# Python NumPy hstack Function

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• Post category:NumPy / Python

NumPy `hstack()` function in Python is used to stack or concatenate arrays in sequence horizontally (column-wise). This process is similar to concatenate arrays along the second axis, except for 1-D arrays where it concatenates along the first axis. It stacks a sequence of two or more arrays into a single NumPy array.

In this article, I will explain `numpy.hstack()` function syntax and how to create an array that is formed by stacking the given arrays horizontally.

## 1. Quick Examples of hstack() Function

Following are quick examples of the hstack() function in Python. For more functions of NumPy refer to NumPy Tutorial.

``````
# Below are the quick examples

# Example 1: Use NumPy.hstack() Functions
arr = np.array([ 2, 3, 4])
arr1 = np.array([5, 6, 7])
arr2 = np.hstack((arr,arr1))

# Example 2: Use numpy.hstack() function to 2-d numpy arrays
arr =  np.array([[ 2, 3, 5], [ -1, -3, -5]])
arr1 = np.array([[ 6, 8, 10], [ -7, -8, -9]])
arr2 = np.hstack((arr,arr1))

# Example 3: stacking arrays horizontally
arr =  np.array([[ 2, 3], [ 4, 6]])
arr1 = np.array([[ 6, 8, 10], [ 8, 10, 12]])
arr2 = np.hstack((arr,arr1))
``````

## 2. Syntax of NumPy hstack()

Following is the Syntax of hstack().

``````
# Syntax of NumPy hstack()
numpy.hstack(tup)
``````

### 2.1 Parameters of hstack()

Following are the Parameters hstack()

• `tup` – It provides the sequence of arrays to be concatenated.

### 2.2 Return Value of hstack()

It returns an array formed by stacking over the given arrays horizontally.

## 3. Use numpy.hstack() Function

Use `numpy.hstack()` function to concatenate arrays horizontally (column wise). When you use hstack() on multiple arrays of 1-D, it combines all arrays and returns the result in a single array.

Let’s create two 1-dimensional arrays of length three and then horizontally stacked them with the `hstack()` function.

``````
import numpy as np

# Create NumPy arrays
arr = np.array([ 2, 3, 4])
arr1 = np.array([5, 6, 7])

# Use NumPy.hstack() Functions
arr2 = np.hstack((arr,arr1))
print(arr2)

# Output
# [2 3 4 5 6 7]
``````

## 4. Use hstack() Function to 2-D Numpy Arrays

To `stack()` the 2-D arrays using `hstack()` function, it will return the horizontally stacked array. Here we stacked two 2d arrays of shape `(2,3) `horizontally resulting in an array of shape` (2,6)`.

``````
# creating an 2D input array
arr =  np.array([[ 2, 3, 5], [ -1, -3, -5]])
arr1 = np.array([[ 6, 8, 10], [ -7, -8, -9]])

# Use numpy.hstack() function to 2-d numpy arrays
arr2 = np.hstack((arr,arr1))
print(arr2)

# Output
# [[ 2  3  5  6  8 10]
#  [-1 -3 -5 -7 -8 -9]]
``````

Below is another example where two arrays have the same shape along all except different second axis.

``````
# creating an 2D input array
arr =  np.array([[ 2, 3], [ 4, 6]])
arr1 = np.array([[ 6, 8, 10], [ 8, 10, 12]])

# stacking arrays horizontally
arr2 = np.hstack((arr,arr1))
print(arr2)

# Output
# [[ 2  3  6  8 10]
# [ 4  6  8 10 12]]
``````

## 5. Conclusion

In this article, I have explained `numpy.hstack()` function and using this function how we can stack the NumPy arrays horizontally.

Happy Learning!!

## References 