Article    /    CC BY-SA 4.0    /    29 novembre 2021    /    : 89    /    Comments: None

Examples of how to get the absolute value of a dataframe column in pandas: [TOC] ### Create a dataframe with pandas Let's create first a dataframe with pandas: import pandas as pd import numpy as np data = np.random.randint(-10,10, size=(5,4)) df = pd.DataFrame(data=data,columns=['a','b','c','d']) returns for example a b c d 0 1 -4 5 -3 1 4 -9 -4 -7 2 -7 6 -10 9 3 3 -5 -2 1 4 5 -5 -8 4 ### Apply abs() to one dataframe column To ge ...

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Article    /    CC BY-SA 4.0    /    29 novembre 2021    /    : 273    /    Comments: None

Examples of how to create a datetime column from year, month and day columns in pandas: [TOC] ### Create a dataframe Let's first create a dataframe in pandas with 3 columns "Year", "Month" and "Day": import pandas as pd import random import itertools year_list = [] month_list = [] day_list = [] for combination in itertools.product([2015], [1,2,3,4], [5,15,25]): #print(combination) year_list.append( combination[0] ) month_list.append( combination[1] ) ...

Article    /    CC BY-SA 4.0    /    24 novembre 2021    /    : 137    /    Comments: None

Examples of how to store a condition expression in a string and use the built-in function eval() in python: [TOC] ### Store a conditional expression in a string Let's consider for example the following condition: x == 2 that we can store a a string representation like so: s = ' x == 2 ' Note that print(type(s)) returns here <class 'str'> ### Use the built-in function eval() To use that string representation of a condition, a solution is to use the buil ...

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To evaluate a two-variable function in python such as for example \begin{equation} f: (x_1,x_2) \rightarrow x_1 * \exp^{-(x_1^2+x_2^2)} \end{equation} a solution is to use the numpy function [meshgrid](https://docs.scipy.org/doc/numpy/reference/generated/numpy.meshgrid.html). [TOC] ### Using meshgrid Example from pylab import figure, cm import matplotlib.pyplot as plt import numpy as np def f(x1,x2): return x1 * np.exp(-(x1**2+x2**2)) x1_min = -2.0 x ...

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Examples of how to create a random sample with a reservoir using pandas in python [TOC] ### Create a list of dataframes To create a sample from a dataframe, a straightforward solution is to use the pandas's function called sample() (see the previous article: [How to select randomly (sample) the rows of a dataframe using pandas in python:](https://www.moonbooks.orgArticles/How-to-select-randomly-sample--the-rows-of-a-dataframe-using-pandas-in-python-/) ). However it does not work if you ...

Article    /    CC BY-SA 4.0    /    13 octobre 2021    /    : 1091    /    Comments: None

Examples of how to plot a heatmap on a global map with cartopy in python [TOC] ### Get gridded data Let's first get some data: import numpy as np data = np.loadtxt('https://raw.githubusercontent.com/benjamin-hg-marchant/teaching/main/datasets/modis_myd06_cpop_2d_hist_1b1_grid.txt') Note that data.shape gives (360, 180) Here longitude [-180,180] are on the first axis (axis =0) and latitude [-90: 90] on the second axis (axis =1): ### Plot a simple map with ca ...

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Examples of how to find the number of occurrences in a list of tuples in python : [TOC] ### Create a list of tuples Let's create a list of tuples: mylist = [ ('a', 'b', 'c',), ('a', 'b', 'c',), ('c', 'b', 'a',), ('a', 'b', ), ('a',) ] note that type(mylist) type(mylist[0]) gives <class 'list'> and <class 'tuple'> respectively ### Find occurrences of tuples To find occurrences of tuples in a list, a solution is to use counter from [coll ...

Article    /    CC BY-SA 4.0    /    08 octobre 2021    /    : 593    /    Comments: None

Examples of how to get the sign of a number (e.g -1 or +1) in python: [TOC] ### Get the sign of a number in python Let's consider the following variables in python: x1 = 42 x2 = -24 To get the sign of those numbers a solution is to use the function [copysign(x, y)](https://docs.python.org/3/library/math.html#math.copysign) from python's math module: import math math.copysign(1,x1) will returns: 1 while math.copysign(1,x2) will returns: ...

Article    /    CC BY-SA 4.0    /    29 septembre 2021    /    : 329    /    Comments: None

Examples of how to remove a character from a string in python: [TOC] ### Remove the n’th character from a string Let's consider the following sentence: sentence = "Hello, how are you ?" To remove the n’th character (for example 4), a solution is to convert the string into a list: sentence_list = list(sentence) remove the element corresponding to index = 4: sentence_list.pop(4) and convert back to a string sentence = "".join(sentence_list) prin ...

Article    /    CC BY-SA 4.0    /    28 septembre 2021    /    : 1177    /    Comments: None

There are multiple solutions to create a matrix of random numbers in python. Let's see some examples here: [TOC] ### Create a matrix of random integers To create a matrix of random integers, a solution is to use [numpy.random.randint](https://numpy.org/doc/stable/reference/random/generated/numpy.random.randint.html) import numpy as np data = np.random.randint(-10,10,10) print(data) gives [-4 9 4 0 -3 -4 8 0 0 -7] Another example with a matrix of ...

Article    /    CC BY-SA 4.0    /    28 septembre 2021    /    : 772    /    Comments: None

Examples of how to replace or remove a word in a string in python: [TOC] ### Create a string in python Let's first create a string in python sentence = "Hello How are you today ?" ### Replace or remove a word in a string To replace or remove a word in a string, a solution is to use [replace()](https://docs.python.org/3/library/stdtypes.html#str.replace) sentence = sentence.replace('Hello', "Hi") print(sentence) gives Hi How are you today ? Note that sent ...

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Examples of how to check if a character from a string is a letter, a special character or a whitespace in python: [TOC] ### Create a string in python Let's create for example the following sentence: sentence = " Hello World !" Note: type(sentence) returns str ### Check character type #### Check if a character is a letter To check if a character is a letter, a solution is to use [isalpha()](https://docs.python.org/3.8/library/stdtypes.html#str.isalnum) ...

Article    /    CC BY-SA 4.0    /    27 septembre 2021    /    : 785    /    Comments: None

Examples of how to shift values in a dataframe column with pandas: [TOC] ### Create a dataframe with pandas Let's first create a dataframe with pandas: import pandas as pd import numpy as np data = np.arange(1,31) data = data.reshape(10,3) df = pd.DataFrame(data=data,columns=['a','b','c']) print(df) gives then a b c 0 1 2 3 1 4 5 6 2 7 8 9 3 10 11 12 4 13 14 15 5 16 17 18 ...

Article    /    CC BY-SA 4.0    /    23 septembre 2021    /    : 1508    /    Comments: None

Examples of how to get unique values in a list in python: [TOC] ### Create a list in python Let's first create a list with unique values: mylist1 = ['a', 'b', 'c', 'd'] and then create another bigger list using those unique values: import random mylist2 = [random.choice(mylist1) for i in range(20)] mylist2 gives for example: ['d', 'c', 'd', 'd', 'b', 'd', 'a', 'b', 'b', 'd', 'a', 'b', 'a', 'b', 'a', 'd', 'b', ' ...

Article    /    CC BY-SA 4.0    /    22 septembre 2021    /    : 297    /    Comments: None

Examples of how to reverse a string in python [TOC] ### Create a string in python Let's create a simple string using python: s = 'abcdef' print(s) gives abcdef Note that type(s) gives str ### Reverse a string in python ### Example 1 A straightforward solution to reverse a string in python is to do s1 = s[::-1] print(s1) gives fedcba ### Example 2 Another example s2 = "".join(reversed(s)) print( ...

Article    /    CC BY-SA 4.0    /    22 septembre 2021    /    : 665    /    Comments: None

Examples of how to remove leading spaces in a string with python: [TOC] ### Create a string variable in python Let's create a string variable in python with 3 spaces at the begining and 4 at the end (after !): s = ' Hello World ! ' Note that: type( s ) gives str and len(s) gives 20 #### Count the number of spaces in a string To count the number of spaces in string s: space_count = 0 for c in s: if c.isspace(): space_ ...

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Examples of how to find common elements between two lists in python: [TOC] ### Create two lists in python Let's create a list in python list_a = ['e','0', 6, 'f', 4, 1, 'g'] and another list_b = ['a','d', 1, 'o', 8, 3, 'e'] ### Find common elements between two lists To find common elements between two lists, a solution is to use [Python Set intersection() Method](https://www.w3schools.com/python/ref_set_intersection.asp). To do so, let's first convert one of the t ...

Article    /    CC BY-SA 4.0    /    21 septembre 2021    /    : 849    /    Comments: None

Examples of how to create a list of US states and and an example of applications with python: [TOC] ### Create a list of US states Let's create a list of US states us_states = ['Alabama', 'Alaska', 'Arizona', 'Arkansas', 'California', 'Colorado', 'Connecticut', 'Washington DC', 'Delaware', 'Florida', 'Georgia', 'Hawaii', 'Idaho', 'Illinois', 'Indiana', 'Iowa', 'Kansas', 'Kentucky', 'Louisiana', 'Maine', 'Maryland', 'Massachusetts', 'Michigan', 'Minnesota', 'Mississippi', 'Missouri', ...

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Examples of how to convert a matrix of floats into a matrix of integers with numpy in python: [TOC] ### Create a matrix of floats with numpy Let's first create a matrix of random floats with numpy: import numpy as np data = np.random.rand(4,5) * 100.0 data returns for example array([[66.09124948, 68.46065421, 22.16897645, 77.84098663, 44.46296743], [16.41751266, 63.95374579, 86.87471262, 93.30928078, 59.99831054], [90.28468277, 72.2 ...

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Examples of how to round all the elements of a matrix with numpy in python: [TOC] ### Create a matrix of float with numpy Let's create first a matrix of float with numpy import numpy as np data = np.random.rand(4,5) * 100.0 gives for example array([[ 0.54545428, 62.89794368, 50.60548232, 57.47598712, 11.78324876], [57.10987983, 19.58684319, 7.67338724, 10.76866391, 90.78997325], [62.25706073, 42.60884215, 58.72412611, 91.84459902, 26.1276 ...

Article    /    CC BY-SA 4.0    /    19 septembre 2021    /    : 312    /    Comments: None

Examples of how to calculate the mean along a given axis with numpy in python [TOC] ### Create a random matrix with numpy import numpy as np data = np.random.randint(0,10,size=(3,3)) gives for exaple array([[4, 1, 9], [1, 6, 5], [9, 9, 5]]) ### Calculate the mean along an axis with numpy To calculate the mean along an axis with numpy, a solution is to use [numpy.mean](https://numpy.org/doc/stable/reference/generated/numpy.mean.html), ...

Article    /    CC BY-SA 4.0    /    19 septembre 2021    /    : 283    /    Comments: None

Examples of how to calculate the sum along an axis with numpy in python: [TOC] ### Create a random matrix with numpy Let's create a random matrix with numpy of size = (3,3): import numpy as np data = np.random.randint(0,5,size=(3,3)) returns for example array([[4, 1, 2], [2, 1, 1], [4, 0, 1]]) ### Sum along the first axis (axis=0) To sum along a given axis of a matrix with numpy, a solution is to use [numpy.sum](https://numpy.org/doc/stable/reference/gen ...