Get index of char in string python

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Get index of char in string python

NEWBEDEVPythonJavascriptLinuxCheat sheet Python has a in-built string method that does the work: index(). string.index(value, start, end) Where: Value: (Required) The value to search for. start: (Optional) Where to start the search. Default is 0. end: (Optional) Where to end the search. Default is to the end of the string. def character_index(): string = "Hello World! This is an example sentence with no meaning." match = "i" return string.index(match) print(character_index()) > 15 Let's say you need all the indexes where the character match is and not just the first one. The pythonic way would be to use enumerate(). def character_indexes(): string = "Hello World! This is an example sentence with no meaning." match = "i" indexes_of_match = [] for index, character in enumerate(string): if character == match: indexes_of_match.append(index) return indexes_of_match print(character_indexes()) # [15, 18, 42, 53] Or even better with a list comprehension: def character_indexes_comprehension(): string = "Hello World! This is an example sentence with no meaning." match = "i" return [index for index, character in enumerate(string) if character == match] print(character_indexes_comprehension()) # [15, 18, 42, 53] In this article, we will discuss how to fetch/access the first N characters of a string in python. This N can be 1 or 3 etc. Python string is a sequence of characters and each character in it has an index number associated with it. For example, we have a string variable sample_str that contains a string i.e. sample_str = "Hello World !!" Each character in this string has a sequence number, and it starts with 0 i.e. `H' has index 0 `e' has index 1 `l' has index 2 `l' has index 3 `o' has index 4 ` ` has index 5 `W' has index 6 `o' has index 7 `r' has index 8 `l' has index 9 `d' has index 10 `!' has index 11 `!' has index 12 In python String provides an [] operator to access any character in the string by index position. We need to pass the index position in the square brackets, and it will return the character at that index. This index position can be a positive or negative int value. Like, sample_str[i] will return a character at index i-th index position. Let's use it. Get the first character of a string in python As indexing of characters in a string starts from 0, So to get the first character of a string pass the index position 0 in the [] operator i.e. # Get first character of string i.e. char at index position 0 first_char = sample_str[0] print('First character : ', first_char) Output:First character : H It returned a copy of the first character in the string. You can use it to check its content or print it etc. In the above example, we fetched the first character or the string, but what if we want more like, get the first three characters of a string or first four, etc. Basically we want to access a substring of given length from the start of the string. How to do that? Get first N characters in a string In python, apart from index position, subscript operator, i.e. [] can accept a range too i.e. string[ start_index_pos: end_index_pos: step_size] It returns a slice the string, i.e. a substring. If we want to get more than one character out of a string i.e. fetch a substring, then we can pass these range elements in [] operator, start_index_pos: Index position, from where it will start fetching the characters, the default value is 0 end_index_pos: Index position till which it will fetch the characters from string, default value is the end of string step_size: Interval between each character, default value is 1. To get the first N characters of the string, we need to pass start_index_pos as 0 and end_index_pos as N i.e. sample_str[ 0 : N ] The value of step_size will be default i.e. 0. It will slice the string from 0th index to n-1-th index and returns a substring with first N characters of the given string. Let's use this, Get first three characters of a string in python # Get First 3 character of a string in python first_chars = sample_str[0:3] print('First 3 character : ', first_chars) Output:First 3 character : Hel We sliced the string from 0 the index position to (3 -1) index position and we got a substring containing the first three characters of the string. Get first four characters of a string in python # Get First 4 character of a string in python first_chars = sample_str[0:4] print('First 4 character : ', first_chars) Output:First 4 character : Hell We sliced the string from 0 the index position to (4 -1) index position, and we got a substring containing the first four characters of the string. IndexError: string index out of range While using [] operator, we need to be careful about out of range error i.e. if we try to access the index position in a string that doesn't exist, like a position that is larger than the size of the string, then it will give IndexError like this, sample_str = "Hello World !!" # Accessing out of range element causes error first_char = sample_str[20] It will give the error,IndexError: string index out of range Because we accessed index position 20 that doesn't exist, it is larger than the size of the string. Therefore, we should always check for the size of string before accessing the character based on the index position. Like this,sample_str = "Hello World !!" if len(sample_str) > 20: # Accessing out of range element causes error first_char = sample_str[20] else: print('Sorry out of range position') Output:Sorry out of range position The complete example is as follows,def main(): sample_str = "Hello World !!" print('**** Get first character of a String in python ****') # Get first character of string i.e. char at index position 0 first_char = sample_str[0] print('First character : ', first_char) print('**** Get first N characters of a String in python ****') print('** Get first 3 characters of a String in python **') # Get First 3 character of a string in python first_chars = sample_str[0:3] print('First 3 character : ', first_chars) print('** Get first 4 characters of a String in python **') # Get First 4 character of a string in python first_chars = sample_str[0:4] print('First 4 character : ', first_chars) print('*** Handle IndexError ***') sample_str = "Hello World !!" if len(sample_str) > 20: # Accessing out of range element causes error first_char = sample_str[20] else: print('Sorry out of range position') if __name__ == '__main__': main() Output:**** Get first character of a String in python **** First character : H **** Get first N characters of a String in python **** ** Get first 3 characters of a String in python ** First 3 character : Hel ** Get first 4 characters of a String in python ** First 4 character : Hell *** Handle IndexError *** Sorry out of range position Python string index() Function is determined if string substring occurs in a string (sentence) or in a substring of a string. This function is the same as find(), but throw an exception if str not found using index() function. In this tutorial, you will learn about the index() function and some examples of use. When you create a string in python, every string that you create Python under the hood, what it does assigns a number to each of the items of your string so it starts from 0. Syntax string.index(value, start, end) Parameter value ? string (substring) to search | Requiredstart ? Where to start the search otherwise Default is 0 | Optionalend ? Where to end the search. The default is to the end of the string | Optional Return Value Index if found otherwise throw an exception given str is not found. like this ? ValueError: substring not found Python string index() Function Examples This is a simple example only using value (search substring) in a sentence and print() the result in the console. sentence = 'Python programming tutorial.' result = sentence.index('programming') print("Substring index is :", result) Output: Substring index is: 7 Another example let's find a latter, the first occurrence of letter return the index value. Searching "p" (lowercase ) in a string sentence = 'Python programming tutorial.' result = sentence.index('p') print("index is:", result) Output : index is : 7 Note: Python is case sensitive language, that why first "P" (uppercase) latter ignored. index() function With start and end Arguments Now let's looks at with all arguments in index() function. sentence = 'Python programming tutorial.' # Substring is searched in 'gramming tutorial.' print(sentence.index('tutorial', 10)) # Substring is searched in 'gramming tuto' print(sentence.index('o', 10, -4)) # Substring is searched in 'programming' print(sentence.index('programming', 7, 18)) # Substring is searched in 'programming' print(sentence.index('easy', 7, 18)) Output: 19227...... ValueError: substring not found Python offers many ways to substring a string. It is often called `slicing'.It follows this template:string[start: end: step]Where,start: The starting index of the substring. The character at this index is included in the substring. If start is not included, it is assumed to equal to 0.end: The terminating index of the substring. The character at this index is NOT included in the substring. If end is not included, or if the specified value exceeds the string length, it is assumed to be equal to the length of the string by default.step: Every `step' character after the current character to be included. The default value is 1. If the step value is omitted, it is assumed to equal to 1.Templatestring[start:end]: Get all characters from index start to end-1string[:end]: Get all characters from the beginning of the string to end-1string[start:]: Get all characters from index start to the end of the stringstring[start:end:step]: Get all characters from start to end-1 discounting every step characterExamplesGet the first 5 characters of a stringstring = "freeCodeCamp" print(string[0:5])Output:> freeCNote: print(string[:5]) returns the same result as print(string[0:5])Get a substring of length 4 from the 3rd character of the stringstring = "freeCodeCamp" print(string[2:6])Output:> eeCoPlease note that the start or end index may be a negative number. A negative index means that you start counting from the end of the string instead of the beginning (i.e from the right to left). Index -1 represents the last character of the string, -2 represents the second to last character and so on...Get the last character of the stringstring = "freeCodeCamp" print(string[-1])Output:> pGet the last 5 characters of a stringstring = "freeCodeCamp" print(string[-5:])Output:> eCampGet a substring which contains all characters except the last 4 characters and the 1st characterstring = "freeCodeCamp" print(string[1:-4])Output:> reeCodeMore examplesstr = "freeCodeCamp" print str[-5:-2] # prints `eCa' print str[-1:-2] # prints `' (empty string)Get every other character from a stringstring = "freeCodeCamp" print(string[::2])Output:> feCdCm Python String find() is a function available in Python library to find the index of the first occurrence of a substring from the given string. The string find() function will return -1 instead of throwing an exception, if the specified substring is not present in the given string. In this Python string find() method tutorial, you will learn: Syntax of Python string find()The basic syntax of Python find() function is as follows: string.find(substring,start,end) Parameters for the find() method Here, are three parameters of the function String find() in Python:substring: The substring you want to search in the given string.start: (optional) The start value from where the search for substring will begin. By default, it is 0. end: (optional) The end value where the search for substring will end. By default, the value is the length of the string. Example of find() method with default valuesThe parameters passed to Python find() method are substring i.e the string you want to search for, start, and end. The start value is 0 by default, and the end value is the length of the string. In this example, we will use the method find() in Python with default values. The find() method will search for the substring and give the position of the very first occurrence of the substring. Now, if the substring is present multiple times in the given string, still it will return you the index or position of the first one. Example: mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Tutorials is at:", mystring.find("Tutorials")) Output: The position of Tutorials is at: 12 Example of find() using start argumentYou can search the substring in the given string and specify the start position, from where the search will begin. The start parameter can be used for the same. The example will specify the start position as 15, and the find() in Python method will begin the search from position 15. Here, the end position will be the length of the string and will search till the end of the string from 15 positions onwards. Example: mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Tutorials is at:", mystring.find("Tutorials", 20)) Output: The position of Tutorials is at 48 Example of find() using start and end argumentsUsing the start and end parameter, we will try to limit the search, instead of searching the entire string. Example: mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Tutorials is at:", mystring.find("Tutorials", 5, 30)) Output: The position of Tutorials is at 12 Example of find() method To find the position of a given substring in a stringWe know that find() helps us to find the index of the first occurrence of substring. It returns -1 if the substring is not present in the given string. The example below shows the index when the string is present and -1 when we don't find the substring we are searching for. Example: mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Best site is at:", mystring.find("Best site", 5, 40)) print("The position of Guru99 is at:", mystring.find("Guru99", 20)) Output: The position of Best site is at: 27 The position of Guru99 is at: -1 Python string rfind()The Python function rfind() is similar to find() function with the only difference is that rfind() gives the highest index for the substring given and find() gives the lowest i.e the very first index. Both rfind() and find() will return -1 if the substring is not present. In the example below, we have a string "Meet Guru99 Tutorials Site. Best site for Python Tutorials!" and will try to find the position of substring Tutorials using find() and rfind(). The occurrence of Tutorials in the string is twice. Here is an example where both find() and rfind() are used. mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Tutorials using find() : ", mystring.find("Tutorials")) print("The position of Tutorials using rfind() : ", mystring.rfind("Tutorials")) Output: The position of Tutorials using find() : 12 The position of Tutorials using rfind() : 48 The output shows that find() gives the index of the very first Tutorials substring that it gets, and rfind() gives the last index of substring Tutorials. Python string index()The Python string index() is function that will give you the position of the substring given just like find(). The only difference between the two is, index() will throw an exception if the substring is not present in the string and find() will return -1. Here is a working example that shows the behaviour of index() and find(). mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Tutorials using find() : ", mystring.find("Tutorials")) print("The position of Tutorials using index() : ", mystring.index("Tutorials")) Output: The position of Tutorials using find() : 12 The position of Tutorials using index() : 12 We are getting same position for both find() and index(). Let us see an example when the substring given is not present in the string. mystring = "Meet Guru99 Tutorials Site.Best site for Python Tutorials!" print("The position of Tutorials using find() : ", mystring.find("test")) print("The position of Tutorials using index() : ", mystring.index("test")) Output: The position of Tutorials using find() : -1 Traceback (most recent call last): File "task1.py", line 3, in print("The position of Tutorials using index() : ", mystring.index("test")) ValueError: substring not found In the above example, we are trying to find the position of substring "test". The substring is not present in the given string, and hence using find(), we get the position as -1, but for index(), it throws an error as shown above. To find the total occurrence of a substringTo find the total number of times the substring has occurred in the given string we will make use of find() function in Python. Will loop through the string using for-loop from 0 till the end of the string. Will make use of startIndex parameter for find(). Variables startIndex and count will be initialized to 0. Inside for ?loop will check if the substring is present inside the string given using find() and startIndex as 0. The value returned from find() if not -1, will update the startIndex to the index where the string is found and also increment the count value. Here is the working example: my_string = "test string test, test string testing, test string test string" startIndex = 0 count = 0 for i in range(len(my_string)): k = my_string.find('test', startIndex) if(k != -1): startIndex = k+1 count += 1 k = 0 print("The total count of substring test is: ", count ) Output: The total count of substring test is: 6 SummaryThe Python string find() method helps to find the index of the first occurrence of the substring in the given string. It will return -1 if the substring is not present.The parameters passed to Python find substring method are substring i.e the string you want to search for, start, and end. The start value is 0 by default, and the end value is the length of the string. You can search the substring in the given string and specify the start position, from where the search will begin. The start parameter can be used for the same.Using the start and end parameter, we will try to limit the search, instead of searching the entire string.The Python function rfind() is similar to find() function with the only difference is that rfind() gives the highest index for the substring given and find() gives the lowest i.e the very first index. Both rfind() and find() will return -1 if the substring is not present.The Python string index() is yet another function that will give you the position of the substring given just like find(). The only difference between the two is, index() will throw an exception if the substring is not present in the string and find() will return -1.We can make use of find() to find the count of the total occurrence of a substring in a given string.Page 2Python main function is a starting point of any program. When the program is run, the python interpreter runs the code sequentially. Main function is executed only when it is run as a Python program. It will not run the main function if it imported as a module. What is the def main() function in Python? To understand this, consider the following example code def main(): print ("hello world!") print ("Guru99") Here, we got two pieces of print- one is defined within the main function that is "Hello World" and the other is independent, which is "Guru99". When you run the function def main (): Only "Guru99" prints out and not the code "Hello World." It is because we did not declare the call function "if__name__== "__main__". It is important that after defining the main function, you call the code by if__name__== "__main__" and then run the code, only then you will get the output "hello world!" in the programming console. Consider the following code def main(): print("hello world!") if __name__ == "__main__": main() print("Guru99") Guru99 is printed in this case. Here is the explanation, When Python interpreter reads a source file, it will execute all the code found in it. When Python runs the "source file" as the main program, it sets the special variable (__name__) to have a value ("__main__"). When you execute the main function in python, it will then read the "if" statement and checks whether __name__ does equal to __main__. In Python "if__name__== "__main__" allows you to run the Python files either as reusable modules or standalone programs. The __name__ variable and Python ModuleTo understand the importance of __name__ variable in Python main function method, consider the following code: def main(): print("hello world!") if __name__ == "__main__": main() print("Guru99") print("Value in built variable name is: ",__name__) Now consider, code is imported as a module import MainFunction print("done") Here, is the code explanation: Like C, Python uses == for comparison while = for assignment. Python interpreter uses the main function in two ways direct run: __name__=__main__ if statement == True, and the script in _main_will be executed import as a module __name__= module's filename if statement == false, and the script in __main__ will not be executed When the code is executed, it will check for the module name with "if." This mechanism ensures, the main function is executed only as direct run not when imported as a module. Above examples are Python 3 codes, if you want to use Python 2, please consider following code def main(): print "Hello World!" if __name__== "__main__": main() print "Guru99" In Python 3, you do not need to use if__name. Following code also works def main(): print("Hello World!") main() print("Guru99") Note: Make sure that after defining the main function, you leave some indent and not declare the code right below the def main(): function otherwise, it will give indent error. Krishna has over 15 years of professional software development and testing experience, as an individual contributor, technical lead, and today as CEO of Guru99.

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