Convert string to float in Python

Question

I have a csv file and I am trying to plot some of the contents and need to convert the strings to floats. My problem is the following:

When I run the code below,

with open('meteors.csv', 'rU') as csvfile:
    reader=csv.reader(csvfile)
    for row in reader:    
        print row[6]

I get this output:

58.58333

When I instead try

print type(row[6])

I get this output:

<type 'str'>

But when I try

 print float(row[6])

I get this error:

ValueError: could not convert string to float: coordinate_1

Anyone know what's going on?

Is there some form of terminator on the end of the string such as \n, \t etc...? — TheMerovingian
– TheMerovingian, Commented Apr 23, 2013 at 4:43
I reckon there probably is but I'm not sure of it sorry. Maybe @jamylak can help, he seems to have a clue about most python stuff — TheMerovingian
– TheMerovingian, Commented Apr 23, 2013 at 4:48
@user1571767: There you go, try print(repr(row[6])) and it should print out the raw string showing all terminating characters — TheMerovingian
– TheMerovingian, Commented Apr 23, 2013 at 4:56

Burhan Khalid · Accepted Answer · 2013-04-23 04:46:11Z

4

Most likely you are seeing the first row, which has the column headers on your CSV file, and coordinate_1 is a header.

You can either skip the first row:

with open('meteors.csv', 'rU') as csvfile:
    reader=csv.reader(csvfile)
    reader.next() # skips the first (header) row
    for row in reader:    
        print row[6]

Or, use DictReader, like this:

with open('meteors.csv', 'rU') as csvfile:
    reader=csv.DictReader(csvfile)
    for row in reader:    
        print row['coordinate_1']

answered Apr 23, 2013 at 4:46

Burhan Khalid

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1 Comment

jamylak Over a year ago

@BurhanKhalid you should also use next() function instead for the reason I specified on the other answer. Otherwise yes this is the cleaner way

Blender · Accepted Answer · 2013-04-23 04:44:07Z

2

Your CSV file probably has a header that lists the name of each column. You can't convert those names into floats, so that's where your error is coming from.

You have to consume the header first:

with open('meteors.csv', 'rU') as csvfile:
    reader = csv.reader(csvfile)
    header = next(reader)

    for row in reader:
        print float(row[6])

answered Apr 23, 2013 at 4:44

Blender

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2 Comments

jamylak Over a year ago

yes because they change .next to the special method .__next__, since the function next was supposed to be used when it was created

Blender Over a year ago

@jamylak: Ah, I never knew that. Thanks.

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