Python。 ValueError无法将字符串转换为浮点型：

Question

我试图在文本文件中的指定列中生成数字的平均值。我收到一个错误，蟒蛇无法将字符串转换为浮点数，尽管我没有看到我可以传递一个无效字符串的位置。

def avg_col(f, col, delim=None, nhr=0): 
    """ 
    file, int, str, int -> float 

    Produces average of data stored in column col of file f 

    Requires: file has nhr header rows of data; data is separated by delim 

    >>> test_file = StringIO('0.0, 3.5, 2.0, 5.8, 2.1') 
    >>> avg_col(test_file, 2, ',', 0) 
    2.0 

    >>> test_file = StringIO('0.0, 3.5, 2.0, 5.8, 2.1') 
    >>> avg_col(test_file, 3, ',', 0) 
    5.8 
    """ 
    total = 0 
    count = 0 


    skip_rows(f, nhr) 
    for line in f: 
     if line.strip() != '': 
      data = line.split(delim) 
      col_data = data[col] 
      total = sum_data(col_data) + total 
      count = len(col_data) + count 
    return total/count 

def sum_data(lod): 
    ''' 
    (listof str) -> Real 

    Consume a list of string-data in a file and produce the sum 

    >>> sum_data(['0']) 
    0.0 
    >>> sum_data(['1.5']) 
    1.5 

    ''' 
    data_sum = 0 
    for number in lod: 
     data_sum = data_sum + float(number) 
    return data_sum 

Answer 1

您传递一个字符串到sum_lod()：

data = line.split(delim) 
col_data = data[col] 
total = sum_data(col_data) + total 

data是一个字符串列表，data[col]然后一个元素。在若干

def sum_data(lod): 
    # ... 
    for number in lod: 

迭代然后给你的单个字符：

在另一方面sum_data()期望一个迭代

>>> for element in '5.8': 
...  print element 
... 
5 
. 
8 

试图把这样的字符串中的每个元素可以很容易导致您试图将而不是号码的字符转换为浮点数：

>>> float('.') 
Traceback (most recent call last): 
    File "<stdin>", line 1, in <module> 
ValueError: could not convert string to float: . 

无论是传递一个字符串的列表：

total += sum_data([col_data]) 
count += 1 

或只是一个元素上使用float()您有：

total += float(col_data) 
count += 1