Life Outside Vr 4

We've looked at external files with the goal of getting useful information out of them. At the same time how about saving something there. We use a program that takes symbols in a certain layer, compares their elevation to an active dtm, and if they are over a specified distance from the elevation displays the error. It is a very useful program and when it was first written had all the variables hard coded, but stored at the beginning of the file so they could be easily found and changed. With the advent of PromBox it just became too easy to allow the user to confirm and change the variables each time. You could even easily use the old hard coded values as defaults. However it seemed like a certain set of values would be used over and over again based on job specifications. The obvious solution would be to have the last values used stored between runs to be used as the new defaults. Storing values like this is easy enough, but what elements would the most elegant solution contain? I have no idea but I did come up with a couple of features I really wanted.

First lets store them in a logical location like /vr/hostdir where all the other function parameter files are stored. Truthfully it would probably be better to make a user parameter directory, but I'll be lazy for now because the next thing I wanted was a test to see if the parameter file existed already and if not, would create one with the original default values. Yes I could test for a folder and if it wasn't there create it but maybe that will come in the next version but for now I didn't really want to change the directory structure.

import os
# os is necessary to test the existence of the parameter file.

ParFileName='c:/vr/hostdir/save_params.par'
# Call it what you want, this is great code to save for later use in other
# programs that require persistence.

if not os.path.isfile(ParFileName): # Does the file exist.
open(ParFileName,'w').write('%d %.3f %.3f'%(86,.2,0.0))
# If not then create it in write mode and write an integer and 2 floating
# point numbers with 3 decimal places.

Params=open(ParFileName,'r').read().split()
# Since it either existed or was created all we need to do now is open, read
# and split the values into a list that would look like ['86','.2','0.0']

PromBox=VrPromBox('Display Sym to Dtm',20,1)
PromBox.AddInt('Layer to Check',int(Params[0]),1,1000)
PromBox.AddDouble('Display delta >',float(Params[1]),2)
PromBox.AddDouble('Text Rotation',float(Params[2]),1)
# Using the values stored in the Params list make a prombox which will allow
# the user to change the values.

if (PromBox.Display(0) == 0):
...LayerToCheck=PromBox.GetInt(0)
...DisplayDelta=PromBox.GetDouble(1)
...TextRot=(PromBox.GetDouble(2))
# Grab the new values.

open(ParFileName,'w').write('%d %.3f %.3f'%(LayerToCheck,DisplayDelta,TextRot))
# And write them out to a file.

I won't spend the time on all the uses for keeping persistent variables between sessions, but this is the basics.

This just about wraps it up for reading and writing files, except for maybe writing parameters as a binary file, just for the fun of it and maybe walking through the single line parser I posted on the forum. It isn't very flexible but offers some interesting lessons. Here's a peek.

alist=[]
for line in open('hughes.pxyzopk','r').readlines():Alist.append(line.split())

Life Outside Vr 3 (well not very far outside)

Actually not at all, but the point isn't that we need to work outside Vr, it is that there is a whole world of files available using some straight python functionality. Now that we have a little file opening and reading under our belts, lets come back into Vr and bring a little of that technique with us. Let's also work on tightening up the code a little. If you are fairly new to VrPython, I think you'll really like this one. We are going to let Vr tell us the name of the file to open (in this case the current function key file) then using standard python we'll open, read, and parse the data we want out of the file, then use Vr to build a prompt box. For this example the data we want is all the function key names, and the prompt box will allow us to pick a function key name. Why? Well how about if you were going to write a parallel function using the existing inspar command but wanted to change the PARMOD= to "Function Key" and wanted to select the function key from a list then send it using the FKEY= key-in. If so, here is one way to get the function key name. It is rather long and convoluted but I'll try to make it worth what you are paying for the look.

Cfig=VrCfg()
# We'll need a Vr config object to access the name of the currently
# loaded function key file name.
FileName=Cfig.GetFkeyFileName()
# Grab the name.
FkeyFile=open(FileName,'r')
# Open the file
FileData=FkeyFile.read()
# We're going to read the file twice. This time as a string so we can count the
# instances of the key-word "KeyName" to figure out how many entries our
# prompt box will need.
NumberOfFunctionKeys=FileData.count('Fkey KeyName')
# Use the string method count to do just that.
PromBox = VrPromBox ("Function Keys", 60, 1)
# Create a PromBox object.
PromBox.AddList ("Select a Function Key",NumberOfFunctionKeys , 40, 0)
# I'll skip helpful text and just get right to the listbox of length equal to
# the number of function keys.
FkeyFile.seek(0,0)
# Rewind the file to read it again.
FileData=FkeyFile.readlines()
# This time read it as a list, each element of which is a line from the file.
for FileLine in FileData:
# For each line in the data list.
..if FileLine.count('Fkey KeyName') != 0:
# Test to see if it is a line containing the function key name.
....FkeyName=FileLine.split()[2]
# If so split the line data into the three words
# 'Fkey','KeyName','ActualName' which is in itself a list, the third element
# of which ( [2] ) is the actual name of the function key.
....PromBox.AddListItem(FkeyName)
# Add that to the list of items to be displayed.
FkeyFile.close()
# Could have closed it after the readlines() but just go ahead and do it now.
if (PromBox.Display(0) == 0):
..FunctionKey=PromBox.GetList (0)
# Standard prompt box usage to grab a list item
print FunctionKey
# and print it out (or use it in an interesting way)

Now as far as tightening it up. I could have read the file only once and built a for loop to count the instances of the KeyName key-word, but I'm going to go in a totally different direction. I'm still going to read the file twice, as a matter of fact I'm going to open it twice. The goal of this exercise is to get as few lines of code as I can. I'm not going to profess that this is the tightest and most efficient. It just has a lot less lines of code and is intended to make you think. Look at it first, then read the explanation.

PromBox = VrPromBox ("Function Keys", 60, 1)
PromBox.AddList ("Select a Function Key", open(VrCfg().GetFkeyFileName (),'r').read().count('Fkey KeyName'), 40, 0)
for FileLine in open(VrCfg().GetFkeyFileName (),'r').readlines():
..if FileLine.count('Fkey KeyName'):PromBox.AddListItem(FileLine.split()[2])
if (PromBox.Display(0) == 0):print PromBox.GetList (0)


Line 1: We have to create a prompt box object because I can't find any way around it and we'll need to use methods of this specific object in the next line.

Line 2: Let's take this a step at a time.
A) Add to the list using the formula AddList (Prompt, NumItems, Height, DefaultItem) Prompt is set to "Select a Function Key", NumItems is the tricky part, Height is 40 and DefaultItem is 0.
B) Look at the NumItems formula and read it as follows
Open a file using the string returned by a VrCfig() object method GetFkeyFileName in read mode ( 'r' ) which open file object you should read() the data, of which data you should count() the number of occurrences of 'Fkey KeyName'. Notice that when all this is said and done it just give a number which represents the number of function key names and therefore the number of function keys

Now that we have a prompt box which contains a list set to the length equal to the number of function key names, let's populate the list with those names.

Line 3: For each element in a list, the list comes from opening the same file again, in read mode again, but this time instead of read() to get one solid string, we use a method that will return a list which is exactly what the for loop is working on. We are looping on a list that was created and populated just for the loop, each element of which is a line from that file.

Line 4: FileLine represents the line of data out of the file which is being examined as we loop over the entire list of lines. count() tells how many occurrences of a work exist in the string 0 means none which in terms of an if means false so any other number that comes back means true (the string exists, therefore it is a function key name). if it is true we want to add the actual name to the list. The actual name as we saw above means splitting the string, which creates a list, from which we want element [2]. So add to the list the third word which is the result of splitting the string because it does (true) contain the phrase 'Fkey KeyName'. I didn't add another line and indent it because there is only one statement to execute if true which allows you to just put it after the colon and be done with it.

Line 5: Same idea, standard prombox syntax, but since there is only one action, just put it on the same line. Ugly code but remember my only goal was to reduce the line count. And why put the string in a variable just to print it on the next line, print it as it returns from GetList()

And I haven't even taken the time to explain the forum post regarding reading and parsing aerosys .pxyzopk files, a great example of the kind of data we mappers work with all the time. Maybe next time.

Life Outside Vr 2

Ok I have a few minutes so let's look at actually reading some data out of an open file. We'll stick with ascii files for now. There are so many fun reasons to work with files outside Vr that it would take too long to enumerate all of them but a few of my favorites include.

• Reading info out of an ImageUtil project and creating all the Vr2Ori project and all models with 1 click.
• Converting all Vr2Ori and VrAt files to or from their corresponding equivalents in orientp3 (yes we still use it and get along fine) so a setup in the analytical equals a setup in softcopy or vice versa.
• Reading a PATB and plotting it graphically in VrOne for visual analysis and blunder checking.
• Reading and plotting AT results including photos, layout data, computed extents and photo scale based on image centers and control averages.

You get the idea, there are just so many files out there that can be of easy use with a click or two. Sometimes there is already a way to do the same things, but a custom program allows you to set things up exactly the way you want without any further intervention. As in the last post we'll be working with vr.cfg in read only mode because it is a file that exists on everyones system (if you are working in Vr, which I assume) and has the same basic patterns. Again I will suggest you work in some kind of python IDE, I suggest pythonwin but you could even work from the python command interpreter if you want.

Let's start by creating a python file object like we did last time, it will be called f (for file).
f=open('/vr/hostdir/vr.cfg','r') # 'r' means read mode.
now if I
print f
there should be some obscure text describing the object. Hopefully there was some time to look over the file object docs to study up on the simple methods, let's look at a few.
Read one line out of the open file
>>> f.readline()
'# VR Configuration File\n'
I should note here that once it reads the line, the address in the file that python is sitting at is the beginning of the next line so if you do it again
>>> f.readline()
'# FileName : c:\\vr\\hostdir\\vr.cfg\n'
if for some reason it is necessary to go back to the beginning of the file we need to "seek" the new position. seek takes two arguments "offset" (how much to jump from the position passed in) and "whence" (where to start the offset). "Whence" defaults to the beginning of the file, so if you want to go the the beginning you can say "see" with an offset of 0 (don't move at all from the position passed in) and a whence of 0 (or if you pass nothing for whence this is the default) so
f.seek(0,0) # 0 offset, 0 whence
f.seek(0) # 0 offset, default whence which is 0
will both take python to the beginning of the file, so do that then readline() again and you should see the first line again. Let's read that first line and store it as a string we'll call "s"
>>> s=f.readline()
>>> print s
# VR Configuration File
It would of course be possible to do this with each line of the file, though there are better methods to be discussed later, but lets look a what we could do with that string now that we have it.
I can split it based on white spaces (the default for split) which returns a list of the words in the string.
>>> s.split()
['#', 'VR', 'Configuration', 'File']
For that matter we could split it based on any character like commas, or in this case the "i"
>>> s.split('i')
['# VR Conf', 'gurat', 'on F', 'le\n']
Note that splitting on white spaces takes off the newline character but splitting on a printable character doesn't. Of course there are plenty of ways to take the newline off, but just take note for now. How about if we needed to test for a sub string inside the string, well let's count occurrences.
>>> s.count('i')
3
>>> s.count('ig')
1
>>> s.count('z')
0
So a test would look something like
>>> if s.count('ig') : print 'yes'
...
yes
I didn't say if s.count equals something or is greater than something because I just wanted to know if it was non zero and anything non zero is true for the if test.

There is no sense typing out all the things you can do with strings because they are documented in the python docs, it is worth noting that there used to be a string module, but now each of these methods is a built-in which is why there was never an import statement above.

Let's go back to the split step and put the resulting list in a variable called "l", then we can loop over the list and do whatever we want, in this example just print each item.
>>> l=s.split()
>>> l
['#', 'VR', 'Configuration', 'File']
>>> for item in l:
... print item
...
#
VR
Configuration
File
or grab individual elements of the list.
>>> l[3]
'File'
or test to see if the list contains certain values.
>>> l.count('File')
1
>>> l.count('Nothing')
0
Like strings I'll leave the explanation of lists and their methods to the professionals, in this case the python tutorials.
But I'll leave with a couple of thoughts:
First, check out readlines() it will come in handy and will probably bear heavily in the next post.
Second, one of the real beauties of python is it's ability to get a lot done with a little code, this can be accomplished by taking whatever data a method returns and acting on it with the appropriate method. Consider the following lines typed into the interpreter.

open() is going to return a file object with all it's accompanying methods.
>>> open('/vr/hostdir/vr.cfg','r')
obscure text describing an open file object

let's just tack the readline() onto the end and let it return the first line which is a string.
>>> open('/vr/hostdir/vr.cfg','r').readline()
'# VR Configuration File\n'

It it is a string we can split it to get a list of words that were included.
>>> open('/vr/hostdir/vr.cfg','r').readline().split()
['#', 'VR', 'Configuration', 'File']

Of course the list is made up of elements that can be addressed lets get the second element whose index is 1.
>>> open('/vr/hostdir/vr.cfg','r').readline().split()[1]
'VR'

Which is a string and can be forced to lower case using the lower() method.
>>> open('/vr/hostdir/vr.cfg','r').readline().split()[1].lower()
'vr'

Is this stuff cool or what.

Life outside Vr

I have had several folks ask about opening reading and parsing external files. There are so many applications that I don't even know where to begin. Whether plain ascii text files, or binary, python is loaded with tools for doing what you want or need. I think I'll just start rambling and see where we head. One thing that you will notice immediately is that there can be literally dozens of ways of accomplishing the same task. I'm not going to pretend that what I am putting forward is the best or most efficient, I'll just mention some tools that come in handy quite often. For today lets just open a file and grab some data from it. Let's pick a file that everyone should have, that being your Vr configuration file \vr\hostdir\vr.cfg. Don't worry, for now we'll just open it in read only mode. In your favorite python editor or pyedi within VrOne type and run the following.

f=open('\vr\hostdir\vr.cfg','r')

Ok, I'll admit that was a trick, it shouldn't have worked, even though everything about the command is totally correct, except that is for the backslashes "\". But wait you say, we are working in windows, and the backslash is what is always used to specify path hierarchy, and you would be correct. The problem is that the backslash is a special character in python (and several other languages, not to mention unix). Python uses a forward slash to separate path elements, or to use backslashes you have to tell python you are sending it a "raw" string and that it should not treat the backslashes as escape characters. I don't start here to be bothersome, but it is important to understand that python expects certain things and may send back something you aren't expecting if you aren't careful. Note the paste of a session where I open the file using 3 valid commands, look at what comes back, and realize that this is outside VrOne in the PythonWin editor.

First I open it with forward slashes because I grew up on unix, the forward slash is always in the same place no matter what keyboard you have, it is what python prefers, and I just like it better. I'll use "f" as the open file object and will open it, display it, and close it. The 'r' means to open it in read-only mode, we'll talk about modes later but I digress.

>>> f=open('/vr/hostdir/vr.cfg','r')
>>> f
Text describing file object which blogger deletes for some reason.
>>> f.close()

Next we'll use backslashes, but they have to be protected by another character so they aren't evaluated as an escape sequence, by coincidence the character that does this is a backslash so each occurence must be entered as
>>> f=open('\\vr\\hostdir\\vr.cfg','r')
>>> f
Text describing file object which blogger deletes for some reason.
>>> f.close()

Next we'll use the backslashes but we'll tell python that the name string is a raw string and that the backslashes shouldn't be evaluated as anything else. Just put an r in front of it and we're good to go.
>>> f=open(r'\vr\hostdir\vr.cfg','r')
>>> f
Text describing file object which blogger deletes for some reason.
>>> f.close()

It is worth noting that if you grab a filename inside Vr using either of the following, it returns forward slashes.
print PyVrGui().OpenFileNameDialog('title','c:/vr/hostdir','*.cfg')
C:/vr/hostdir/vr.cfg
print PyVrGui().OpenFileNameDialog('title','c:\\vr\\hostdir','*.cfg')
C:/vr/hostdir/vr.cfg

That will do for now. Yeah it's pretty slim, but I need to go and we'll actually read something next time. For now though it wouldn't hurt to get familiar with some of the python file methods, from the python docs. You can also get a little more information on the open function from the built-in functions docs (just scroll down to open()), this will come in handy when we move to appending, writing, and binary files.

Still breathing

Just a quick apology that with one graduating college, one high school, and another 2 with year end activities to take in, I just haven't had time to get into anything. I even know where I would like to head but will have to put it on hold for a few weeks. Check back.