How To Handle Pandas Value Error Could Not Convert String To Float

Pandas is an open-source Python library used for data manipulation and analysis. It is a powerful package that provides fast, flexible, and meaningful data structures that help with practical, real-world data analysis. Its functions help in working with structured and labelled data like spreadsheets, tables, etc. It is considered a powerful tool by the developers for data analysis, as it helps in importing the data efficiently. It is used in various fields like data science, machine learning, etc., as it provides many functions for data analysis.

Value error in Python:

Valueerror in Python occurs when an invalid type of value is passed to a function other than the type specified in the function argument. Developers generally handle this type of error using the try-except block or the raise keyword.

Python3

#importing pandas library
import pandas as pd

# Create a DataFrame with strings containing symbols
df = pd.DataFrame({'Prices': ['23-00', '56-50', '78-10', '9-05']})

#Convert the column to float
df['Prices'] = df['Prices'].astype(float)

#Print the DataFrame
print(df)

Output:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-d082d5acca01> in <cell line: 11>()
      9 
     10 #Convert the column to float
---> 11 df['Prices'] = df['Prices'].astype(float)
     12 
     13 #Print the DataFrame
    168     if copy or is_object_dtype(arr.dtype) or is_object_dtype(dtype):
    169         # Explicit copy, or required since NumPy can't view from / to object.
--> 170         return arr.astype(dtype, copy=True)
    171 
    172     return arr.astype(dtype, copy=copy)
ValueError: could not convert string to float: '23-00'

Handling the Pandas Value error Could Not Convert String To Float:

Handling these types of errors comes under the data preprocessing phase of data analysis. In this article, we will discuss a few approaches to handling it using some examples.

1. replace() method:

When the string in the dataframe contains inappropriate characters that cause problems in converting the string to a float type, the replace() method is a good and easy way to remove those characters from the string.

Python3

#importing pandas library
import pandas as pd

# Create a DataFrame with strings containing symbols
df = pd.DataFrame({'Prices': ['23-00', '56-50', '78-10', '9-05']})

# Use the `replace()` function to remove symbols
df['Prices'] = df['Prices'].replace('-', '', regex=True)


#Convert the column to float
df['Prices'] = df['Prices'].astype(float)

#Print the DataFrame
print(df)

Output:

   Prices
0  2300.0
1  5650.0
2  7810.0
3   905.0

In the above, we removed the symbols by using replace() method and then by using astype() method we converted strings to float.

2. to_numeric() method:

The to_numeric() method is a good option to consider when the string contains special characters and forbid to convert into float.

Python3

#importing pandas library
import pandas as pd

#Creating some random data with strings containing special characters
data = {
    'Prices': ['$12', '#8', '12%', '1.21']
}

#Converting the data into dataframe by using pandas
df = pd.DataFrame(data)

#Use the `to_numeric()` function to convert the column to float
df['Prices'] = pd.to_numeric(df['Prices'], errors='coerce')

#print the data frame
print(df)

Output:

   Prices
0     NaN
1     NaN
2     NaN
3    1.21

In the above, by using to_numeric() method and errors = ‘coerce’ parameter we converted the ‘Prices‘ column which contains numeric values to float and replaced all non numeric values with NaN.

3. apply() method:

This is a flexible method that allows us to use custom expressions to convert string into float. For instance, our string contains both numeric and special characters then apply() method allows us to convert the numeric part of the string into float with a custom expression.

Python3

# importing pandas library
import pandas as pd

# Creating some data with strings containing special characters and numbers
data = {
    'Prices': ['12$', '-514%', '$82', '19.1%']
}

# Converting the data into dataframe by using pandas
df = pd.DataFrame(data)

# implementing the apply() method
# Using lambda expression for extracting the float values
df['Prices'] = df['Prices'].apply(
    lambda p: float(''.join(filter(str.isdigit, p)))
    if not p.isnumeric() else float(p))

# printing the dataframe
print(df)

Output:

   Prices
0    12.0
1   514.0
2    82.0
3   191.0

Here, by using apply() method along with a custom lambda function we have extracted and converted the numeric parts of the string into float.

Conclusion:

In conclusion, the valueerror could not convert string to float in python can be addressed by using replace(), to_numeric(), or apply() functions. It is a challenge that occurs in the data analysis under data preprocessing. These functions explained above provide effective way to handle these errors.