After conversion to float, matplotlib outputs cannot convert string to float

Question

I have a program that inputs 2 txt files.

deaths.txt

29.0
122.0
453.0

years.txt

1995
1996
1997

I make lists from the data

deaths = open("deaths.txt").read().splitlines()
years = open("years.txt").read().splitlines()

then I convert the lists to int and floats

for x in years[:-1]:
    x = int(x)

for x in deaths[:-1]:
    x = float(x)

and then the part where it gives the error: ValueError: could not convert string to float

plt.plot(years, deaths)

So it says it cannot convert strings to floats. But I thought I allready did that. What could be the reason?

Can you provide the content of deaths and years before you convert the lists? I don't get an error with these arrays: deaths = ["29.0", "122.0", "453.0"] years = ["1995", "1996", "1997"] — Ohumeronen
– Ohumeronen, Commented Jun 24, 2016 at 8:28
why are you not converting the last element, what is it, and are you sure you want to plot it (because that's what your are doing)? — Julien
– Julien, Commented Jun 24, 2016 at 8:38
also you can use list comprehension to convert, or even better map — Julien
– Julien, Commented Jun 24, 2016 at 8:40
You get error because you skip last element during conversion and after conversation you do not save x element back to arrays years and deaths. Try to write deaths_int= map(int,deaths). And plot deaths_int array. — Serenity
– Serenity, Commented Jun 24, 2016 at 9:28
Thands @StanleyR I did indeed skip the last element of the list. The last element would be " ". So it couldn't change it. But I did tell matplotlib to plot it and that is when the error came. — Tristan
– Tristan, Commented Jun 24, 2016 at 10:26

Martin Evans · Accepted Answer · 2016-06-24 10:58:09Z

3

The following should get you going. Rather than using readlines() to read the whole file, a better approach would be to convert each row as it is read in.

As your two data files have a different number of elements, the code makes use of zip_longest to fill in any missing death data with 0.0:

from itertools import zip_longest
import matplotlib.pyplot as plt
import matplotlib.ticker as ticker

with open('deaths.txt') as f_deaths:
    deaths = [float(row) for row in f_deaths]

with open('years.txt') as f_years:
    years = [int(row) for row in f_years]

# Add these to deal with missing data in your files, (see Q before edit)    
years_deaths = list(zip_longest(years, deaths, fillvalue=0.0))
years = [y for y, d in years_deaths]
deaths = [d for y, d in years_deaths]

print(deaths)
print(years)

plt.xlabel('Year')
plt.ylabel('Deaths')

ax = plt.gca()
ax.xaxis.set_major_formatter(ticker.FormatStrFormatter('%d'))
ax.set_xticks(years)

plt.plot(years, deaths)
plt.show()

This will display the following on the screen, showing that the conversions to ints and floats were correct:

[29.0, 122.0, 453.0, 0.0]
[1995, 1996, 1997, 1998]

And the following graph will then be displayed:

edited Jun 24, 2016 at 10:58

answered Jun 24, 2016 at 9:53

Martin Evans

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2 Comments

Tristan Over a year ago

I figured it out. I like you approach. The thing was that I didn't convert the last character because it was "". The last line of the file. But I did ask matplot lib to plot it. And that is when the error came. I gave it an upvote because it was very helpfull. But it did not solve my problem.

Martin Evans Over a year ago

If you are trying to deal with missing data, one approach would be to use zip_longest to pad out missing entries with a fill value. For example 0.0

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