# numpy.concatenate() – Python

In this article, we will dsicuss how to join a sequence of numpy arrays along any given axis using concatenate() function.

## Overview of numpy.concatenate()

Numpy library in python provides a function to concatenate two or more arrays along a given axis.

`numpy.concatenate((a1, a2, …), axis=0, out=None)`

Arguments:

• a1, a2,…: A Sequence of array_like like objects
• The arrays in sequence must be of shape same shape.
• axis: int, optional | Default value is 0.
• The axis along which we want the arrays to be joined.
• If axis is None, then all arrays are flattened and the..
• If axis is 0, then all arrays are joined row wise.
• If axis is 1, then all arrays are joined column wise.
• out: ndarray, optional
• If provided, place the result in the our array. The shape of our must match with expected result value of concatenate() function.

Returns:

• Returns a new ndarray i.e. a Numpy array containing the concatenated values from all input arrays.

## Examples of numpy.concatenate()

### Concatenate two 1D Numpy Arrays

Suppose we have two 1D NumPy Arrays and we want to join them one after another and create a merged array. For that we need to create a sequence of both the arrays and pass them to the numpy.concatenate() function. For example,

```import numpy as np

# A Numpy Array of integers
first = np.array([1, 1, 1, 1, 1])
# Another Numpy Array of integers
second = np.array([2, 2, 2, 2, 2])

# Concatenate two arrays to create a merged array
merged_arr = np.concatenate( (first, second) )

print(merged_arr)```

Output:

`[1 1 1 1 1 2 2 2 2 2]`

We created a tuple of 2 arrays and passed that to the concatenate() function. It returned a new merged array containing the contents of both the arrays.

### Concatenate multiple 1D Numpy Arrays

To join multiple 1D Numpy Arrays, we can create a sequence of all these arrays and pass that sequence to concatenate() function. For example, let’s see how to join three numpy arrays to create a single merged array,

```import numpy as np

# Create three Numpy Arrays of integers
first =  np.array([1, 1, 1, 1, 1])
second = np.array([2, 2, 2, 2, 2])
third =  np.array([3, 3, 3, 3, 3])

# Concatenate three arrays to create a merged array
merged_arr = np.concatenate( (first, second, third) )

print(merged_arr)```

Output:

`[1 1 1 1 1 2 2 2 2 2 3 3 3 3 3]`

## Concatenate 2D Numpy Arrays row wise

To join two 2D Numpy Arrays row-wise, we need pass a sequence of arrays to concatenate() function along with value 0 for the axis parameter. It will insert all the rows of arrays one after another into a new array and returns the merged array. For example,

```import numpy as np

# Create 2D Numpy array of hard coded numbers
first = np.array([[1, 1, 1],
[2, 2, 2],
[3, 3, 3]])

# Create another 2D Numpy array of hard coded numbers
second = np.array([ [4, 4, 4],
[6, 5, 5],
[6, 6, 6]])

# Concatenate 2D Numpy Arrays row wise

# Merge two 2D arrays row-wise
merged_arr = np.concatenate( (first, second), axis=0 )

print(merged_arr)```

Output:

```[[1 1 1]
[2 2 2]
[3 3 3]
[4 4 4]
[6 5 5]
[6 6 6]]```

## Concatenate 2D Numpy Arrays column wise

To join two 2D Numpy Arrays column-wise, we need pass a sequence of arrays to concatenate() function along with value 1 for the axis parameter. It will insert all the columns of arrays, one after another into a new array and returns the merged array. For example,

```import numpy as np

# Create 2D Numpy array of hard coded numbers
first = np.array([[1, 1, 1],
[2, 2, 2],
[3, 3, 3]])

# Create another 2D Numpy array of hard coded numbers
second = np.array([ [4, 4, 4],
[6, 5, 5],
[6, 6, 6]])

# Concatenate 2D Numpy Arrays column wise
merged_arr = np.concatenate( (first, second), axis=1 )

print(merged_arr)```

Output:

```[[1 1 1 4 4 4]
[2 2 2 6 5 5]
[3 3 3 6 6 6]]```

## Concatenate 2D Numpy Arrays by flattening the shape

If we pass None as value for axis parameter, then concatenate() function will flatten all the arrays and join them. For example,

```import numpy as np

# Create 2D Numpy array of hard coded numbers
first = np.array([[1, 1, 1],
[2, 2, 2],
[3, 3, 3]])

# Create another 2D Numpy array of hard coded numbers
second = np.array([ [4, 4, 4],
[6, 5, 5],
[6, 6, 6]])

# Concatenate 2D Numpy Arrays by flattening the shape
merged_arr = np.concatenate( (first, second), axis=None )

print(merged_arr)```

Output:

`[1 1 1 2 2 2 3 3 3 4 4 4 6 5 5 6 6 6]`

Summary:

In this article, we learned that how we can join NumPy Arrays of different size and shapes.

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