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train(x,y,w,steps,lr=.1,l2=.01):\n    w=w.copy()\n    for _ in range(steps):\n        p=1/(1+np.exp(-np.clip(x@w,-30,30))); w-=lr*(x.T@(p-y)/len(y)+l2*np.r_[w[:2],0])\n    return w\ndef acc(w,d): x,y=d; return float(((x@w>=0)==y).mean())\nsource=data(220,512,1); source_test=data(221,512,1); adapt=data(222,32,-1); target_test=data(223,512,-1)\nw=train(*source,np.zeros(3),500); adapted=train(*adapt,w,150); scratch=train(*adapt,np.zeros(3),150)\nprint('source-trained weights stable/spurious/bias',w.round(3).tolist())\nresult={'source_holdout':acc(w,source_test),'target_zero_shot':acc(w,target_test),'target_adapt32':acc(adapted,target_test),'target_scratch32':acc(scratch,target_test)}\nprint(result); print('adaptation gain over zero-shot',round(result['target_adapt32']-result['target_zero_shot'],4),'transfer advantage vs scratch',round(result['target_adapt32']-result['target_scratch32'],4))\nprint('test seeds 221/223 never used by optimizer; adaptation seed222 only')\n```","sha256":"27b60dcda94fba59f6af2ff7c1b50efba3c5b073354ca4122c0762fb8a70a6ef","chapter":{"id":"course","title":"LAB 19 · 新域：捷径反转与少标签适配 · 实现代码"},"unit":{"id":"notebook-blocks/57aeca83ffea4a7c9f6d46529f58fa18","title":"LAB 19 · 新域：捷径反转与少标签适配 · 实现代码"},"page":{"id":"reading-opening","title":"LAB 19 · 新域：捷径反转与少标签适配 · 实现代码","number":1},"sectionTitle":"LAB 19 · 新域：捷径反转与少标签适配 · 实现代码","currentQuestion":"LAB 19 · 新域：捷径反转与少标签适配 · 实现代码","sourceSlides":[],"knowledgeBlock":null,"formulas":[],"previousBlock":"b-70193352516c59f7d1d5a884","nextBlock":null,"prerequisites":[{"id":"b-5e5c2d29997541afdc4a7fe6","url":"#b-5e5c2d29997541afdc4a7fe6","role":"section-title"},{"id":"b-3626839797c696d3a26d09c7","url":"#b-3626839797c696d3a26d09c7","role":"problem-context"}],"relatedBlocks":[]}]}