一列数据是类似 1234,2345,3122,3442 这样的一个字符串，我要把这列数据分割成四列数据，分完后再进另一个去查找，之前写的逻辑代码类似如下：
for u in range(0,products.shape[0]):
start=time.time()
cat1=products.catIds[u].split(',')[0]
#print(categories[categories['catId']==int(cat1)].index[0])
category1=categories['category'][categories[categories['catId']==int(cat1)].index[0]]
cat2=products.catIds[u].split(',')[1]
category2=categories['category'][categories[categories['catId']==int(cat2)].index[0]]
cat3=products.catIds[u].split(',')[-1]
category3=categories['category'][categories[categories['catId']==int(cat3)].index[0]]
df=df.append({"catIds":products.catIds[u],"cat1":category1,"cat2":category2,"cat3":category3})
end=time.time()
print(end-start)
ndf = pd.merge(products,df,how='left',on='catIds'),