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

Question

I wonder how to best solve the following problem in my script: "ValueError: x and y must have same first dimension, but have shapes (1531,) and (1532,)".

What is the problem here? The problem is that the x and y axis of the plot don't share the exact same number of values (input) to plot. The result is the error message above.

Let us look at the code first:

# Initialize
import numpy as np
import matplotlib.pyplot as plt
from scipy import signal
from matplotlib.pyplot import cm

# Numpy.loadtxt – Loads data from a textfile.
# Scipy.signal.welch – Creation of the power-spectrum via welch method. f, Welch creates the ideal frequencies (f, Welch = Power Spectrum or Power Spectral Density)
Subjects = ["Subject1" "Subject2"]

for Subject in Subjects:
    Txt = np.loadtxt("/datadir.../{0}/filename...{0}.txt".format(Subject), comments="#", delimiter=None,
                         converters=None, skiprows=0, usecols=0, unpack=False, ndmin=0, encoding=None, max_rows=None, like=None)

    f, Welch = signal.welch(Txt, fs=1.0, window="hann", nperseg=None, noverlap=None, nfft=3062, detrend="constant", return_onesided=True, scaling="density", axis=-1, average="mean")

    BypassZero1 = f[f > 0.00000000000001] # Avoids "RuntimeWarning: divide by zero encountered in log"
    BypassZero2 = Welch[Welch > 0.00000000000001]

    Log_f = np.log(BypassZero1, out=BypassZero1, where=BypassZero1 > 0)
    Log_Welch = np.log(BypassZero2, out=BypassZero2, where=BypassZero2 > 0)

    plt.plot(Log_f, Log_Welch)

The code lines "BypassZero1" and "BypassZero2" tell Python to only use values above 0.00000000000001 for both "f" and "Welch". Otherwise the problem "RuntimeWarning: divide by zero encountered in log" would occur in the following step where I apply the logarithm for both axes (Log_f and Log_Welch).

This is where the problem occurs for the last plt.plot line of the code. It seems that a different number of numeric values are "left over" for "f" and "Welch" after the previous step of using the Welch method and applying the logarithm for both axes.

I wonder if there is a possibility to deal with the 0.xxx values provided in the.txt file. Currently, only values above 0.00000000000001 for both f and Welch are used. This will lead to the different number of values for x and y, hence resulting in the impossibility of plotting the data.

What could be a solution for this problem?

Answer 1

As you pointed out, the error message indicates that your two arrays are of different length. This is because the mask of the second array should be the same as the mask of the first. Therefore, replacing BypassZero2 = Welch[Welch > 0.00000000000001] with BypassZero2 = Welch[f > 0.00000000000001] should fix the issue.

Answer 2

#Plot
plt.figure(figsize = (16,8))
plt.plot(anos, ratings)
plt.xlabel('\nAno')
plt.ylabel('Mediana de Avaliação')
plt.title('\nMediana de Avaliação dos Filmes Em Relação ao Ano de Estréia\n')
plt.show()

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

Answer 3

Basically, x and y coordinates we are plotting must be of same length, so that we can make sure it plots one on one. Thus, ensure their lengths are equal.