Make a Face with D3.js and SVG

This tutorial is a good introduction to using D3.js with SVG. It will cover selecting DOM elements with D3, creating DOM elements with D3, customizing them, and more.

At the end of the tutorial, you will have created a smiley face that looks like this:

Loading D3.js

Load D3.js from CDN by adding the following script tag to the header of your index.html file: 

Now, you can check to make sure that D3 was successfully added to your project by simply typing “d3” in the console while running your html file. It should output the d3 object, which looks like this:

Now, we can use D3 to manipulate the DOM by using the d3 object in our JavaScript code. Here’s an example:'div');

Creating a face using SVG and D3

Make an SVG in your index.html file:

<svg width="500" height="250"></svg>

In your app.js file, get access to the SVG element you just created using d3:

const svg ='svg');

Creating the Head

Now, add a circle to the SVG:

const circle = svg.append('circle');

Now, we will give the circle a radius and (x, y) position for its center. We will use D3 method chaining:

    .attr('r', 250 / 2)   // radius
    .attr('cx', 500 / 2)  // center x
    .attr('cy', 250 / 2); // center y

I set the radius and center-y values to be half of the height of the SVG and the center-x to be half of the width.

Instead of hard-coding the values in, we can create variables for these width and height values in our JavaScript. To access the width and height values from the SVG in our index.html file, we can use svg.attr(). If you call svg.attr() without a second attribute, it returns the value of the attribute that you specify:

const width = svg.attr('width')

SVG attributes are strings by default, so we can parse the value into a float using parseFloat:

const width = parseFloat(svg.attr('width'));

You can also use the unary operator to parse into a float:

const height = +svg.attr('height');

Now, we can use the variables instead of hard-coded values to define our SVG elements:

    .attr('r', height / 2)   
    .attr('cx', width / 2)
    .attr('cy', height / 2); 

We can also add fill and stroke attributes to give the circle a fill color and outline color:

const circle = svg.append('circle') 
    .attr('r', height / 2)    
    .attr('cx', width / 2)  
    .attr('cy', height / 2)
    .attr('fill', 'yellow')
    .attr('stroke', 'black')

Now, we have the circle for the head:

Creating the Eyes and Mouth

We will make the eyes using circles, similar to how we made the head:

const leftEye = svg.append('circle')
    .attr('r', 10)    
    .attr('cx', width / 2 - 40)     
    .attr('cy', height / 2 - 40)
    .attr('fill', 'black')

const rightEye = svg.append('circle')
    .attr('r', 10)    
    .attr('cx', width / 2 + 40)     
    .attr('cy', height / 2 - 40)
    .attr('fill', 'black')

As you can see, we are defining the leftEye and rightEye variables in the same line in which we are method chaining the attributes.

For the mouth, we will create a group element and add a transform property to the group element to translate it to the center of the face:

const g = svg.append('g')    
    .attr('transform', `translate(${width / 2}, ${height / 2})`);

Then, we will use a d3.arc:

const mouth = g.append('path')
    .attr('d', d3.arc()({     
        innerRadius: 40,
        outerRadius: 50, 
        startAngle: Math.PI / 2, 
        endAngle: Math.PI * 3/2

Group Elements

Instead of constantly repeating width / 2 and height / 2 to move elements to the center, we can put everything in the group element so that we can leverage that fact that it’s translated by (width/2) and (height/2).

This update doesn’t change the look or functionality, but it just makes our code a little bit cleaner.

First, move the group element to the top of the screen, under the width and height variable declarations.

Then, append the face, eye and mouth elements to the group element instead of to the svg element directly.

const circle = g.append('circle') 

Now, we can remove the cx and cy attributes from the face because it is put in the center by default because its parent, g, is translated into the center:

const circle = g.append('circle') 
    .attr('r', height / 2)    
    .attr('fill', 'yellow')
    .attr('stroke', 'black')

We can also update the eyes:

const leftEye = g.append('circle')
    .attr('r', 10)    
    .attr('cx', -40)     
    .attr('cy', -40)
    .attr('fill', 'black')

const rightEye = g.append('circle')
    .attr('r', 10)    
    .attr('cx', 40)     
    .attr('cy', -40)
    .attr('fill', 'black')

Final Code

The final code is at

The final code in index.html:

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script src="" defer></script> 
    <script src="app.js" defer></script>
    <title>Let's Make a Face </title>
    <svg width="500" height="250"></svg>

The final code in app.js:

const svg ='svg');

const width = parseFloat(svg.attr('width')); 
const height = +svg.attr('height'); 

const g = svg.append('g')  
    .attr('transform', `translate(${width / 2}, ${height / 2})`);

const circle = g.append('circle') 
    .attr('r', height / 2)    
    .attr('fill', 'yellow')
    .attr('stroke', 'black')

const leftEye = g.append('circle')
    .attr('r', 15)    
    .attr('cx', -50)     
    .attr('cy', -40)
    .attr('fill', 'black')

const rightEye = g.append('circle')
    .attr('r', 15)    
    .attr('cx', 50)     
    .attr('cy', -40)
    .attr('fill', 'black')

const mouth = g.append('path')
    .attr('d', d3.arc()({     
        innerRadius: 70,
        outerRadius: 80, 
        startAngle: Math.PI / 2, 
        endAngle: Math.PI * 3/2

How to Connect to a Remote Server with SSH on VSCode

Install VSCode

Please check the VSCode homepage.

Install remote-ssh extension in VSCode

  1. Open the VSCode. On the left, there’s an Extension icon.
  2. Search for remote-ssh. Choose the first one and click install.
  3. You may need to reload VSCode to let the extension initialize.

Connect to the remote server

  1. Click View > Command Palette… or use the shortcut cmd+shift+p for Mac and ctrl+shift+p Windows. Type ssh and choose remote-ssh: connect to host.

2. Type the URL of the remote server

3. After that, it will open a new VSCode window.

Add your workspace

  1. After connect to the remote server, we need to cd to our workspace. Call your Command Palette again and type folder. Choose Add folder to your workspace in my case and enter your path to workspace.

Overlapping Histograms with Matplotlib Library in Python

In this tutorial, you will learn how to plot overlapping histograms on the same graph. This is helpful when you want to show a comparison between two sets of data.

Step 1: Import the matplotlib library and matplotlib.pyplot interface

import pandas as pd

import matplotlib
%matplotlib inline
import matplotlib.pyplot as plt

Step 2: Load the dataset

baby_df = pd.read_csv('baby.csv')

Step 3: Plot overlapping histograms

# Split the dataframe by column value
smoker_df = baby_df.loc[baby_df['Maternal Smoker'] == True]
nonsmoker_df = baby_df.loc[baby_df['Maternal Smoker'] == False]

# Generate histogram plot
         label='Maternal Smokers BMI')
         label='Maternal Non-smokers BMI')
plt.legend(loc='upper right')
plt.title('Mother BMI for smokers and non-smokers')

We’ve generated overlapping histograms! But, in this graph, it’s hard to see the blue histogram. We can give the histograms an opacity value less than 1.0 so that they become translucent, or see-through. This will allow us to see both of them.

Set alpha values

The only difference is adding the optional alpha parameter to the hist method. The alpha value can be any decimal between 0 and 1. Each plot can have a unique alpha value.

# Generate histogram plot
         label='Maternal Smokers BMI')
         label='Maternal Non-smokers BMI')

You can also have more than 2 overlapping plots. In this case, it can be helpful to manually set the color for each histogram. We can do this by adding the color parameter to each hist call:

         label='val 1',
         color='red') # customized color parameter
         label='val 2',

How to Split Pandas DataFrame into Multiple DataFrames by Column Value in Python

In this tutorial, I will show you how to create a separate DataFrame for each value for a given column.

Use the pandas.DataFrame.groupby(column) method to group the DataFrame by the values found in the column named column.

grouped_df = df.groupby('Column')

This method returns a GroupBy object. You can see this yourself by printing the new dataframe by running print grouped_df:

<pandas.core.groupby.generic.DataFrameGroupBy object at 0x7efe3ec55670>

With the GroupBy object from the previous function, call the DataFrameGroupBy.get_group(group) method on the new dataframe grouped_df

This method will return a Dataframe of all the rows that have the value group in the column named column.


Alternatively, you can call both methods in one line:

value_df = df.groupby('Column').get_group('column_value')

Return the n largest/smallest values for column in DataFrame

Get n-largest values from a particular column in Pandas DataFrame

df.nlargest(5, 'Gross')

Return the first n rows with the smallest values for column in DataFrame

df.nsmallest(5, ['Age'])

To order by the smallest values in column “Age” and then “Salary”, we can specify multiple columns like in the next example.

df.nsmallest(5, ['Age', 'Salary'])

There is also an optional keep parameter for the nlargest and nsmallest functions. keep has three possible values: {'first', 'last', 'all'}. The default is 'first'

Where there are duplicate values:

  • first : take the first occurrence.
  • last : take the last occurrence.
  • all : do not drop any duplicates, even it means selecting more than n items.
df.nlargest(5, 'Gross', keep='last')

Working with a New Dataset / DataFrame

When you are working with a new Pandas DataFrame, these attributes and methods will give you insights into key aspects of the data.

The dir function let’s you look at all of the attributes that a Python object has.


The shape attribute returns a tuple of integers indicating the number of elements that are stored along each dimension of an array. For a 2D-array with N rows and M columns, shape will be (N,M). 


You may be working with a dataframe that has hundreds or thousands of rows. To get a glimpse of the data inside a dataframe without printing out all of the values you can use the head and tail methods.

Returns the first n rows in the dataframe

df.head() # returns rows 0-4
df.head(n) # returns the first n rows

Returns the last n rows in the dataframe


The count method of a dataframe shows you the number of entries for each column


Check if there are any missing values in any of the columns


The info method of the dataframe gives a bunch of information. It tells

  1. The number of entries in the df
  2. The names of the columns
  3. The number of columns
  4. The number of entries in each column
  5. The dtype of each column
  6. If there are null values in a column

Different Ways to Create Pandas DataFrames

A Pandas DataFrame is a 2D labeled data structure with columns of potentially different types.

There are a variety of different methods and syntaxes that can be used to create a pd.DataFrame.

Firstly, make sure you import the pandas module:

import pandas as pd

Method 1: Creating DataFrame from list of lists

# initialize list of lists
data = [['bob', 20], ['jane', 30], ['joe', 40]]
# Create the pandas DataFrame
df = pd.DataFrame(data, columns = ['Name', 'Age'])


Method #2: Creating DataFrame from dictionary of lists

In this method, you define a dictionary which has the column name as the key which corresponds to an array of row values.

# initialize dictionary of lists
data = {'Name': ['Bob', 'Joe', 'Jane', 'Jack'],
        'Age': [30, 30, 21, 40]}
# Create DataFrame
df = pd.DataFrame(data)


You can use custom index values for the DataFrame by adding a parameter to the pd.DataFrame function. Set the optional index parameter of the pd.DataFrame function to an array of strings for the index values.

df = pd.DataFrame(data, index=['first',


In the same way that we just defined the index values, you can also define the column names separately. Set the optional columns parameter of the pd.DataFrame function to an array of strings for the column values.

Notice that the row values are now defined as a list of lists rather than a dictionary of lists. This is because the column values are no longer being defined with them.

df = pd.DataFrame(
    index = ['row_one','row_two','row_three'],



Method #3: Creating DataFrame using zip() function.

The zip function returns an iterator of tuples where the corresponding items in each passed iterator is paired together. By calling the list function on the object returned from the zip function, we convert the object to a list which can be passed into the pd.DataFrame function.

name = ["Bob", "Sam", "Sally", "Sue"]
age = [19, 17, 51, 49]

data = list(zip(name, age))

df = pd.DataFrame(data,
                  columns = ['Name', 'Age'])



Setup Firebase in Next.js Project

In this tutorial, I will show you how to set up Firebase in a Next.js project. It’s actually pretty simple.

Setup Next.js Project

First, create a new Next.js project:

npx create-next-app myproject

You can verify that everything is working okay by quickly running npm run dev.

Then, install the Firebase package into your new project:

npm install firebase

Setup Firebase Project

Now, head over to the Firebase website to set up a new Firebase project:

Then, select the option to add Firebase to your web app:

Next, register your app with a name and copy the code that is created for you. This code will initialize Firebase in your project.

Add Firebase config to Next.js Project

In your project, create a new file to hold the Firebase configuration code:

import { initializeApp } from "firebase/app";

const firebaseConfig = {
  apiKey: "...",
  authDomain: "...",
  projectId: "...",
  storageBucket: "...",
  messagingSenderId: "...",
  appId: "..."

export const firebaseApp = initializeApp(firebaseConfig);

Now, you can import the Firebase app into other files within your project and thus access Firebase in your project.

Import the app const from the Firebase config file you just created:

import firebaseApp from '../firebase_config';

Working with Firestore

Firestore is a powerful Cloud, NoSQL database within Firebase. There is a good chance you will use it in your web application.

To setup Firestore within your project, add the following code to your Firebase config file:

First, import the getFirestore function at the top:

import { getFirestore } from "firebase/firestore";

Then, create a database using the function, passing in your firebaseApp from before:

const db = getFirestore(firebaseApp);

Lastly, export the database along with your firebase app:

export { firebaseApp, db }

Now, you can work with Firestore in your project. Here’s an example:

import { firestore } from '../firebase_config';
import { useState } from 'react';

import { collection, QueryDocumentSnapshot, DocumentData } from "@firebase/firestore";

const todosCollection = collection(firestore,'todos');

const [todos,setTodos] = useState<QueryDocumentSnapshot<DocumentData>[]>([]);

This was just a very simple, bare-bones introduction to using Firebase in Next.js. Hope it helped.

How to Split Training and Test Data in Python

In this article, I’ll be explaining why you should split your dataset into training and testing data and showing you how to split up your data using a function in the scikitlearn library.

If you are training a machine learning model using a limited dataset, you should split the dataset into 2 parts: training and testing data.

The training data will be the data that is used to train your model. Then, use the testing data to see how the algorithm performs on a dataset that it hasn’t seen yet.

If you use the entire dataset to train the model, then by the time you are testing the model, you will have to re-use the same data. This provides a slightly biased outcome because the model is somewhat “used” to the data.

We will be using the train_test_split function from the Python scikitlearn library to accomplish this task. Import the function using this statement:

from sklearn.model_selection import train_test_split

This is the function signature for the train_test_split function:

sklearn.model_selection.train_test_split(*arrays, test_size=None, train_size=None, random_state=None, shuffle=True, stratify=None)

The first parameters to the function are a sequence of arrays. The allowed inputs are lists, numpy arrays, scipy-sparse matrices or pandas dataframes.

So the first argument is gonna be our features variable and the second argument is gonna be our targets.

# X = the features array
# y = the targets array
train_test_split(X, y, ...)

The next parameter test_size represents the proportion of the dataset to include in the test split. This parameter should be either a floating point number or None (undefined). If it is a float, it should be between 0.0 and 1.0 because it represents the percentage of the data that is for testing. If it is not specified, the value is set to the complement of the train size.

This is saying that I want the test data set to be 20% of the total:

train_test_split(X, y, test_size=0.2)

train_size is the proportion of the dataset that is for training. Since test_size is already specified, there is no need to specify the train_size parameter because it is automatically set to the complement of the test_size parameter. That means the train_size will be set to 1 – test_size. Since the test_size is 0.2, train_size will be 0.8.

The function has a shuffle property, which is set to True by default. If shuffle is set to True, the function will shuffle the dataset before splitting it up.

What’s the point of shuffling the data before splitting it? If your dataset is formatted in an ordered way, it could affect the randomness of your training and testing datasets which could hurt the accuracy of your model. Thus, it is recommended that you shuffle your dataset before splitting it up.

We could leave the function like this or add another property called random_state.

random_state controls the shuffling applied to the data before applying the split. Pass an int for reproducible output across multiple function calls. We are using the arbitrary number 10. You can really use any number.

train_test_split(X, y, test_size=0.2, random_state=10)

The function will return four arrays to us: a training and testing dataset for the feature(s), and a training and testing dataset for the target.

We can use tuple unpacking to store the four values that the function returns:

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=10)

Now, you can verify that the splitting was successful.

The percent of the training set will be the number of rows in X_train divided by the total number of rows in the dataset as a whole:


The percent of the testing dataset will be the number of rows in X_test divided by the total number of rows in the dataset:


The numbers returned by these calculations will probably not be exact numbers. For example, if you are using an 80/20 split, then this division by give you numbers like 0.7934728 instead of 0.80 and 0.1983932 instead of 0.20.

That’s it!

How to set Python default version to 3.x on macOS

In this tutorial, I’ll show you how to update your Mac’s default version of Python from 2.x to 3.x.

If Python 3.X is already installed on your system, even if it’s not the default version, then you should be able to run version 3.X using the python3 command in your Terminal. But in this tutorial, I’ll be showing you how you can run version 3.X using the default python command.

Step 0: Install Homebrew

Homebrew is a very popular package manager for macOS. If you don’t already have it installed, you can install it by running this command:

/bin/bash -c "$(curl -fsSL"

Step 1: Install Python with Homebrew

This command will Python 3.

brew install python

Step 2:

Next, we’ll check the symlinks for Python 3.x. A symlink or a Symbolic Link is simply enough a shortcut to another file.

ls -l /usr/local/bin/python*

This should output something like the following:

Look at the first line. It shows default python being symlinked to the brew installed python3. If it does not show this exactly, then set it as the default python symlink run the following:

ln -s -f /usr/local/bin/python3 /usr/local/bin/python

You should probably run this command just to be sure. Run ls -l /usr/local/bin/python* again and make sure you have the desired output.

Step 3: Verify Python 3.X install

Run the following command to see which executable Python binary will launch when you issue a python command to the shell:

which python

The output should be this: /usr/local/bin/python

Finally, check if the default Python version has changed:

python --version

The terminal should output the newest version of Python 3.x. For me, it is Python 3.9.7.

That’s it! Hope this helps.