Course Content
Ultimate NumPy
3. Commonly used NumPy Functions
Ultimate NumPy
Multidimensional Indexing
Now that you are able to access elements in 1D arrays, it’s time to learn about indexing in higher-dimensional arrays.
2D Arrays Indexing
Let's first take a look at a 2D array example:
This is a 2x3 array, which means it consists of 2 1D arrays along axis 0, and each of these 1D arrays has 3 elements along axis 1.
The images below will clarify positive and negative indexing in 2D arrays:
As you can see, indexing along each of the axes is identical to indexing in 1D arrays.
Accessing Elements in 2D Arrays
In 1D arrays, we accessed elements by specifying the index of the element in square brackets. If we do the same in 2D arrays, we retrieve a 1D array at the specified index, which may be exactly what we need.
However, if we want to retrieve a particular element of an inner 1D array, we should specify the index of the 1D array (along axis 0) and the index of its element (along axis 1), e.g., array[0, 1]. We could also write array[0][1] as we do with Python list, but this is less efficient since it performs the search twice for each index instead of once.
Note
If a specified index is out of bounds, an
IndexErroris thrown, so be cautious of that.
Let's take a look at an example:
Task
stock_prices contains simulated stock prices over five days for five different companies. Each row corresponds to a particular company, and each column corresponds to a particular day. Consequently, each element in the matrix represents the closing price of a certain company's stock on a given day.
Your task is the following:
- Retrieve all the stock prices of the first company over five days with a positive index.
- Retrieve the stock price of the third company on the second day with positive indices.
- Retrieve the stock price of the last company on the last day with negative indices.
Everything was clear?
Course Content
Ultimate NumPy
3. Commonly used NumPy Functions
Ultimate NumPy
Multidimensional Indexing
Now that you are able to access elements in 1D arrays, it’s time to learn about indexing in higher-dimensional arrays.
2D Arrays Indexing
Let's first take a look at a 2D array example:
This is a 2x3 array, which means it consists of 2 1D arrays along axis 0, and each of these 1D arrays has 3 elements along axis 1.
The images below will clarify positive and negative indexing in 2D arrays:
As you can see, indexing along each of the axes is identical to indexing in 1D arrays.
Accessing Elements in 2D Arrays
In 1D arrays, we accessed elements by specifying the index of the element in square brackets. If we do the same in 2D arrays, we retrieve a 1D array at the specified index, which may be exactly what we need.
However, if we want to retrieve a particular element of an inner 1D array, we should specify the index of the 1D array (along axis 0) and the index of its element (along axis 1), e.g., array[0, 1]. We could also write array[0][1] as we do with Python list, but this is less efficient since it performs the search twice for each index instead of once.
Note
If a specified index is out of bounds, an
IndexErroris thrown, so be cautious of that.
Let's take a look at an example:
Task
stock_prices contains simulated stock prices over five days for five different companies. Each row corresponds to a particular company, and each column corresponds to a particular day. Consequently, each element in the matrix represents the closing price of a certain company's stock on a given day.
Your task is the following:
- Retrieve all the stock prices of the first company over five days with a positive index.
- Retrieve the stock price of the third company on the second day with positive indices.
- Retrieve the stock price of the last company on the last day with negative indices.
Everything was clear?
