[Tutor] Reading in large .dat file

[Peter Otten]

Colin Ross wrote:

> Good afternoon,
> 
> I have a .dat file that is 214 rows by 65 columns that I would like to
> read into python and plot the 1st column versus all other columns.
> 
> My code:
> 
> #########################################################################
> 
> import numpy as np
> import scipy
> import pylab as pl
> import matplotlib
> from matplotlib.ticker import ScalarFormatter, FormatStrFormatter
> import sys
> 
> # Load in text from .dat file
> 
> sed = np.loadtxt('spectra.dat', unpack = True)

for flux in sed:
    pl.plot(wavelength, flux)

> pl.xscale('log')
> 
> pl.show()
> 
> #########################################################################
> 
> This is fine if I want to write out a separate line for each of the 65
> columns, but I would like to simplify the code by looping over the data.
> Can someone please help me formatting the loop correctly?
 
To understand why the above works (assuming it does) consider that for many 
data types

for item in data:
    ... # use item

is equivalent to

for index in range(len(data)):
    item = data[index]
    ... # use item

Unrolling the above for len(data) == 3 gives:

item = data[0]
... # use first item
item = data[1]
... # use second item
item = data[2]
... # use third item