ValueError: x and y must have same first dimension, but have shapes

# ValueError: x and y must have same first dimension, but have shapes

The error "ValueError: x and y must have same first dimension, but have shapes" occurs when the values you pass to the pyplot.plot() method have different dimensions.

To solve the error, make sure the x and y values you pass to pyplot.plot() have the same length.

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import matplotlib.pyplot as plt

x = [1, 2, 3]
y = [1, 2, 3, 4]

# ⛔️ ValueError: x and y must have same first dimension, but have shapes (3,) and (4,)
plt.plot(x, y, '-', color='green')

plt.show()
plt.close()

The first two arguments the pyplot.plot() method takes are the x and y values.

The length of the x and y values must be the same.

The error is caused because the x value has a length of 3 and the y value has a length of 4.

The part of the error message "but have shapes (3,) and (4,)" tells us that one of the lists has a length of 3 and the other one has a length of 4.

One way to solve the error is to add a value to the x list so that the length of x and y matches.

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import matplotlib.pyplot as plt


# 👇️ both have a length of 4
x = [1, 2, 3, 4]
y = [1, 2, 3, 4]

plt.plot(x, y, '-', color='green')

plt.show()
plt.close()

You can use the len() function if you need to get the length of a list.

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x = [1, 2, 3, 4]
y = [1, 2, 3, 4]

print(len(x))  # 👉️ 4
print(len(y))  # 👉️ 4

Both lists in the example have a length of 4, so they can safely be used as the x and y values when calling plot().

Alternatively, you can remove a value from the y, list so that both x and y have a length of 3.

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import matplotlib.pyplot as plt


x = [1, 2, 3]
y = [1, 2, 3]

plt.plot(x, y, '-', color='green')

plt.show()
plt.close()

Both lists in the example have a length of 3, so everything works as expected.

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x = [1, 2, 3]
y = [1, 2, 3]

print(len(x))  # 👉️ 3
print(len(y))  # 👉️ 3

If x or y is a 2D array, a separate data set is drawn for each column.

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import matplotlib.pyplot as plt

x = [1, 2, 3]
y = [[1, 2], [3, 4], [5, 6]]

plt.plot(x, y, '-', color='green')

plt.show()
plt.close()

Notice that the lengths of the 1D and 2D arrays are the same - they both have 3 elements.

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x = [1, 2, 3]
y = [[1, 2], [3, 4], [5, 6]]

print(len(x))  # 👉️ 3
print(len(y))  # 👉️ 3

print(len(y[0])) # 👉️ 2

I've written a detailed article on how to get the length of a 2D array.

If both x and y are 2D arrays, they must have the same shape.

In some cases, you might be generating x or y based on the output of a custom function.

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import numpy as np
import matplotlib.pyplot as plt


def plot(func):
    x = np.linspace(2.0, 4.0, num=5)
    y = func(x)

    # 👇️ [2., 2.5, 3., 3.5, 4.], length is 5
    print(x, len(x))

    # 👇️ [ 4., 6.25, 9., 12.25, 16.], length is 5
    print(y, len(y))

    plt.plot(x, y, '-', color='green')

    plt.show()
    plt.close()


plot(lambda x: x ** 2)

We used the numpy.linspace() method to get evenly spaced numbers over a given interval and then assigned the result to the x variable.

We then calculated the y variable with a custom lambda function that takes each value in a sequence and squares it.

Notice that both x and y have a length of 5, so plotting the two lists is allowed.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• ModuleNotFoundError: No module named 'matplotlib' in Python
• ValueError: numpy.ndarray size changed, may indicate binary incompatibility
• Convert a NumPy array to 0 or 1 based on threshold in Python
• How to get the length of a 2D Array in Python
• TypeError: 'numpy.ndarray' object is not callable in Python
• TypeError: Object of type ndarray is not JSON serializable
• IndexError: too many indices for array in Python [Solved]
• How to filter a JSON array in Python