{ "source_file": "0930", "source_platform": "Medium", "author": "Shashank Kapadia", "title_listed": "Building Blocks: Text Pre-Processing", "keywords": "", "domain": "文本处理", "content_type": "深度技术解析", "is_paid_flag": "否", "url": "https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/", "publish_date": "2019-03-02", "corpus_stage": "", "notes": "", "id": "82f958f84132", "collected_at": "2026-10-02T10:09:36", "title": "Building Blocks: Text Pre-Processing | Towards Data Science", "body_markdown": "![Revisit consent button](../images/body-df29beacd7496220cc94.svg)\nWe value your privacy\nWe use cookies to enhance your browsing experience, serve personalised ads or content, and analyse our traffic. By clicking \"Accept All\", you consent to our use of cookies.\nCustomise Reject All Accept All\nCustomise Consent Preferences ![](../images/body-2159bb67f3de8ad870eb.svg)\nWe use cookies to help you navigate efficiently and perform certain functions. 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It also helps in adverstising on the website.\n\n\n * Cookie\nedgebucket\n * Duration\nsession\n * Description\nReddit sets this cookie to save the information about a log-on Reddit user, for the purpose of advertisement recommendations and updating the content.\n\n\n * Cookie\ndid\n * Duration\n1 year\n * Description\nArbor sets this cookie to show targeted ads to site visitors.This cookie expires after 2 months or 1 year.\n\n\nUncategorised\nOther uncategorised cookies are those that are being analysed and have not been classified into a category as yet.\nNo cookies to display.\nReject All Save My Preferences Accept All\nToggle Search[](https://towardsdatascience.com/)Toggle Mobile Navigation\n[](https://towardsdatascience.com/)\nPublish AI, ML & data-science insights to a global community of data professionals.\nSign in[Submit an Article](https://write.insightmediagroup.io)\n[Latest](https://towardsdatascience.com/latest/)[Editor's Picks](https://towardsdatascience.com/tag/editors-pick/)[Newsletter](https://towardsdatascience.com/tag/the-variable/)[ShipAI](https://towardsdatascience.com/shipai/)\n* * *\n[Write For TDS](https://towardsdatascience.com/submissions/)\n[LinkedIn](https://www.linkedin.com/company/towards-data-science/)[X](https://x.com/TDataScience)Toggle Search\n[Data Science](https://towardsdatascience.com/category/data-science/)\n# Building Blocks: Text Pre-Processing\nKey concepts and practical implementation\n[Shashank Kapadia](https://towardsdatascience.com/author/shashank%20kapadia/)\nMarch 2, 20198 min read\n![An image of pale code stretching across a black background](../images/000.jpg)\nImage via Pixabay\nIn the last article of our series, we introduced the concept of Natural Language Processing. You can [read it here](https://medium.com/@shashank.kapadia/introduction-to-natural-language-processing-nlp-2a8fae09ed03), and now you probably want to try it yourself, right? Great! Without further ado, let’s dive into the building blocks for statistical natural language processing.\nIn this article, we’ll introduce the key concepts, along with practical implementation in Python and the challenges to keep in mind at the time of application. The complete code is available as a [Jupyter Notebook on GitHub](https://github.com/kapadias/mediumposts/blob/master/nlp/Building%20Blocks%20Text%20Pre-Processing.ipynb).\n## Text normalization\nNormalizing the text means converting it to a more convenient, standard form before performing turning it into features for higher-level modeling. Think of this step as converting human-readable language into a form that is machine-readable.\nThe standard framework to normalize the text includes:\n 1. Tokenization\n 2. Stop words removal\n 3. Morphological normalization\n 4. Collocation\n\n\n> _Data preprocessing consists of a number of steps, any number of which may or not apply to a given task. More generally, in this article we’ll discuss some predetermined body of text, and perform some basic transformative analysis that can be used for performing further, more meaningful natural language processing._[_[1]_](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#1)\nLearn this step by step with the interactive [AI and Data Scientist](https://roadmap.sh/ai-data-scientist) roadmap.\n## Tokenization\nGiven a character sequence and a defined document unit (blurb of texts), _tokenization_ is the task of chopping it up into pieces, called _**tokens**_ , perhaps at the same time throwing away certain characters/words, such as punctuation [[2]](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#2). Ordinarily, there are two types of tokenization:\n 1. **Word tokenization:** Used to separate words via unique _space character._ Depending on the application, word tokenization may also tokenize multi-word expressions like _New York_. This is often times is closely tied to a process called _Named Entity Recognition_. Later in this tutorial, we will look at _Collocation (Phrase) Modeling_ that helps address part of this challenge.\n 2. **Sentence tokenization/segmentation:** Along with word tokenization, sentence segmentation is a crucial step in text processing. This is usually performed based on punctuations such as “.”, “?”, “!” as they tend to mark the sentence boundaries.\n\n\n**Challenges:**\n * The use of abbreviations can prompt the tokenizer to detect a sentence boundary where there is none.\n * Numbers, special characters, hyphenation, and capitalization. In the expressions “don’t,” “I’d,” “John’s” do we have one, two or three tokens?\n\n\n**Implementation example:**\npython\n\n```\nfrom nltk.tokenize import sent_tokenize, word_tokenize\n#Sentence Tokenization\nprint ('Following is the list of sentences tokenized from the sample review\\n')\nsample_text = \"\"\"The first time I ate here I honestly was not that impressed. I decided to wait a bit and give it another chance. \nI have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and \nswiss burgers I had were honestly very good. The shakes were also tasty. Although Mad Mikes is still my favorite burger around, \nyou can do a heck of a lot worse than Smashburger if you get a craving\"\"\"\ntokenize_sentence = sent_tokenize(sample_text)\nprint (tokenize_sentence)\nprint ('---------------------------------------------------------\\n')\nprint ('Following is the list of words tokenized from the sample review sentence\\n')\ntokenize_words = word_tokenize(tokenize_sentence[1])\nprint (tokenize_words)\n```\n\n**Output:**\nmarkup\n\n```\nFollowing is the list of sentences tokenized from the sample review['The first time I ate here I honestly was not that impressed.', 'I decided to wait a bit and give it another chance.', 'I have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and \\nswiss burgers I had were honestly very good.', 'The shakes were also tasty.', 'Although Mad Mikes is still my favorite burger around, \\nyou can do a heck of a lot worse than Smashburger if you get a craving']---------------------------------------------------------Following is the list of words tokenized from the sample review sentence['I', 'decided', 'to', 'wait', 'a', 'bit', 'and', 'give', 'it', 'another', 'chance', '.']\n```\n\n## Stop words removal\nOften, a few ubiquitous words which would appear to be of little value in helping the purpose of analysis but increase the dimensionality of feature set are excluded from the vocabulary entirely as part of the stop words removal process. There are two considerations that usually motivate this removal.\n 1. **Irrelevance:** Allows one to analyze only on content-bearing words. Stopwords, also called empty words because they generally do not bear much meaning, introduce noise in the analysis/modeling process.\n 2. **Dimension:** Removing the stopwords also allows one to reduce the tokens in documents significantly, and thereby decreasing feature dimension.\n\n\n**Challenges:**\nConverting all characters into lowercase letters before stopwords removal process can introduce ambiguity in the text, and sometimes entirely changing the meaning of it. For example, with the expressions “US citizen” will be viewed as “us citizen” or “IT scientist” as “it scientist”. Since both *us* and *it* are normally considered stop words, it would result in an inaccurate outcome. The strategy regarding the treatment of stopwords can thus be refined by identifying that “US” and “IT” are not pronouns in the above examples, through a _part-of-speech_ tagging step.\n**Implementation example:**\npython\n\n```\nfrom nltk.corpus import stopwords\nfrom nltk.tokenize import word_tokenize# define the language for stopwords removal\nstopwords = set(stopwords.words(\"english\"))\nprint (\"\"\"{0} stop words\"\"\".format(len(stopwords)))tokenize_words = word_tokenize(sample_text)\nfiltered_sample_text = [w for w in tokenize_words if not w in stopwords]print ('\\nOriginal Text:')\nprint ('------------------\\n')\nprint (sample_text)\nprint ('\\n Filtered Text:')\nprint ('------------------\\n')\nprint (' '.join(str(token) for token in filtered_sample_text))\n```\n\n**Output:**\nmarkup\n\n```\n179 stop wordsOriginal Text:------------------The first time I ate here I honestly was not that impressed. I decided to wait a bit and give it another chance. I have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and swiss burgers I had were honestly very good. The shakes were also tasty. Although Mad Mikes is still my favorite burger around, you can do a heck of a lot worse than Smashburger if you get a craving Filtered Text:------------------The first time I ate I honestly impressed . I decided wait bit give another chance . I recently eaten couple times although I convinced pricing particularly point two mushroom swiss burgers I honestly good . The shakes also tasty . Although Mad Mikes still favorite burger around , heck lot worse Smashburger get craving\n```\n\n## Morphological normalization\nMorphology, in general, is the study of the way words are built up from smaller meaning-bearing units, _morphomes._ For example, _dogs_ consists of two morphemes: _**dog**_ and _**s**_.\nTwo commonly used techniques for text normalization are:\n 1. **Stemming:** The procedure aims to identify the stem of a word and use it in lieu of the word itself. The most popular algorithm for stemming English, and one that has repeatedly been shown to be empirically very effective, is Porter’s algorithm. The entire algorithm is too long and intricate to present here [[3]](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#3), but you can find details [here](http://people.scs.carleton.ca/~armyunis/projects/KAPI/porter.pdf).\n 2. **Lemmatization:** This process refers to doing things correctly with the use of vocabulary and morphological analysis of words, typically aiming to remove _inflectional_ endings only and to return the base or dictionary form of a word, which is known as the _**lemma**_.\n\n\n> _If confronted with the token_**saw** _, stemming might return just_**s** _, whereas lemmatization would attempt to return either_**see** _or_**saw** _depending on whether the use of the token was as a verb or a noun._[_[4]_](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#4)\n**Implementation example:**\npython\n\n```\nfrom nltk.stem import PorterStemmer\nfrom nltk.stem import WordNetLemmatizer\nfrom nltk.tokenize import word_tokenize\nps = PorterStemmer()\nlemmatizer = WordNetLemmatizer()\ntokenize_words = word_tokenize(sample_text)\nstemmed_sample_text = []\nfor token in tokenize_words:\n stemmed_sample_text.append(ps.stem(token))\nlemma_sample_text = []\nfor token in tokenize_words:\n lemma_sample_text.append(lemmatizer.lemmatize(token))\n \nprint ('\\nOriginal Text:')\nprint ('------------------\\n')\nprint (sample_text)\nprint ('\\nFiltered Text: Stemming')\nprint ('------------------\\n')\nprint (' '.join(str(token) for token in stemmed_sample_text))\nprint ('\\nFiltered Text: Lemmatization')\nprint ('--------------------------------\\n')\nprint (' '.join(str(token) for token in lemma_sample_text))\n```\n\n**Output:**\nmarkup\n\n```\nOriginal Text:------------------The first time I ate here I honestly was not that impressed. I decided to wait a bit and give it another chance. I have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and swiss burgers I had were honestly very good. The shakes were also tasty. Although Mad Mikes is still my favorite burger around, you can do a heck of a lot worse than Smashburger if you get a craving.Filtered Text: Stemming:------------------the first time I ate here I honestli wa not that impress . I decid to wait a bit and give it anoth chanc . I have recent eaten there a coupl of time and although I am not convinc that the price is particularli on point the two mushroom and swiss burger I had were honestli veri good . the shake were also tasti . although mad mike is still my favorit burger around , you can do a heck of a lot wors than smashburg if you get a crave .Filtered Text: Lemmatization--------------------------------The first time I ate here I honestly wa not that impressed . I decided to wait a bit and give it another chance . I have recently eaten there a couple of time and although I am not convinced that the pricing is particularly on point the two mushroom and swiss burger I had were honestly very good . The shake were also tasty . Although Mad Mikes is still my favorite burger around , you can do a heck of a lot worse than Smashburger if you get a craving .\n```\n\n**Challenges:**\nOften, full morphological analysis produces, at most, very modest benefits for analysis. Neither form of normalization improves language information performance in aggregate, both from relevance and dimensionality reduction standpoint — at least not for the following situations:\n**Implementation example:**\npython\n\n```\nfrom nltk.stem import PorterStemmer\nwords = [\"operate\", \"operating\", \"operates\", \"operation\", \"operative\", \"operatives\", \"operational\"]\nps = PorterStemmer()\nfor token in words:\n print (ps.stem(token))\n```\n\n**Output:**\nmarkup\n\n```\noperoperoperoperoperoperoper\n```\n\nAs an example of what can go wrong, note that the Porter stemmer stems all of the following words to _**oper**_.\nHowever, since _operate_ in its various forms is a common verb, we would expect to lose considerable precision [4]:\n * operational AND research\n * operating AND system\n * operative AND dentistry\n\n\nFor cases like these, moving to using a lemmatizer would not completely fix the problem because particular inflectional forms are used in specific collocations. Getting better value from term normalization depends more on pragmatic issues of word use than on formal issues of linguistic morphology. [[4]](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#4)\nFor all the codes used to generate above results, click [here](https://github.com/kapadias/mediumposts/blob/master/nlp/Building%20Blocks%20Text%20Pre-Processing.ipynb).\nAnd that’s it! Now, you know all about gradient descent, linear regression, and logistic regression.\n## Coming up..\nIn the next article, we’ll discuss the concept of collocation (phrase) modeling in detail and walk through its implementation together. Stay tuned, and keep learning!\n## References:\n[1] A General Approach to Preprocessing Text Data — KDnuggets. \n[2] Tokenization — Stanford NLP Group. \n[3] Text Mining in Bovine Diseases — ijcaonline.org. \n[4] Stemming and lemmatization — Stanford NLP Group. \n_If you have any feedback, please reach out by messaging me on_[ _LinkedIn_](https://www.linkedin.com/in/shashankkapadia/) _or shooting me an email (shmkapadia[at]gmail.com)_.\n## Register Now to Continue Reading\n### Unlock Towards Data Science with a Free Account\nAll articles on Towards Data Science are available to registered users. 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You can [read it here](https://medium.com/@shashank.kapadia/introduction-to-natural-language-processing-nlp-2a8fae09ed03), and now you probably want to try it yourself, right? Great! Without further ado, let’s dive into the building blocks for statistical natural language processing.\nIn this article, we’ll introduce the key concepts, along with practical implementation in Python and the challenges to keep in mind at the time of application. The complete code is available as a [Jupyter Notebook on GitHub](https://github.com/kapadias/mediumposts/blob/master/nlp/Building%20Blocks%20Text%20Pre-Processing.ipynb).\n## Text normalization\nNormalizing the text means converting it to a more convenient, standard form before performing turning it into features for higher-level modeling. Think of this step as converting human-readable language into a form that is machine-readable.\nThe standard framework to normalize the text includes:\n 1. Tokenization\n 2. Stop words removal\n 3. Morphological normalization\n 4. Collocation\n\n\n> _Data preprocessing consists of a number of steps, any number of which may or not apply to a given task. More generally, in this article we’ll discuss some predetermined body of text, and perform some basic transformative analysis that can be used for performing further, more meaningful natural language processing._[_[1]_](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#1)\nLearn this step by step with the interactive [AI and Data Scientist](https://roadmap.sh/ai-data-scientist) roadmap.\n## Tokenization\nGiven a character sequence and a defined document unit (blurb of texts), _tokenization_ is the task of chopping it up into pieces, called _**tokens**_ , perhaps at the same time throwing away certain characters/words, such as punctuation [[2]](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#2). Ordinarily, there are two types of tokenization:\n 1. **Word tokenization:** Used to separate words via unique _space character._ Depending on the application, word tokenization may also tokenize multi-word expressions like _New York_. This is often times is closely tied to a process called _Named Entity Recognition_. Later in this tutorial, we will look at _Collocation (Phrase) Modeling_ that helps address part of this challenge.\n 2. **Sentence tokenization/segmentation:** Along with word tokenization, sentence segmentation is a crucial step in text processing. This is usually performed based on punctuations such as “.”, “?”, “!” as they tend to mark the sentence boundaries.\n\n\n**Challenges:**\n * The use of abbreviations can prompt the tokenizer to detect a sentence boundary where there is none.\n * Numbers, special characters, hyphenation, and capitalization. In the expressions “don’t,” “I’d,” “John’s” do we have one, two or three tokens?\n\n\n**Implementation example:**\npython\n\n```\nfrom nltk.tokenize import sent_tokenize, word_tokenize\n#Sentence Tokenization\nprint ('Following is the list of sentences tokenized from the sample review\\n')\nsample_text = \"\"\"The first time I ate here I honestly was not that impressed. I decided to wait a bit and give it another chance. \nI have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and \nswiss burgers I had were honestly very good. The shakes were also tasty. Although Mad Mikes is still my favorite burger around, \nyou can do a heck of a lot worse than Smashburger if you get a craving\"\"\"\ntokenize_sentence = sent_tokenize(sample_text)\nprint (tokenize_sentence)\nprint ('---------------------------------------------------------\\n')\nprint ('Following is the list of words tokenized from the sample review sentence\\n')\ntokenize_words = word_tokenize(tokenize_sentence[1])\nprint (tokenize_words)\n```\n\n**Output:**\nmarkup\n\n```\nFollowing is the list of sentences tokenized from the sample review['The first time I ate here I honestly was not that impressed.', 'I decided to wait a bit and give it another chance.', 'I have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and \\nswiss burgers I had were honestly very good.', 'The shakes were also tasty.', 'Although Mad Mikes is still my favorite burger around, \\nyou can do a heck of a lot worse than Smashburger if you get a craving']---------------------------------------------------------Following is the list of words tokenized from the sample review sentence['I', 'decided', 'to', 'wait', 'a', 'bit', 'and', 'give', 'it', 'another', 'chance', '.']\n```\n\n## Stop words removal\nOften, a few ubiquitous words which would appear to be of little value in helping the purpose of analysis but increase the dimensionality of feature set are excluded from the vocabulary entirely as part of the stop words removal process. There are two considerations that usually motivate this removal.\n 1. **Irrelevance:** Allows one to analyze only on content-bearing words. Stopwords, also called empty words because they generally do not bear much meaning, introduce noise in the analysis/modeling process.\n 2. **Dimension:** Removing the stopwords also allows one to reduce the tokens in documents significantly, and thereby decreasing feature dimension.\n\n\n**Challenges:**\nConverting all characters into lowercase letters before stopwords removal process can introduce ambiguity in the text, and sometimes entirely changing the meaning of it. For example, with the expressions “US citizen” will be viewed as “us citizen” or “IT scientist” as “it scientist”. Since both *us* and *it* are normally considered stop words, it would result in an inaccurate outcome. The strategy regarding the treatment of stopwords can thus be refined by identifying that “US” and “IT” are not pronouns in the above examples, through a _part-of-speech_ tagging step.\n**Implementation example:**\npython\n\n```\nfrom nltk.corpus import stopwords\nfrom nltk.tokenize import word_tokenize# define the language for stopwords removal\nstopwords = set(stopwords.words(\"english\"))\nprint (\"\"\"{0} stop words\"\"\".format(len(stopwords)))tokenize_words = word_tokenize(sample_text)\nfiltered_sample_text = [w for w in tokenize_words if not w in stopwords]print ('\\nOriginal Text:')\nprint ('------------------\\n')\nprint (sample_text)\nprint ('\\n Filtered Text:')\nprint ('------------------\\n')\nprint (' '.join(str(token) for token in filtered_sample_text))\n```\n\n**Output:**\nmarkup\n\n```\n179 stop wordsOriginal Text:------------------The first time I ate here I honestly was not that impressed. I decided to wait a bit and give it another chance. I have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and swiss burgers I had were honestly very good. The shakes were also tasty. Although Mad Mikes is still my favorite burger around, you can do a heck of a lot worse than Smashburger if you get a craving Filtered Text:------------------The first time I ate I honestly impressed . I decided wait bit give another chance . I recently eaten couple times although I convinced pricing particularly point two mushroom swiss burgers I honestly good . The shakes also tasty . Although Mad Mikes still favorite burger around , heck lot worse Smashburger get craving\n```\n\n## Morphological normalization\nMorphology, in general, is the study of the way words are built up from smaller meaning-bearing units, _morphomes._ For example, _dogs_ consists of two morphemes: _**dog**_ and _**s**_.\nTwo commonly used techniques for text normalization are:\n 1. **Stemming:** The procedure aims to identify the stem of a word and use it in lieu of the word itself. The most popular algorithm for stemming English, and one that has repeatedly been shown to be empirically very effective, is Porter’s algorithm. The entire algorithm is too long and intricate to present here [[3]](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#3), but you can find details [here](http://people.scs.carleton.ca/~armyunis/projects/KAPI/porter.pdf).\n 2. **Lemmatization:** This process refers to doing things correctly with the use of vocabulary and morphological analysis of words, typically aiming to remove _inflectional_ endings only and to return the base or dictionary form of a word, which is known as the _**lemma**_.\n\n\n> _If confronted with the token_**saw** _, stemming might return just_**s** _, whereas lemmatization would attempt to return either_**see** _or_**saw** _depending on whether the use of the token was as a verb or a noun._[_[4]_](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#4)\n**Implementation example:**\npython\n\n```\nfrom nltk.stem import PorterStemmer\nfrom nltk.stem import WordNetLemmatizer\nfrom nltk.tokenize import word_tokenize\nps = PorterStemmer()\nlemmatizer = WordNetLemmatizer()\ntokenize_words = word_tokenize(sample_text)\nstemmed_sample_text = []\nfor token in tokenize_words:\n stemmed_sample_text.append(ps.stem(token))\nlemma_sample_text = []\nfor token in tokenize_words:\n lemma_sample_text.append(lemmatizer.lemmatize(token))\n \nprint ('\\nOriginal Text:')\nprint ('------------------\\n')\nprint (sample_text)\nprint ('\\nFiltered Text: Stemming')\nprint ('------------------\\n')\nprint (' '.join(str(token) for token in stemmed_sample_text))\nprint ('\\nFiltered Text: Lemmatization')\nprint ('--------------------------------\\n')\nprint (' '.join(str(token) for token in lemma_sample_text))\n```\n\n**Output:**\nmarkup\n\n```\nOriginal Text:------------------The first time I ate here I honestly was not that impressed. I decided to wait a bit and give it another chance. I have recently eaten there a couple of times and although I am not convinced that the pricing is particularly on point the two mushroom and swiss burgers I had were honestly very good. The shakes were also tasty. Although Mad Mikes is still my favorite burger around, you can do a heck of a lot worse than Smashburger if you get a craving.Filtered Text: Stemming:------------------the first time I ate here I honestli wa not that impress . I decid to wait a bit and give it anoth chanc . I have recent eaten there a coupl of time and although I am not convinc that the price is particularli on point the two mushroom and swiss burger I had were honestli veri good . the shake were also tasti . although mad mike is still my favorit burger around , you can do a heck of a lot wors than smashburg if you get a crave .Filtered Text: Lemmatization--------------------------------The first time I ate here I honestly wa not that impressed . I decided to wait a bit and give it another chance . I have recently eaten there a couple of time and although I am not convinced that the pricing is particularly on point the two mushroom and swiss burger I had were honestly very good . The shake were also tasty . Although Mad Mikes is still my favorite burger around , you can do a heck of a lot worse than Smashburger if you get a craving .\n```\n\n**Challenges:**\nOften, full morphological analysis produces, at most, very modest benefits for analysis. Neither form of normalization improves language information performance in aggregate, both from relevance and dimensionality reduction standpoint — at least not for the following situations:\n**Implementation example:**\npython\n\n```\nfrom nltk.stem import PorterStemmer\nwords = [\"operate\", \"operating\", \"operates\", \"operation\", \"operative\", \"operatives\", \"operational\"]\nps = PorterStemmer()\nfor token in words:\n print (ps.stem(token))\n```\n\n**Output:**\nmarkup\n\n```\noperoperoperoperoperoperoper\n```\n\nAs an example of what can go wrong, note that the Porter stemmer stems all of the following words to _**oper**_.\nHowever, since _operate_ in its various forms is a common verb, we would expect to lose considerable precision [4]:\n * operational AND research\n * operating AND system\n * operative AND dentistry\n\n\nFor cases like these, moving to using a lemmatizer would not completely fix the problem because particular inflectional forms are used in specific collocations. Getting better value from term normalization depends more on pragmatic issues of word use than on formal issues of linguistic morphology. [[4]](https://towardsdatascience.com/building-blocks-text-pre-processing-641cae8ba3bf/#4)\nFor all the codes used to generate above results, click [here](https://github.com/kapadias/mediumposts/blob/master/nlp/Building%20Blocks%20Text%20Pre-Processing.ipynb).\nAnd that’s it! Now, you know all about gradient descent, linear regression, and logistic regression.\n## Coming up..\nIn the next article, we’ll discuss the concept of collocation (phrase) modeling in detail and walk through its implementation together. Stay tuned, and keep learning!\n## References:\n[1] A General Approach to Preprocessing Text Data — KDnuggets. \n[2] Tokenization — Stanford NLP Group. \n[3] Text Mining in Bovine Diseases — ijcaonline.org. \n[4] Stemming and lemmatization — Stanford NLP Group. \n_If you have any feedback, please reach out by messaging me on_[ _LinkedIn_](https://www.linkedin.com/in/shashankkapadia/) _or shooting me an email (shmkapadia[at]gmail.com)_.\n## Register Now to Continue Reading\n### Unlock Towards Data Science with a Free Account\nAll articles on Towards Data Science are available to registered users. 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