Python Strings - Module 4 - Data Structures PDF

**Module-IV: Data Structures** **Strings** Strings in python are surrounded by either single quotation marks, or double quotation marks. \'hello\' is the same as \"hello\". You can display a string literal with the print() function: Example print(\"Hello\")\ print(\'Hello\') **Quotes Inside Q...

**Module-IV: Data Structures** **Strings** Strings in python are surrounded by either single quotation marks, or double quotation marks. \'hello\' is the same as \"hello\". You can display a string literal with the print() function: Example print(\"Hello\")\ print(\'Hello\') **Quotes Inside Quotes** You can use quotes inside a string, as long as they don\'t match the quotes surrounding the string: Example print(\"It\'s alright\")\ print(\"He is called \'Johnny\'\")\ print(\'He is called \"Johnny\"\') **Assign String to a Variable** Assigning a string to a variable is done with the variable name followed by an equal sign and the string: Example a = \"Hello\"\ print(a) **Multiline Strings** You can assign a multiline string to a variable by using three quotes: Example You can use three double quotes: a = \"\"\"Lorem ipsum dolor sit amet,\ consectetur adipiscing elit,\ sed do eiusmod tempor incididunt\ ut labore et dolore magna aliqua.\"\"\"\ print(a) **Or three single quotes:** Example a = \'\'\'Lorem ipsum dolor sit amet,\ consectetur adipiscing elit,\ sed do eiusmod tempor incididunt\ ut labore et dolore magna aliqua.\'\'\'\ print(a) **Note:** in the result, the line breaks are inserted at the same position as in the code. **Strings are Arrays** Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters. However, Python does not have a character data type, a single character is simply a string with a length of 1. Square brackets can be used to access elements of the string. Example Get the character at position 1 (remember that the first character has the position 0): a = \"Hello, World!\"\ print(a\[1\]) **Looping Through a String:** Since strings are arrays, we can loop through the characters in a string, with a for loop. Example Loop through the letters in the word \"banana\": for x in \"banana\":\   print(x) **String Length:** To get the length of a string, use the len() function. Example The len() function returns the length of a string: a = \"Hello, World!\"\ print(len(a)) **Check String:** To check if a certain phrase or character is present in a string, we can use the keyword in. Example Check if \"free\" is present in the following text: txt = \"The best things in life are free!\"\ print(\"free\" in txt) Use it in an if statement: Example Print only if \"free\" is present: txt = \"The best things in life are free!\"\ if \"free\" in txt:\   print(\"Yes, \'free\' is present.\") **Check if NOT:** To check if a certain phrase or character is NOT present in a string, we can use the keyword not in. Example Check if \"expensive\" is NOT present in the following text: txt = \"The best things in life are free!\"\ print(\"expensive\" not in txt) Use it in an if statement: Example print only if \"expensive\" is NOT present: txt = \"The best things in life are free!\"\ if \"expensive\" not in txt:\   print(\"No, \'expensive\' is NOT present.\") Python has a set of built-in methods that you can use on strings. **Upper Case** Example The upper() method returns the string in upper case: a = \"Hello, World!\"\ print(a.upper()) **Lower Case** Example The lower() method returns the string in lower case: a = \"Hello, World!\"\ print(a.lower()) **Remove Whitespace:** Whitespace is the space before and/or after the actual text, and very often you want to remove this space. Example The strip() method removes any whitespace from the beginning or the end: a = \" Hello, World! \"\ print(a.strip()) \# returns \"Hello, World!\" **Replace String** Example The replace() method replaces a string with another string: a = \"Hello, World!\"\ print(a.replace(\"H\", \"J\")) **Split String** The split() method returns a list where the text between the specified separator becomes the list items. Example The split() method splits the string into substrings if it finds instances of the separator: a = \"Hello, World!\"\ print(a.split(\",\")) \# returns \[\'Hello\', \' World!\'\] **String Methods** Learn more about String Methods with our  **Note:** All string methods return new values. They do not change the original string. **Method** **Description** ---------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- [[capitalize()]](https://www.w3schools.com/python/ref_string_capitalize.asp) Converts the first character to upper case [[casefold()]](https://www.w3schools.com/python/ref_string_casefold.asp) Converts string into lower case [[center()]](https://www.w3schools.com/python/ref_string_center.asp) Returns a centered string [[count()]](https://www.w3schools.com/python/ref_string_count.asp) Returns the number of times a specified value occurs in a string [[encode()]](https://www.w3schools.com/python/ref_string_encode.asp) Returns an encoded version of the string [[endswith()]](https://www.w3schools.com/python/ref_string_endswith.asp) Returns true if the string ends with the specified value [[expandtabs()]](https://www.w3schools.com/python/ref_string_expandtabs.asp) Sets the tab size of the string [[find()]](https://www.w3schools.com/python/ref_string_find.asp) Searches the string for a specified value and returns the position of where it was found [[format()]](https://www.w3schools.com/python/ref_string_format.asp) Formats specified values in a string format\_map() Formats specified values in a string [[index()]](https://www.w3schools.com/python/ref_string_index.asp) Searches the string for a specified value and returns the position of where it was found [[isalnum()]](https://www.w3schools.com/python/ref_string_isalnum.asp) Returns True if all characters in the string are alphanumeric [[isalpha()]](https://www.w3schools.com/python/ref_string_isalpha.asp) Returns True if all characters in the string are in the alphabet [[isascii()]](https://www.w3schools.com/python/ref_string_isascii.asp) Returns True if all characters in the string are ascii characters [[isdecimal()]](https://www.w3schools.com/python/ref_string_isdecimal.asp) Returns True if all characters in the string are decimals [[isdigit()]](https://www.w3schools.com/python/ref_string_isdigit.asp) Returns True if all characters in the string are digits [[isidentifier()]](https://www.w3schools.com/python/ref_string_isidentifier.asp) Returns True if the string is an identifier [[islower()]](https://www.w3schools.com/python/ref_string_islower.asp) Returns True if all characters in the string are lower case [[isnumeric()]](https://www.w3schools.com/python/ref_string_isnumeric.asp) Returns True if all characters in the string are numeric [[isprintable()]](https://www.w3schools.com/python/ref_string_isprintable.asp) Returns True if all characters in the string are printable [[isspace()]](https://www.w3schools.com/python/ref_string_isspace.asp) Returns True if all characters in the string are whitespaces [[istitle()]](https://www.w3schools.com/python/ref_string_istitle.asp) Returns True if the string follows the rules of a title [[isupper()]](https://www.w3schools.com/python/ref_string_isupper.asp) Returns True if all characters in the string are upper case [[join()]](https://www.w3schools.com/python/ref_string_join.asp) Joins the elements of an iterable to the end of the string [[ljust()]](https://www.w3schools.com/python/ref_string_ljust.asp) Returns a left justified version of the string [[lower()]](https://www.w3schools.com/python/ref_string_lower.asp) Converts a string into lower case [[lstrip()]](https://www.w3schools.com/python/ref_string_lstrip.asp) Returns a left trim version of the string [[maketrans()]](https://www.w3schools.com/python/ref_string_maketrans.asp) Returns a translation table to be used in translations [[partition()]](https://www.w3schools.com/python/ref_string_partition.asp) Returns a tuple where the string is parted into three parts [[replace()]](https://www.w3schools.com/python/ref_string_replace.asp) Returns a string where a specified value is replaced with a specified value [[rfind()]](https://www.w3schools.com/python/ref_string_rfind.asp) Searches the string for a specified value and returns the last position of where it was found [[rindex()]](https://www.w3schools.com/python/ref_string_rindex.asp) Searches the string for a specified value and returns the last position of where it was found [[rjust()]](https://www.w3schools.com/python/ref_string_rjust.asp) Returns a right justified version of the string [[rpartition()]](https://www.w3schools.com/python/ref_string_rpartition.asp) Returns a tuple where the string is parted into three parts [[rsplit()]](https://www.w3schools.com/python/ref_string_rsplit.asp) Splits the string at the specified separator, and returns a list [[rstrip()]](https://www.w3schools.com/python/ref_string_rstrip.asp) Returns a right trim version of the string [[split()]](https://www.w3schools.com/python/ref_string_split.asp) Splits the string at the specified separator, and returns a list [[splitlines()]](https://www.w3schools.com/python/ref_string_splitlines.asp) Splits the string at line breaks and returns a list [[startswith()]](https://www.w3schools.com/python/ref_string_startswith.asp) Returns true if the string starts with the specified value [[strip()]](https://www.w3schools.com/python/ref_string_strip.asp) Returns a trimmed version of the string [[swapcase()]](https://www.w3schools.com/python/ref_string_swapcase.asp) Swaps cases, lower case becomes upper case and vice versa [[title()]](https://www.w3schools.com/python/ref_string_title.asp) Converts the first character of each word to upper case [[translate()]](https://www.w3schools.com/python/ref_string_translate.asp) Returns a translated string [[upper()]](https://www.w3schools.com/python/ref_string_upper.asp) Converts a string into upper case [[zfill()]](https://www.w3schools.com/python/ref_string_zfill.asp) Fills the string with a specified number of 0 values at the beginning List: ----- Lists are used to store multiple items in a single variable. Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are [Tuple](https://www.w3schools.com/python/python_tuples.asp), [Set](https://www.w3schools.com/python/python_sets.asp), and [Dictionary](https://www.w3schools.com/python/python_dictionaries.asp), all with different qualities and usage. Lists are created using square brackets: ### Example Create a List: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ print(thislist) List Items: ----------- List items are ordered, changeable, and allow duplicate values. List items are indexed, the first item has index \[0\], the second item has index \[1\] etc. Ordered ------- When we say that lists are ordered, it means that the items have a defined order, and that order will not change. If you add new items to a list, the new items will be placed at the end of the list. **Note:** There are some [list methods](https://www.w3schools.com/python/python_lists_methods.asp) that will change the order, but in general: the order of the items will not change. Changeable: ----------- The list is changeable, meaning that we can change, add, and remove items in a list after it has been created. Allow Duplicates ---------------- Since lists are indexed, lists can have items with the same value: ### Example Lists allow duplicate values: thislist = \[\"apple\", \"banana\", \"cherry\", \"apple\", \"cherry\"\]\ print(thislist) List Length ----------- To determine how many items a list has, use the len() function: ### Example Print the number of items in the list: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ print(len(thislist)) List Items - Data Types ----------------------- List items can be of any data type: ### Example String, int and boolean data types: list1 = \[\"apple\", \"banana\", \"cherry\"\]\ list2 = \[1, 5, 7, 9, 3\]\ list3 = \[True, False, False\] **A list can contain different data types:** ### Example A list with strings, integers and boolean values: list1 = \[\"abc\", 34, True, 40, \"male\"\] type() ------ From Python\'s perspective, lists are defined as objects with the data type \'list\': \ ### Example What is the data type of a list? mylist = \[\"apple\", \"banana\", \"cherry\"\]\ print(type(mylist)) The list() Constructor ---------------------- It is also possible to use the list() constructor when creating a new list. ### Example Using the list() constructor to make a List: thislist = list((\"apple\", \"banana\", \"cherry\")) \# note the double round-brackets\ print(thislist) Python Collections (Arrays) --------------------------- There are four collection data types in the Python programming language: - **List** is a collection which is ordered and changeable. Allows duplicate members. - [**Tuple**](https://www.w3schools.com/python/python_tuples.asp) is a collection which is ordered and unchangeable. Allows duplicate members. - [**Set**](https://www.w3schools.com/python/python_sets.asp) is a collection which is unordered, unchangeable\*, and unindexed. No duplicate members. - [**Dictionary**](https://www.w3schools.com/python/python_dictionaries.asp) is a collection which is ordered\*\* and changeable. No duplicate members. \*Set *items* are unchangeable, but you can remove and/or add items whenever you like. \*\*As of Python version 3.7, dictionaries are *ordered*. In Python 3.6 and earlier, dictionaries are *unordered*. When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security. Access Items ------------ List items are indexed and you can access them by referring to the index number: ### Example[Get your own Python Server](https://www.w3schools.com/python/python_server.asp) Print the second item of the list: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ print(thislist\[1\]) **Note: **The first item has index 0. ### Negative Indexing Negative indexing means start from the end -1 refers to the last item, -2 refers to the second last item etc. ### Example Print the last item of the list: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ print(thislist\[-1\]) ### Range of Indexes You can specify a range of indexes by specifying where to start and where to end the range. When specifying a range, the return value will be a new list with the specified items. ### Example Return the third, fourth, and fifth item: thislist = \[\"apple\", \"banana\", \"cherry\", \"orange\", \"kiwi\", \"melon\", \"mango\"\]\ print(thislist\[2:5\]) **Note:** The search will start at index 2 (included) and end at index 5 (not included).Remember that the first item has index 0. By leaving out the start value, the range will start at the first item: ### Example This example returns the items from the beginning to, but NOT including, \"kiwi\": thislist = \[\"apple\", \"banana\", \"cherry\", \"orange\", \"kiwi\", \"melon\", \"mango\"\]\ print(thislist\[:4\]) By leaving out the end value, the range will go on to the end of the list: ### Example This example returns the items from \"cherry\" to the end: thislist = \[\"apple\", \"banana\", \"cherry\", \"orange\", \"kiwi\", \"melon\", \"mango\"\]\ print(thislist\[2:\]) ### Range of Negative Indexes: Specify negative indexes if you want to start the search from the end of the list: ### Example This example returns the items from \"orange\" (-4) to, but NOT including \"mango\" (-1): thislist = \[\"apple\", \"banana\", \"cherry\", \"orange\", \"kiwi\", \"melon\", \"mango\"\]\ print(thislist\[-4:-1\]) Check if Item Exists -------------------- To determine if a specified item is present in a list use the in keyword: ### Example Check if \"apple\" is present in the list: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ if \"apple\" in thislist:\   print(\"Yes, \'apple\' is in the fruits list\") Change Item Value ----------------- To change the value of a specific item, refer to the index number: ### Example Change the second item: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist\[1\] = \"blackcurrant\"\ print(thislist) Change a Range of Item Values ----------------------------- To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values: ### Example Change the values \"banana\" and \"cherry\" with the values \"blackcurrant\" and \"watermelon\": thislist = \[\"apple\", \"banana\", \"cherry\", \"orange\", \"kiwi\", \"mango\"\]\ thislist\[1:3\] = \[\"blackcurrant\", \"watermelon\"\]\ print(thislist) If you insert *more* items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly: ### Example Change the second value by replacing it with *two* new values: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist\[1:2\] = \[\"blackcurrant\", \"watermelon\"\]\ print(thislist) **Note:** The length of the list will change when the number of items inserted does not match the number of items replaced. If you insert *less* items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly: ### Example Change the second and third value by replacing it with *one* value: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist\[1:3\] = \[\"watermelon\"\]\ print(thislist) Insert Items ------------ To insert a new list item, without replacing any of the existing values, we can use the insert() method. The insert() method inserts an item at the specified index: ### Example Insert \"watermelon\" as the third item: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.insert(2, \"watermelon\")\ print(thislist) **Note:** As a result of the example above, the list will now contain 4 items. Append Items ------------ To add an item to the end of the list, use the append() method: ### Example Using the append() method to append an item: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.append(\"orange\")\ print(thislist) Insert Items ------------ To insert a list item at a specified index, use the insert() method. The insert() method inserts an item at the specified index: ### Example Insert an item as the second position: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.insert(1, \"orange\")\ print(thislist) **Note:** As a result of the examples above, the lists will now contain 4 items. Extend List ----------- To append elements from *another list* to the current list, use the extend() method. ### Example Add the elements of tropical to thislist: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ tropical = \[\"mango\", \"pineapple\", \"papaya\"\]\ thislist.extend(tropical)\ print(thislist) The elements will be added to the *end* of the list. Add Any Iterable ---------------- The extend() method does not have to append *lists*, you can add any iterable object (tuples, sets, dictionaries etc.). ### Example Add elements of a tuple to a list: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thistuple = (\"kiwi\", \"orange\")\ thislist.extend(thistuple)\ print(thislist) Remove Specified Item --------------------- The remove() method removes the specified item. ### Example Remove \"banana\": thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.remove(\"banana\")\ print(thislist) If there are more than one item with the specified value, the remove() method removes the first occurrence: ### Example Remove the first occurrence of \"banana\": thislist = \[\"apple\", \"banana\", \"cherry\", \"banana\", \"kiwi\"\]\ thislist.remove(\"banana\")\ print(thislist) Remove Specified Index ---------------------- The pop() method removes the specified index. ### Example Remove the second item: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.pop(1)\ print(thislist) If you do not specify the index, the pop() method removes the last item. ### Example Remove the last item: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.pop()\ print(thislist) The del keyword also removes the specified index: ### Example Remove the first item: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ del thislist\[0\]\ print(thislist) The del keyword can also delete the list completely. ### Example Delete the entire list: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ del thislist Clear the List -------------- The clear() method empties the list. The list still remains, but it has no content. ### Example Clear the list content: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ thislist.clear()\ print(thislist) Loop Through a List ------------------- You can loop through the list items by using a for loop: ### Example Print all items in the list, one by one: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ for x in thislist:\   print(x) Loop Through the Index Numbers ------------------------------ You can also loop through the list items by referring to their index number. Use the range() and len() functions to create a suitable iterable. ### Example Print all items by referring to their index number: thislist = \[\"apple\", \"banana\", \"cherry\"\]\ for i in range(len(thislist)):\   print(thislist\[i\]) The iterable created in the example above is \[0, 1, 2\]. Using a While Loop ------------------ You can loop through the list items by using a while loop. Use the len() function to determine the length of the list, then start at 0 and loop your way through the list items by referring to their indexes. Remember to increase the index by 1 after each iteration. ### Example Print all items, using a while loop to go through all the index numbers thislist = \[\"apple\", \"banana\", \"cherry\"\]\ i = 0\ while i \