Practice

Implement the primitives that ML engineering interviews and real systems are built on. Every problem runs in your browser. No setup, no submission queue — just write Python and press Run.

165 problems254 topics10 companies

Difficulty:Topic:Company:

Title	Difficulty	Topics	Companies	Tests
Accuracy score	Easy	numpymetricsevaluationclassificationfundamentals		4
AdaBoost fit (decision stumps)	Medium	numpyensembleboostingadaboostclassical-mlclassification		4
Adadelta optimizer	Medium	numpyoptimizationoptimizeradaptiveadadelta		4
Adagrad optimizer	Easy	numpyoptimizationoptimizeradaptiveadagrad		4
Adam optimiser step	Medium	numpyoptimizationadam	Meta OpenAI Anthropic	6
Adamax optimizer	Medium	numpyoptimizationoptimizeradaptiveadamax		4
AdamW step (decoupled weight decay)	Medium	numpyoptimizationadamwweight-decay		5
Affine coupling layer (normalizing flow)	Medium	numpynormalizing-flowgenerativerealnvpinvertible		5
Autocorrelation function	Medium	numpytime-seriesautocorrelationstatistics		5
Autoencoder forward pass	Easy	numpyautoencoderneural-networkrepresentation-learning		4
Bag-of-words encoding	Easy	numpynlptext-featuresbag-of-words		4
Basic autograd operations	Medium	autogradbackpropagationneural-networkcomputational-graph		4
BatchNorm forward (train + eval modes)	Medium	numpynormalizationbatch-normregularization	Meta Google Nvidia Tesla	5
Beam search decoding	Medium	numpydecodingbeam-searchllmsequence		5
Bellman equation for value iteration	Medium	numpyreinforcement-learningvalue-iterationbellmanmdp		5
Bernoulli Naive Bayes classifier	Medium	numpynaive-bayesclassical-mlclassificationprobability		4
Best Gini-based split (decision tree)	Medium	numpydecision-treeclassical-mlginiclassification		4
Binary classification with logistic regression	Easy	numpyclassificationlogistic-regressioninferencedecision-boundary		4
BLEU score (unigram)	Medium	nlpmetricsmachine-translationevaluationbleu		4
BPE: apply merges	Medium	tokenizationBPEalgorithms		8
Train BPE merges from a corpus	Hard	pythontokenizationbpe		6
Conv2D forward (naive, stride 1, no pad)	Medium	numpycnnconvolutionbuild-cnn		5
Conv2D forward (padding + stride)	Medium	numpycnnconvolutionbuild-cnn		5
Max pooling 2D forward	Easy	numpycnnpoolingbuild-cnn		6
Average pooling 2D forward	Easy	numpycnnpoolingbuild-cnn		6
Conv2D backward (dx, dW)	Hard	numpycnnbackpropbuild-cnn		5
Mini-CNN forward (capstone)	Medium	numpycnnclassificationcapstonebuild-cnn		5
Token embedding lookup	Easy	numpytransformerembeddingsbuild-gpt		5
Sinusoidal positional encoding	Easy	numpytransformerpositional-encodingbuild-gpt		6
Scaled dot-product self-attention	Medium	numpytransformerattentionbuild-gpt	OpenAI Anthropic Meta Google Cohere	6
Causal mask: build + apply	Easy	numpytransformermaskingbuild-gpt		5
Multi-head split + combine	Medium	numpytransformerreshapebuild-gpt		6
Multi-Head Attention (full layer)	Medium	numpytransformermulti-head-attentionbuild-gpt		5
LayerNorm forward	Easy	numpytransformernormalizationbuild-gpt		5
Transformer block forward (pre-LN, residual)	Hard	numpytransformercapstonebuild-gpt		5
Linear forward (Wx + b)	Easy	numpyneural-netfundamentalsbuild-nn	Meta Google Amazon Apple	5
ReLU forward + backward	Easy	numpyneural-netactivationbackpropbuild-nn		5
Sigmoid forward + backward	Easy	numpyneural-netactivationbackpropbuild-nn		6
MSE loss + backward	Easy	numpyneural-netlossbackpropbuild-nn		6
Linear backward (chain rule)	Medium	numpyneural-netbackpropcalculusbuild-nn		6
SGD step (in place)	Easy	numpyneural-netoptimizationsgdbuild-nn		6
Train a 2-layer MLP on XOR	Hard	numpyneural-nettraining-loopcapstonebuild-nn		5
Byte-level UTF-8 tokenizer	Medium	pythontokenizationunicode		6
Cohen's kappa score	Medium	numpymetricsevaluationagreementstatistics		4
Confusion matrix	Easy	numpymetricsevaluationclassification		4
Contrastive loss	Medium	numpylossmetric-learningembeddingssiamese		4
Warmup + cosine decay LR schedule	Easy	numpytraininglr-scheduleoptimization		6
Covariance matrix	Easy	numpylinear-algebrastatisticscovariance		4
Cross-entropy gradient	Medium	calculusbackpropnumpyfundamentals		6
DBSCAN clustering	Medium	numpyclusteringclassical-mldensity-basedunsupervised		4
Decision tree learning (ID3)	Hard	decision-treeclassical-mlentropyinformation-gainclassification		5
Dense block with 2D convolutions	Medium	numpycnndensenetconvolutionneural-network		5
Dice loss	Easy	numpylosssegmentationcomputer-visiondice		4
Diffusion forward process	Medium	numpydiffusiongenerativeddpm		5
Divide dataset by feature threshold	Easy	numpydecision-treeclassical-mldata-splitting		4
DPO loss	Medium	numpyllmalignmentdpopreference-learningloss		5
Dropout layer (forward & backward)	Medium	numpyregularizationdropoutneural-networkbackpropagation		6
Dynamic-Tanh (DyT)	Medium	numpyactivationtransformernormalization-freedyt		4
Early stopping on validation loss	Easy	trainingregularizationearly-stoppingmodel-evaluation		4
Efficient sparse window attention	Medium	numpyattentiontransformersparselong-context		5
Elastic-Net regression (gradient descent)	Medium	numpyregressionregularizationlassoridgegradient-descent		4
Euclidean distance matrix	Easy	numpylinear-algebradistancesvectorization		4
Exponential moving average of weights	Easy	numpytrainingemaweight-averagingregularization		5
Exponential LR scheduler	Easy	optimizationlearning-rateschedulertraining		4
F1 score (binary classification)	Easy	numpymetricsevaluationfundamentals		6
Fisher-Yates shuffle	Easy	numpysamplingshufflepermutationalgorithm		5
FlashAttention tiled forward	Hard	numpyattentiontransformeronline-softmaxflash-attention		6
Focal loss (multiclass)	Medium	numpylossimbalanceclassification		6
Gaussian process regression (RBF)	Hard	numpygaussian-processregressionkernel-methodsbayesian		4
GCN layer (message passing)	Medium	numpygraph-neural-networkgcnmessage-passing		5
GELU backward (tanh approximation)	Medium	numpyactivationbackpropcalculusgelu		6
GELU forward (tanh approximation)	Easy	numpyactivationtransformergelu		6
Generate random subsets (bootstrap)	Easy	numpysamplingbootstrapbaggingresampling		5
GMM E-step (responsibilities)	Medium	numpyclusteringgmmemunsupervisedprobability		4
Gradient checkpointing forward	Easy	numpytrainingmemory-optimizationgradient-checkpointing		4
Clip gradients by global L2 norm	Easy	numpytrainingstabilityoptimization		6
Graph Laplacian	Medium	numpygraphlinear-algebraspectrallaplacian		4
Greedy decoding loop	Easy	numpydecodingautoregressive		5
Group normalization	Medium	numpynormalizationdeep-learningcnngroup-norm		4
Grouped-query attention	Medium	numpyattentiontransformergqainference		5
GRPO objective	Hard	numpyreinforcement-learninggrporlhfllm		5
He weight initialization	Easy	numpyinitializationneural-networktrainingrelu		4
Huber & Hinge losses	Easy	numpylossregressionsvmrobust		4
Instance normalization	Medium	numpynormalizationcomputer-visionstyle-transferinstance-norm		4
Inverse-transform sampling	Easy	numpysamplingprobabilitymonte-carlo		5
IoU of bounding boxes	Easy	computer-visionobject-detectionmetriciou		5
Jaccard similarity	Easy	numpymetricssimilaritysetsiou		4
k-fold cross-validation	Easy	numpymodel-evaluationcross-validationresampling		5
KL divergence (discrete)	Easy	numpylossinformation-theoryfundamentals		6
K-means: one iteration	Medium	numpyclassical-mlclustering		6
k-NN classification (majority vote)	Easy	numpyclassical-mlclassificationfundamentals		6
KV cache for autoregressive inference	Medium	numpyattentiontransformerinferencekv-cache		4
KV cache compression (MLA)	Hard	numpyattentiontransformerinferencemlakv-cache		4
Label-smoothed cross-entropy	Medium	numpylossregularizationclassification		5
Leaky ReLU activation	Easy	numpyactivationneural-netrelu		4
Learning-rate range test	Medium	numpytraininglearning-ratehyperparameter-tuning		4
Linear regression — gradient descent	Easy	numpyregressionlinear-regressiongradient-descentoptimization		4
Linear regression — normal equation	Medium	numpyregressionlinear-regressionclassical-mlleast-squares		3
Lion optimizer step	Medium	numpyoptimizationoptimizerlionsign-momentum		4
Log-softmax	Easy	numpysoftmaxnumerical-stabilitylossfundamentals		5
Logistic regression — gradient descent	Medium	numpyregressionlogistic-regressionclassificationgradient-descent		4
LoRA forward pass	Easy	numpylorafine-tuningpeftllm		5
Matmul backward	Medium	numpybackpropmatmulcalculus		5
Mean reciprocal rank (MRR)	Easy	rankingmetricsretrievalevaluation		4
METEOR score	Medium	nlpmetricsmachine-translationevaluationmeteor		4
Multi-class cross-entropy loss	Easy	numpylossclassificationcross-entropyfundamentals		4
Mutual information	Medium	numpyinformation-theorymutual-informationfeature-selectionstatistics		5
Nesterov accelerated gradient	Medium	numpyoptimizationoptimizermomentumnesterov		4
Neural ODE forward Euler	Medium	numpyneural-odenumerical-integrationeulergenerative		5
Noisy top-k gating	Medium	numpymoegatingroutingtransformer		5
Orthonormal basis (Gram-Schmidt)	Medium	numpylinear-algebragram-schmidtorthogonalization		4
PageRank (power iteration)	Medium	numpygraphpagerankpower-iterationlinear-algebra		4
Pairwise cosine-similarity matrix	Easy	numpyembeddingssimilarityretrievalcosine		4
Pearson correlation coefficient	Easy	numpystatisticscorrelationfundamentals		4
Pegasos kernel SVM	Medium	numpysvmkernel-methodsclassical-mlclassification		4
Perplexity from log-probs	Easy	numpynlplanguage-modelsmetricsevaluation		4
Pointwise mutual information (PMI)	Medium	numpynlpinformation-theorystatisticspmi		4
Policy gradient with REINFORCE	Medium	numpyreinforcement-learningpolicy-gradientreinforce		5
PPO clipped objective	Medium	numpyreinforcement-learningpporlhfpolicy-gradient		5
Precision@k and NDCG@k	Medium	numpyrankingmetricsretrievalndcg		4
Precision metric	Easy	numpymetricsevaluationclassificationfundamentals		4
Principal Component Analysis (PCA)	Medium	numpypcadimensionality-reductionlinear-algebraunsupervised		4
Prioritized experience replay	Medium	numpyreinforcement-learningexperience-replaydqnimportance-sampling		5
Q-learning for MDPs	Medium	numpyreinforcement-learningq-learningmdpvalue-based		4
Rejection sampling	Medium	numpysamplingmonte-carloprobability		5
Repetition penalty (HuggingFace-style)	Medium	numpydecodingsampling		6
Reservoir sampling	Medium	numpysamplingstreamingalgorithm		5
Residual block with shortcut	Easy	numpyneural-networkresnetresidualcnn		4
Ridge regression loss	Easy	numpyregressionregularizationridgeloss		4
RMSNorm	Easy	numpynormalizationtransformerllmrmsnorm		4
RMSprop optimizer	Easy	numpyoptimizationoptimizeradaptivermsprop		4
Vanilla RNN cell forward	Medium	numpyrnnsequence-models		5
ROC-AUC from scratch	Medium	numpymetricsevaluationclassificationroc-auc		4
Rotary Position Embedding (RoPE)	Hard	numpytransformerpositional-encodingrope		6
ROUGE-1 score	Easy	nlpmetricssummarizationevaluationrouge		4
Scaled dot-product attention	Medium	numpytransformersattention		5
SELU activation	Easy	numpyactivationneural-netself-normalizingselu		4
Sequence padding & masking	Easy	numpynlpsequencepreprocessingmasking		5
SGD with momentum (one step)	Easy	numpyoptimizationsgdmomentum		5
Shannon entropy	Easy	numpyinformation-theoryentropystatistics		5
Singular Value Decomposition (2x2)	Hard	numpylinear-algebrasvdmatrix-factorization		4
Softmax backward	Medium	numpybackpropsoftmaxcalculus		5
Softmax from scratch	Easy	numpyfundamentalsnumerical-stability	Meta Google Amazon Microsoft Apple	6
Softmax (multinomial) regression	Medium	numpyregressionclassificationsoftmaxmulticlassgradient-descent		4
Sorted polynomial features	Medium	numpyfeature-engineeringpolynomialpreprocessing		5
Sparse matrix multiplication	Medium	linear-algebramatmulsparsealgorithms		4
Sparse mixture-of-experts layer	Hard	numpymoetransformerroutingsparse		5
Speculative decoding verification	Medium	numpyllminferencespeculative-decodingsampling		5
Step LR scheduler	Easy	optimizationlearning-rateschedulertraining		4
SwiGLU activation	Medium	numpyactivationtransformerllmglu		4
Swish / SiLU activation	Easy	numpyactivationneural-netsiluswish		4
TD(0) value update	Easy	numpyreinforcement-learningtemporal-differencevalue-function		5
Temperature scaling	Easy	numpydecodingsamplingfundamentals		5
TF-IDF	Medium	numpynlptext-featurestf-idfinformation-retrieval		4
Time-series anomaly detection	Medium	numpytime-seriesanomaly-detectionrobust-statistics		5
Top-k sampling	Easy	numpydecodingsampling		6
Top-p (nucleus) sampling	Medium	samplingdecodingnumpy		6
Triplet loss	Medium	numpylossmetric-learningembeddingstriplet		4
UCB1 multi-armed bandit	Easy	numpyreinforcement-learningbanditucbexploration		5
Unigram probability from a corpus	Easy	nlplanguage-modelsprobabilityfundamentals		4
VAE ELBO loss	Medium	numpyvaegenerativeelbolosskl-divergence		5
Viterbi algorithm	Medium	numpyhmmdynamic-programmingsequence-modelingdecoding		4
Wasserstein distance (1-D)	Medium	numpyinformation-theorywassersteindistribution-distancestatistics		5
Weighted cross-entropy	Easy	numpylossclassificationimbalancecross-entropy		4
Weighted multinomial sampling	Easy	numpysamplingprobabilitymultinomial		5

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