위험.
- 대안: target / mean encoding, hashing trick, embedding.

### 매 응용
1. tabular ML 의 categorical preprocessing.
2. NLP token → vocab vector (sparse).
3. RL action / state space 의 discrete encoding.

## 💻 패턴

### sklearn OneHotEncoder
```python
from sklearn.preprocessing import OneHotEncoder
import numpy as np

X = np.array([["red"], ["blue"], ["green"], ["red"]])
enc = OneHotEncoder(sparse_output=False, handle_unknown="ignore")
enc.fit(X)
print(enc.transform([["red"], ["yellow"]]))
# [[0. 0. 1.]
#  [0. 0. 0.]]   <- unknown -> all zeros
print(enc.get_feature_names_out())   # ['x0_blue' 'x0_green' 'x0_red']
```

### pandas get_dummies
```python
import pandas as pd
df = pd.DataFrame({"color": ["red", "blue", "green", "red"]})
ohe = pd.get_dummies(df, columns=["color"], drop_first=True, dtype=int)
#    color_green  color_red
# 0            0          1
# 1            0          0
# 2            1          0
# 3            0          1
```

### ColumnTransformer (production pipeline)
```python
from sklearn.compose import ColumnTransformer
from sklearn.preprocessing import OneHotEncoder, StandardScaler
from sklearn.pipeline import Pipeline
from sklearn.linear_model import LogisticRegression

pre = ColumnTransformer([
    ("num", StandardScaler(), ["age", "income"]),
    ("cat", OneHotEncoder(handle_unknown="ignore"), ["city", "plan"]),
])
pipe = Pipeline([("pre", pre), ("clf", LogisticRegression(max_iter=1000))])
pipe.fit(X_train, y_train)
```

### Sparse matrix 의 high-cardinality
```python
enc = OneHotEncoder(sparse_output=True, handle_unknown="ignore")
X_sparse = enc.fit_transform(df[["zip_code"]])   # 40k columns sparse
# scipy.sparse.csr_matrix — memory-efficient
```

### vs label encoding (decision)
```python
from sklearn.preprocessing import LabelEncoder, OrdinalEncoder
# DON'T: feed LabelEncoder output to linear model
le = LabelEncoder()
y = le.fit_transform(["red", "blue", "green"])   # [2, 0, 1] — fake order!

# DO: OrdinalEncoder when order is real
oe = OrdinalEncoder(categories=[["low", "med", "high"]])
```

### Frequency / target encoding (high-cardinality 대안)
```python
import category_encoders as ce
te = ce.TargetEncoder(cols=["city"], smoothing=10)
X_tr = te.fit_transform(X_train, y_train)
X_te = te.transform(X_test)
```

### Hashing trick (constant memory)
```python
from sklearn.feature_extraction import FeatureHasher
h = FeatureHasher(n_features=256, input_type="string")
X_h = h.transform([["zip=" + z] for z in df["zip_code"]])
```

## 매 결정 기준
| cardinality | model | encoding |
|---|---|---|
| <10 | any | one-hot |
| 10–50 | linear / NN | one-hot or embedding |
| 50–1000 | tree | target / frequency |
| >1000 | any | hashing / embedding |
| 매 ordinal | any | OrdinalEncoder |

**기본값**: `OneHotEncoder(handle_unknown="ignore")` in ColumnTransformer.

## 🔗 Graph
- 부모: [[Feature Engineering|Feature-Engineering]]
- 변형: [[Target-Encoding]]
- 응용: [[Logistic-Regression-Foundations|Logistic-Regression]]
- Adjacent: [[Sparse-Matrix]] · [[Curse-of-Dimensionality]]

## 🤖 LLM 활용
**언제**: 매 quick prototype, low-cardinality categorical, linear / tree baseline.
**언제 X**: 매 high-cardinality (>1000), text tokens (use embedding), online learning with new categories.

## ❌ 안티패턴
- **Train-only fit**: test set 의 unseen category 에 crash → `handle_unknown="ignore"`.
- **Drop-first with regularized model**: 불필요한 정보 손실.
- **OHE on high-cardinality without sparse**: memory blowup.
- **LabelEncoder for features**: fake ordinal 강제, linear model 망가짐.
- **Leak via target encoding without fold**: target encoding 사용 시 K-fold 필수.

## 🧪 검증 / 중복
- Verified (sklearn 1.4 docs, pandas 2.2 docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — sklearn/pandas patterns + cardinality decision matrix |