CODEWITHSUNDEEP
pythonpandasdataframe
alcarnielo
alcarnielo

Reputation: 45

Error when creating dataframe "*** ValueError: If using all scalar values, you must pass an index"

I was trying to create a plot with seaborn, but I faced an error:

*** ValueError: If using all scalar values, you must pass an index

In my program I have read an output file from XFOIL and was trying to plot its results to check it out.

XFOIL File format:

#      x          Cp  
     1.00000    0.24276
     0.99818    0.23883
     0.99591    0.22657
     0.99342    0.21421
     0.99066    0.20128
     0.98759    0.18802
     0.98413    0.17434
     0.98020    0.16018
     0.97569    0.14544
     0.97044    0.12999
     0.96429    0.11374
     0.95703    0.09661
### **(the file is big, and I will not transcript it completely here)** ###

I decided to create a dataframe to enable the plotting process easily.

lst = fl.readlines()
lst_splt = [ s.replace('#','').split() for s in lst] 
cp = pd.DataFrame.from_records(lst_splt[1:], columns=lst_splt[:1]).astype(float)

and finally I tried to plot it using seaborn:

sns.lineplot(x='x',y='Cp', data=cp)

but as I said on the beginning of the question, an error appeared:

*** ValueError: If using all scalar values, you must pass an index

What can I do to fix this error?

Upvotes: 0

Views: 730

Answers (2)

JacoSolari
JacoSolari

Reputation: 1404

The problem is related to the columns argument passed to the DataFrame constructor.

When printing lst_splt[:1], which is the value you pass to the columns argument, I get this:

print(lst_splt[:1])
# [['x', 'Cp']]

The Dataframe constructor in this case needs a flat list, not a nested one. The problem is solved when you change lst_splt[:1] to lst_splt[:1][0] which when printed gives of course:

print(lst_splt[:1][0])
# ['x', 'Cp']
The modified verrsion of your code below works fine: 
import matplotlib.pyplot as plt
import seaborn as sns
import pandas as pd

fl = open('data.txt', 'r')

lst = fl.readlines()
lst_splt = [ s.replace('#','').split() for s in lst]
cp = pd.DataFrame.from_records(lst_splt[1:], columns=lst_splt[:1][0]).astype(float)

sns.lineplot(data=cp, x='x',y='Cp')

plt.show()

out: enter image description here

Upvotes: 1

qmeeus
qmeeus

Reputation: 2412

Not sure why you are having this error, but you can simply do:

import matplotlib.pyplot as plt

plt.plot(cp["x"], cp["Cp"])
plt.show()

EDIT: After some experimentation, it seems that your method for creating the dataframe is probably the culprit. You can replace it with:

cp = pd.read_csv(filename, sep="\s+", skiprows=2, names=["x", "Cp"])
# Make sure that you have the right value for skiprows (should ignore the header and that's it)

# Then this works:
sns.lineplot(x="x", y="Cp", data=cp)

Upvotes: 2

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