# Variational Classifier: Error when using scikit dataset

**URL:** <https://discuss.pennylane.ai/t/variational-classifier-error-when-using-scikit-dataset/661>\
**Category:** PennyLane Help\
**Created:** [November 6, 2020, 11:51am UTC](https://discuss.pennylane.ai/t/variational-classifier-error-when-using-scikit-dataset/661 "2020-11-06T11:51:07Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![NikSchet](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/nikschet/32/786_2.png) [@NikSchet](https://discuss.pennylane.ai/u/NikSchet)\
**Post date:** [November 6, 2020, 11:51am UTC](https://discuss.pennylane.ai/t/variational-classifier-error-when-using-scikit-dataset/661/1 "2020-11-06T11:51:07Z")

</div>

Hello all,

So, i have builded a 2 qubit Variational Classifier as in the demo. It runs very well with Iris datasets but when i try to import a different dataset from scikit although dataset has exactly the same format i get error

```
TypeError: float() argument must be a string or a number, not 'ArrayBox'
ValueError: setting an array element with a sequence. 

```

The code i used to generate dataset is the following

```
from sklearn.datasets import make_blobs
from matplotlib import pyplot
from pandas import DataFrame
# generate 2d classification dataset
X, y = make_blobs(n_samples=100, centers=2, n_features=2)
# scatter plot, dots colored by class value
df = DataFrame(dict(x=X[:,0], y=X[:,1], label=y))

y[y == 0] = -1 # map zeroes to -1 

Xnorm = minmax_scale(X, feature_range=(0, np.pi))
X_train_val, X_test, y_train_val, y_test = train_test_split(Xnorm, y, test_size=0.5)

```

Any ideas on how to solve this? Thanks in advance!!

Complete code in the next comment.

---

<div class="post-metadata">

**Author:** ![NikSchet](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/nikschet/32/786_2.png) [@NikSchet](https://discuss.pennylane.ai/u/NikSchet)\
**Post date:** [November 6, 2020, 11:51am UTC](https://discuss.pennylane.ai/t/variational-classifier-error-when-using-scikit-dataset/661/2 "2020-11-06T11:51:14Z")

</div>

```
from itertools import chain
import numpy as np
import matplotlib.pyplot as plt
from sklearn.model_selection import train_test_split
from sklearn.preprocessing import minmax_scale
from sklearn.datasets import make_blobs
from matplotlib import pyplot
from pandas import DataFrame
import pennylane as qml
from pennylane.templates.embeddings import AngleEmbedding
from pennylane.templates.layers import StronglyEntanglingLayers
from pennylane.init import strong_ent_layers_uniform
from pennylane.optimize import GradientDescentOptimizer

# generate 2d classification dataset
X, y = make_blobs(n_samples=100, centers=2, n_features=2)

# scaling and splitting dataset
Xnorm = minmax_scale(X, feature_range=(0, np.pi))
X_train_val, X_test, y_train_val, y_test = train_test_split(Xnorm, y, test_size=0.5)

# number of qubits is equal to the number of features 
n_qubits = X.shape[1] 
n_qubits= 2
# quantum device handle
dev = qml.device("default.qubit", wires=n_qubits)

# quantum circuit
@qml.qnode(dev)
def circuit(weights, x=None):
    AngleEmbedding(x, wires = range(n_qubits))
    StronglyEntanglingLayers(weights, wires = range(n_qubits))
    return qml.expval(qml.PauliZ(0))

# variational quantum classifier
def variational_classifier(theta, x=None):
    weights = theta[0]
    bias = theta[1]
    return circuit(weights, x=x) + bias

def cost(theta, X, expectations):
    e_predicted = \
        np.array([variational_classifier(theta, x=x) for x in X])
    loss = np.mean((e_predicted - expectations)**2)    
    return loss

# number of quantum layers
n_layers = 3

# split into train and validation
X_train, X_validation, y_train, y_validation = \
    train_test_split(X_train_val, y_train_val, test_size=0.50) 

# convert classes to expectations: 0 to -1, 1 to +1
e_train = np.empty_like(y_train)
e_train[y_train == -1] = -1
e_train[y_train == 1] = +1

# select learning batch size
batch_size = 5

# calculate numbe of batches
batches = len(X_train) // batch_size

# select number of epochs
n_epochs = 10

# draw random quantum node weights
theta_weights = strong_ent_layers_uniform(n_layers, n_qubits, seed=42)
theta_bias = 0.0
theta_init = (theta_weights, theta_bias) # initial weights

theta = theta_init

# start of main learning loop
# build the optimizer object
pennylane_opt = GradientDescentOptimizer()

# split training data into batches
X_batches = np.array_split(np.arange(len(X_train)), batches)

for it, batch_index in enumerate(chain(*(n_epochs * [X_batches]))):
    # Update the weights by one optimizer step
    batch_cost = \
        lambda theta: cost(theta, X_train[batch_index], e_train[batch_index])
    theta = pennylane_opt.step(batch_cost, theta)
     # end of learning loop

    print(
        "Iter: {:5d} | "
        "".format(it + 1))
```

---

<div class="post-metadata">

**Author:** ![antalszava](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/antalszava/32/171_2.png) [@antalszava](https://discuss.pennylane.ai/u/antalszava)\
**Post date:** [November 6, 2020, 10:50pm UTC](https://discuss.pennylane.ai/t/variational-classifier-error-when-using-scikit-dataset/661/3 "2020-11-06T22:50:08Z")

</div>

Hi @NikSchet,

In PennyLane, internally we are using a custom `ndarray` class (called `tensor`) which subclasses `numpy`'s `ndarray`. It is useful because differentiable parameters can be tracked easily (note the `requires_grad` attribute).

In order to use the custom `tensor` class internally, `numpy` has to be imported from PennyLane: `from pennylane import numpy as np` .

This should help out with your case, this was also a subtle difference between your code and the demo.

---

<div class="post-metadata">

**Author:** ![NikSchet](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/nikschet/32/786_2.png) [@NikSchet](https://discuss.pennylane.ai/u/NikSchet)\
**Post date:** [November 7, 2020, 10:18am UTC](https://discuss.pennylane.ai/t/variational-classifier-error-when-using-scikit-dataset/661/4 "2020-11-07T10:18:12Z")

</div>

Thank you very much that fixed the problem.