matplotlib.pyplot.plot，ValueError：无法将字符串转换为float：f

Question

I'm trying to use python to make plots. Below is the simplified version of my code that cause error.

import numpy as np
import matplotlib   
import matplotlib.pyplot as plt
matplotlib.use("AGG")

distance = np.array('f')
depth = np.array('f')# make sure these two arrays store float type value

with open('Line671.txt','r') as datafile:
  for line in datafile:
    word_count = 0
    for word in line.split():
        word = float(word)#convert string type to float
        if word_count == 0:
            distance = np.append(distance, word)
        elif word_count == 1:
            depth = np.append(depth, word)
        else:
          print 'Error'
        word_count += 1

datafile.closed


print depth
print distance #outputs looks correct 
# original data is like this: -5.3458000e+00
# output of the array is :['f' '-5.3458' '-5.3463' ..., '-5.4902' '-5.4912' '-5.4926']

plt.plot(depth, distance)# here comes the problem  

The error message says that in line for plt.plot(depth, distance): ValueError: could not convert string to float: f
I don't understand this because it seems I converted all string values into float type. I tried to search this problem on stackoverflow but they all seem to solve the problem once they cast all string values into float or int. Can anyone give any suggestion on this problem? I would be really appreciate for any help.

Answer 1

You confused the value with the type. If you're trying to declare the type, you need to use "dtype=". What you actually did was to stick a single character into the array.

To answer a later question, your line

word = float(word)

likely worked just fine. However, we can't tell because you didn't do anything with the resulting value. Are you expecting this to alter the original inside the variable "line"? Common variables don't work that way.