- Join List with Separator
- 4 Answers 4
- Python String join() Explained
- 1. Syntax of join()
- 1.1 Parameters of join()
- 2. Join String
- 3. Join List of Strings
- 4. Join a Set of Strings
- 5. Joining Numbers into String
- 6. Join Dictionaries
- Conclusion
- You may also like reading:
- Python String join() function
- Understanding Python string join() method
- Python numpy.join() method
- Python Pandas str.join() method
- Summary
- Conclusion
- References
Join List with Separator
\$\begingroup\$ Ok in general appending N arbitrary strings iteratively would be O(N^2) in most languages and implementations, because it requires a malloc/realloc() call in each loop, but cPython special-cases this, so it’s only N*O(1) = O(N). In native Python. string.join or sep.join are faster because they’re one Python call, not N. See Is the time-complexity of iterative string append actually O(n^2), or O(n)? \$\endgroup\$
\$\begingroup\$ @bhathiya-perera «delimiter» is broader than «separator», and is also technical jargon. \$\endgroup\$
4 Answers 4
Strings in Python are immutable, and so ‘string a’ + ‘string b’ has to make a third string to combine them. Say you want to clone a string, by adding each item to the string will get \$O(n^2)\$ time, as opposed to \$O(n)\$ as you would get if it were a list.
And so, the best way to join an iterable by a separator is to use str.join .
If you want to do this manually, then I’d accept the \$O(n^2)\$ performance, and write something easy to understand. One way to do this is to take the first item, and add a separator and an item every time after, such as:
def join(iterator, seperator): it = map(str, iterator) seperator = str(seperator) string = next(it, '') for s in it: string += seperator + s return string \$\begingroup\$ Very nice, thanks! Yeah I know about str.join , I was just implementing something slightly different and wondered how to do it nicely. I like your approach with using next at the beginning! Do you know where I can find the source of str.join though? Google didn’t help.. \$\endgroup\$
\$\begingroup\$ @fabian789 The source for str.join is probably this. It looks about right, and is written in C. \$\endgroup\$
Let’s take that step by step:
def join(l, sep): out_str = '' for i, el in enumerate(l): Here, why do you need the enumerate ? You could write for el in l:
.format is not super efficient, there are other methods. You can have a look at This question for some researches and benchmarks on performances.
This makes little sense for l = [] if len(sep) > 1 . »[:-1] is valid, and returns » , because python is nice, but it is not a very good way of getting around that limit case.
In general, adding something just to remove it at the end is not great.
Creating an iter , looking at the first value, then adding the rest, as it has been suggested in other answers, is much better.
I would also recommend writing some unit tests, so that you can then play around with the implementation, and stay confident that what you write still works.
Typically, you could write:
# Empty list join([], '') == '' # Only one element, -> no separator in output join(['a'], '-') == 'a' # Empty separator join(['a', 'b'], '') == 'ab' # "Normal" case join(['a', 'b'], '--') == 'a--b' # ints join([1, 2], 0) == '102' Python String join() Explained
The Python str.join() method is used to join elements or values from a sequence object (list, set, tuple e.t.c) into a string by comma, space, or any custom separator. This method is called on the separator string and passed the sequence of strings as an argument and returns a string after joining.
1. Syntax of join()
Following is the syntax of the string join() method.
1.1 Parameters of join()
- The join() method takes the iterable objects as an argument, these include List, Tuple, String, Dictionary, and Set.
- The type of iterable should hold string values. Using it on an integer will return an error.
2. Join String
When you join a string with a separator in python, it separates each character in a string by the specified delimiter and returns a new string. Here is an example.
3. Join List of Strings
Using the list of strings as an argument to the python str.join() method, you can join the list of strings into a single string by separating each string by the specified separator. The below example uses the space separator to just list of string values. The join() is most commonly used to convert a list to a string.
Note that you should have a list with strings, using join() on a list with numbers returns an error.
In this example, the join() method is called on the string ” ” (a space), and the list [‘Welcome’,’to’,’spark’,’by’,’examples’] is passed as an argument. This joins the strings in the list, using the space as a separator, to create the final string “Welcome to spark by examples”.
Let’s see another example of joining the list of strings by a comma separator in Python.
4. Join a Set of Strings
Similarly, to join a set of strings in python use the set as an argument to the join() method. The below example uses the space separator to convert a set to a string. Note that you should have a set with strings, using join() on a set with numbers also returns an error.
print(myList) # Use join() with set of strings myString = " ".join(myList) print(myString) # Output: # # examples to spark Welcome by 5. Joining Numbers into String
When you use join() with an iterable object, you need to have a list with the string type, in case you have numbers and you wanted to join, use the map() along with the join().
6. Join Dictionaries
Finally, let’s join the keys of the dictionaries into a single string by a comma separator. You can’t use this to merge dictionaries.
Conclusion
In this article, you have learned the Python str.join() method that is used to join a sequence of strings into a single string by comma, space, or any custom separator. This method can be used only on with sequence of strings, if you wanted to join a sequence of integers into a string, use map().
You may also like reading:
Python String join() function
In this article, we will have a look at the Python String join() function. As the name suggests, it is used to join strings together and works for the data of string type.
Understanding Python string join() method
Python String has various in-built functions to deal with the string type of data.
The join() method basically is used to join the input string by another set of separator/string elements. It accepts iterables such as set, list, tuple, string, etc and another string(separable element) as parameters.
The join() function returns a string that joins the elements of the iterable with the separator string passed as an argument to the function.
separator-string.join(iterable)
inp_str='JournalDev' insert_str='*' res=insert_str.join(inp_str) print(res)
inp_str='PYTHON' insert_str='#!' res=insert_str.join(inp_str) print(res)
Hey, Folks! The most important point to be taken into consideration is that the join() function operates only on string type input values. If we input any of the parameters of non-string type, it raises a TypeError exception .
inp_str=200 #non-string type input insert_str='S' res=insert_str.join(inp_str) print(res)
In the above example, the separator string i.e. insert_str has been assigned an integer value. Thus, it would raise a TypeError exception.
--------------------------------------------------------------------------- TypeError Traceback (most recent call last) in 1 inp_str=200 #non-string type input 2 insert_str='S' ----> 3 res=insert_str.join(inp_str) 4 print(res) TypeError: can only join an iterable
Python string join() method with list as an iterable:
inp_lst=['10','20','30','40'] sep='@@' res=sep.join(inp_lst) print(res)
In the above example, the separator string “@@” gets joined to every element of the input list i.e. inp_lst.
Python join() method with set an iterable:
In the above example, the separator string “**” and “
Python join() method with dictionary as an iterable:
Python string join() method can also be applied to the dictionary as an iterable.
But, the important thing to note is that the join() method works only on the keys of the dictionary data structure and not the values associated with the keys.
inp_dict= <'Python':'1','Java':'2','C++':'3'>sep='##' res=sep.join(inp_dict) print(res)
As seen in the above example, the join() method only considers the keys of the dict for manipulation. It completely neglects the values of the dict.
inp_dict= <'Python':1,'Java':2,'C++':3>sep='##' res=sep.join(inp_dict) print(res)
In the above example, the values in the dict are of non-string type. Still, it would cause no error to the execution of the code because join() method deals only with the keys of the dictionary.
inp_dict= <1:'Python',2:'Java',3:'C++'>sep='##' res=sep.join(inp_dict) print(res)
The above code returns a TypeError because, the key values associated with the dictionary are of non-string type.
TypeError Traceback (most recent call last) in 1 inp_dict= <1:'Python',2:'Java',3:'C++'>2 sep='##' ----> 3 res=sep.join(inp_dict) 4 print(res) TypeError: sequence item 0: expected str instance, int found
Python numpy.join() method
Python NumPy module has got in-built functions to deal with the string data in an array.
The numpy.core.defchararray.join(sep-string,inp-arr) is used to join the elements of the array with the passed separator string as an argument.
It accepts an array containing string type elements and separator string as arguments and returns an array containing elements separated by the input separator string (delimiter).
numpy.core.defchararray.join(separator-string,array)
import numpy as np inp_arr=np.array(["Python","Java","Ruby","Kotlin"]) sep=np.array("**") res=np.core.defchararray.join(sep,inp_arr) print(res) In the above example, we have generated an array out of the passed list elements using numpy.array() method. Further, by using the join() function, it joins the string “**” to every element of the array.
import numpy as np inp_arr=np.array(["Python","Java","Ruby","Kotlin"]) sep=np.array(["**","++","&&","$$"]) res=np.core.defchararray.join(sep,inp_arr) print(res)
In the above example, we have used a different separate string for every element of the array. The only condition stays that the count of the separable string(delimiter) in the array should match with the number of elements in the input array.
Python Pandas str.join() method
Python Pandas module has in-built pandas.str.join() method to join the elements of the data-set with the provided delimiter.
The pandas.str.join() method works on the particular column(data) values of the data set or input series and returns the series with joined data items with the separator string or delimiter.
Series.str.join(delimiter or separator-string)
Input .csv file: Book1.csv
import pandas info=pandas.read_csv("C:\\Book1.csv") info["Name"]=info["Name"].str.join("||") print(info) In the above example, we have used pandas.read_csv() method to read the contents of the data set. Further, we join the separator string i.e “||” to the data values of the column “Name” of the input data set.
Name Age 0 J||i||m 21 1 J||e||n||n||y 22 2 B||r||a||n 24 3 S||h||a||w||n 12 4 R||i||t||i||k 26 5 R||o||s||y 24 6 D||a||n||n||y 25 7 D||a||i||s||y 15 8 T||o||m 27
Summary
- The join() method is used to join elements or iterables of string type with a string delimiter element.
- It is mandatory for the arguments: iterable elements and the delimiter to be of string type .
- Moreover, Python join() method can also be applied to iterables such as a set, list, dict, etc.
- The join() method raises a TypeError exception if the delimiter or the input iterable contains elements of non-string type.
Conclusion
Thus, in this article, we have understood the working of Python String join() method with different iterables such as a set, list, tuple, dict, etc.