NUMPY Tutorial

🔹 NumPy Array Join (Concatenation)

Joining or concatenating arrays means combining two or more NumPy arrays into a single array. NumPy provides several functions to do this efficiently.

✅ Common Functions to Join Arrays

• np.concatenate() — Joins arrays along an existing axis.
• np.vstack() — Stacks arrays vertically (row wise).
• np.hstack() — Stacks arrays horizontally (column wise).
• np.stack() — Joins arrays along a new axis.

🎯 Basic Syntax

joined_array = np.concatenate((arr1, arr2, ...), axis=axis_value)

axis specifies the axis along which the arrays will be joined. The arrays must have compatible shapes.

🧰 Important Notes

• Arrays must have the same shape except in the dimension corresponding to axis.
• For 1D arrays, axis=0 concatenates end to end.
• vstack() and hstack() are convenient shortcuts for common concatenations.

✅ Examples

import numpy as np

# Example 1: Concatenate 1D arrays
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
c = np.concatenate((a, b))
print("Concatenate 1D arrays:")
print(c)  # Output: [1 2 3 4 5 6]

# Example 2: Concatenate 2D arrays along rows (axis=0)
a2 = np.array([[1, 2], [3, 4]])
b2 = np.array([[5, 6]])
c2 = np.concatenate((a2, b2), axis=0)
print("\nConcatenate 2D arrays along rows (axis=0):")
print(c2)
# Output:
# [[1 2]
#  [3 4]
#  [5 6]]

# Example 3: Concatenate 2D arrays along columns (axis=1)
d2 = np.array([[7], [8]])
c3 = np.concatenate((a2, d2), axis=1)
print("\nConcatenate 2D arrays along columns (axis=1):")
print(c3)
# Output:
# [[1 2 7]
#  [3 4 8]]

# Example 4: Vertical stack (vstack)
vstacked = np.vstack((a2, b2))
print("\nVertical stack (vstack):")
print(vstacked)

# Example 5: Horizontal stack (hstack)
hstacked = np.hstack((a2, d2))
print("\nHorizontal stack (hstack):")
print(hstacked)

# Example 6: Stack along a new axis (axis=0)
stacked = np.stack((a, b), axis=0)
print("\nStack along a new axis (axis=0):")
print(stacked)
# Output:
# [[1 2 3]
#  [4 5 6]]

⚠️ Common Errors

• ValueError: all the input arrays must have same shape — arrays have incompatible shapes.
• Check dimensions carefully before joining arrays.

🔎 Summary

• Use np.concatenate() for flexible array joining along existing axes.
• vstack() and hstack() are convenient for vertical and horizontal joining.
• stack() adds a new dimension and is useful for higher-dimensional arrays.