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Python Numerical Methods

This notebook contains an excerpt from the Python Programming and Numerical Methods - A Guide for Engineers and Scientists , the content is also available at Berkeley Python Numerical Methods .

The copyright of the book belongs to Elsevier. We also have this interactive book online for a better learning experience. The code is released under the MIT license . If you find this content useful, please consider supporting the work on Elsevier or Amazon !

< 2.0 Variables and Basic Data Structures | Contents | 2.2 Data Structure - Strings >

Variables and Assignment ¶

When programming, it is useful to be able to store information in variables. A variable is a string of characters and numbers associated with a piece of information. The assignment operator , denoted by the “=” symbol, is the operator that is used to assign values to variables in Python. The line x=1 takes the known value, 1, and assigns that value to the variable with name “x”. After executing this line, this number will be stored into this variable. Until the value is changed or the variable deleted, the character x behaves like the value 1.

TRY IT! Assign the value 2 to the variable y. Multiply y by 3 to show that it behaves like the value 2.

A variable is more like a container to store the data in the computer’s memory, the name of the variable tells the computer where to find this value in the memory. For now, it is sufficient to know that the notebook has its own memory space to store all the variables in the notebook. As a result of the previous example, you will see the variable “x” and “y” in the memory. You can view a list of all the variables in the notebook using the magic command %whos .

TRY IT! List all the variables in this notebook

Note that the equal sign in programming is not the same as a truth statement in mathematics. In math, the statement x = 2 declares the universal truth within the given framework, x is 2 . In programming, the statement x=2 means a known value is being associated with a variable name, store 2 in x. Although it is perfectly valid to say 1 = x in mathematics, assignments in Python always go left : meaning the value to the right of the equal sign is assigned to the variable on the left of the equal sign. Therefore, 1=x will generate an error in Python. The assignment operator is always last in the order of operations relative to mathematical, logical, and comparison operators.

TRY IT! The mathematical statement x=x+1 has no solution for any value of x . In programming, if we initialize the value of x to be 1, then the statement makes perfect sense. It means, “Add x and 1, which is 2, then assign that value to the variable x”. Note that this operation overwrites the previous value stored in x .

There are some restrictions on the names variables can take. Variables can only contain alphanumeric characters (letters and numbers) as well as underscores. However, the first character of a variable name must be a letter or underscores. Spaces within a variable name are not permitted, and the variable names are case-sensitive (e.g., x and X will be considered different variables).

TIP! Unlike in pure mathematics, variables in programming almost always represent something tangible. It may be the distance between two points in space or the number of rabbits in a population. Therefore, as your code becomes increasingly complicated, it is very important that your variables carry a name that can easily be associated with what they represent. For example, the distance between two points in space is better represented by the variable dist than x , and the number of rabbits in a population is better represented by nRabbits than y .

Note that when a variable is assigned, it has no memory of how it was assigned. That is, if the value of a variable, y , is constructed from other variables, like x , reassigning the value of x will not change the value of y .

EXAMPLE: What value will y have after the following lines of code are executed?

WARNING! You can overwrite variables or functions that have been stored in Python. For example, the command help = 2 will store the value 2 in the variable with name help . After this assignment help will behave like the value 2 instead of the function help . Therefore, you should always be careful not to give your variables the same name as built-in functions or values.

TIP! Now that you know how to assign variables, it is important that you learn to never leave unassigned commands. An unassigned command is an operation that has a result, but that result is not assigned to a variable. For example, you should never use 2+2 . You should instead assign it to some variable x=2+2 . This allows you to “hold on” to the results of previous commands and will make your interaction with Python must less confusing.

You can clear a variable from the notebook using the del function. Typing del x will clear the variable x from the workspace. If you want to remove all the variables in the notebook, you can use the magic command %reset .

In mathematics, variables are usually associated with unknown numbers; in programming, variables are associated with a value of a certain type. There are many data types that can be assigned to variables. A data type is a classification of the type of information that is being stored in a variable. The basic data types that you will utilize throughout this book are boolean, int, float, string, list, tuple, dictionary, set. A formal description of these data types is given in the following sections.

CS-IP-Learning-Hub

Important Questions and Notes

String Assignment in Python Set 1- Important Practice Exercise

String assignment in python.

CHAPTER: STRING IN PYTHON

STRING ASSIGNMENT IN PYTHON SET – 1

Time: 30 min                                                                                   M.M. – 20

Instructions:

• All Questions are compulsory
• Q1 to Q6 carry 1 mark
• Q7 to Q10 carry 2 marks
• Q11 to Q12 carry 3 marks

Q1. Write the code to create an empty string named “str”.

Q2. name the function which count the total number of characters in a string., q3. fill the index value in place of ‘’ so that the output should come ‘o’,        str = “csiplearninghub.com”,        print(str[]), q4. write the function which removes the spaces from the left side of the string., q5. what type of error is returned by the following statement,         str = “csiplearninghub.com”,         str[7] = ‘e’, q6. what do you mean by concatenation of string, q7. write a program to accept a string and display in reverse., q8. write a function count( ) in python which takes string as argument and return total number of words., q9. write the output of the following:.

• print(“Suman” > “Sumati”)
• print(“amit”.isalpha())

Q10. Write the output of the following code :

str = “CSIPLearningHuB”

newstr = ” “

for i in range(len(str)):

    if str[i].isupper():

        newstr =newstr + str[i].lower()

    if str[i].islower():

        newstr =newstr + str[i].upper()

print(newstr)

Q11. Write a program in python that accept a string from the user and display the following:

• Total number of vowels
• Total number of digits
• Total number of words.

Q12. Write the output of the following:

print(str[: :-1])

print(str[-7: -1: 2])

print(str[3 : 14 : 3])

Disclaimer : I tried to give you the correct questions of “String Assignment in Python ” , but if you feel that there is/are mistakes in the questions of “String Assignment in Python “ given above, you can directly contact me at [email protected] . Also Share your feedback so that I can give better content to you .

100 Practice Questions on String

90+ Practice Questions on List

50+ Output based Practice Questions

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70 Practice Questions on Loops

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Answer to Question #188200 in Python for Hari nadh babu

Given a string, write a program to print a secret message that replaces characters with numbers 'a' with 1, 'b' with 2, ..., 'z' with 26 where characters are separated by '-'.

Note: You need to replace both uppercase and lowercase characters. You can ignore replacing all characters that are not letters.

The input will be a string in the single line containing spaces and letters (both uppercase and lowercase).

The output should be a single line containing the secret message. All characters in the output should be in lower case.

For Example -

16-25-20-8-15-14

Foundations

6-15-21-14-4-1-20-9-15-14-19

python learning

16-25-20-8-15-14 12-5-1-18-14-9-14-7

Note:- there is a space between two strings that means 16-25-20-8-15-14 12-5-1-18-14-9-14-7

We want given both three inputs they can get both three outputs we code was run one by one input and output

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Ask your question, related questions.

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Many times, while working with strings we come across this issue in which we need to get all the numeric occurrences. This type of problem generally occurs in competitive programming and also in web development. Let’s discuss certain ways in which this problem can be solved in Python . 

Extract Numbers from a String in Python

Below are the methods that we will cover in this article:

• Using List comprehension and isdigit() method
• Using re.findall() method
• Using isnumeric() method
• Using Filter() function
• Using a loop and isdigit() method
• Using str.translate() with str.maketrans() 
• Using numpy module

Extract numbers from string using list comprehension and isdigit() method

This problem can be solved by using the split function to convert string to list and then the list comprehension which can help us iterate through the list and isdigit function helps to get the digit out of a string. 

Time Complexity: O(n), where n is the number of elements in the input string. Auxiliary Space: O(n), where n is the number of numbers in the input string.

Extract Digit from string using re.findall() method

This particular problem can also be solved using Python regex, we can use the findall function to check for the numeric occurrences using a matching regex string. 

Extract Interger from string using isnumeric() method

In Python, we have isnumeric function which can tell the user whether a particular element is a number or not so by this method we can also extract the number from a string.

Time Complexity : O(N) Auxiliary Space : O(N)

Extract Digit from string using Filter() function

First, we define the input string then print the original string and split the input string into a list of words using the split() method. Use the filter() function to filter out non-numeric elements from the list by applying the lambda function x .isdigit() to each elementConvert the remaining elements in the filtered list to integers using a list comprehension

Print the resulting list of integers

Time complexity: O(n), where n is the length of the input string. The split() method takes O(n) time to split the input string into a list of words, and the filter() function takes O(n) time to iterate over each element in the list and apply the lambda function. The list comprehension takes O(k) time, where k is the number of elements in the filtered list that are digits, and this is typically much smaller than n. Therefore, the overall time complexity is O(n).

Auxiliary space complexity: O(n), as the split() method creates a list of words that has the same length as the input string, and the filter() function creates a filtered list that can be up to the same length as the input list. The list comprehension creates a new list of integers that is typically much smaller than the input list, but the space complexity is still O(n) in the worst case. Therefore, the overall auxiliary space complexity is O(n)

Extract Interger from string using a loop and isdigit() method

Use a loop to iterate over each character in the string and check if it is a digit using the isdigit() method. If it is a digit, append it to a list.

Time complexity: O(n), where n is the length of the string. Auxiliary space: O(k), where k is the number of digits in the string.

Extract Numbers from string using str.translate() with str.maketrans() 

Define the input string then Initialize a translation table to remove non-numeric characters using str. maketrans() . Use str. translate() with the translation table to remove non-numeric characters from the string and store the result in a new string called numeric_string . Use str. split() to split the numeric_string into a list of words and store the result in a new list called words. Initialize an empty list called numbers to store the resulting integers and then iterate over each word in the list of words. Check if the word is a numeric string using str. isdigit() .If the word is a numeric string, convert it to an integer using int() and append it to the list of numbers.

Print the resulting list of integers.

Below is the implementation of the above approach:

Time complexity: O(n), where n is the length of the input string. The str.translate() method and str.split() method take O(n) time, and iterating over each word in the list of words takes O(k) time, where k is the number of words in the list that are numeric strings. Auxiliary Space: O(n), as we create a new string and a new list of words that each have the same length as the input string, and we create a new list of integers that has a maximum length of k, where k is the number of words in the list that are numeric strings.

Extract Numbers from string using numpy module

Initialize the string test_string then split the string into a list of words using the split method and create a numpy array x from the resulting list. Use np.char .isnumeric to create a boolean mask indicating which elements of x are numeric. Use this boolean mask to index x and extract only the numeric elements. Convert the resulting array of strings to an array of integers using astype.

Print the resulting array of integers.

Time complexity:  O(n), where n is the length of the original string test_string. This is because the split method takes O(n) time to split the string into a list of words, and the np.char.isnumeric method takes O(n) time to create the boolean mask. The remaining operations take constant time.

Auxiliary Space: O(n), where n is the length of the original string test_string. This is because we create a numpy array x to store the words of the string, which takes O(n) space. The space used by the resulting numpy array of integers is also O(n), since it contains all the numeric elements of the string.

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Validation in Python

Problem statement:.

We are given a string, we need to check whether the string is a valid username or not. To be a valid username, the string should satisfy the following conditions:

• The string should only contain letters, numbers, or underscore(s).
• It should not start with a number.
• It should not end with an underscore.
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