Converting a String to a Double (with SI prefixes)

Engineers and scientists will often use SI prefixes to make writing down very large, and very small, numbers easier. Writing down 3 GV is much better than 3000000000 V :). I’m currently working on a couple of software projects, both at home and work, where I’d like the ability to enter numbers with SI prefixes for convenience.

First I decided to write down the different styles of input my code will have to support, below is the list of styles I came up with.

Now I know what I want the code to do I can start writing it. I created a dictionary containing the prefixe character and the associated multiplier. I am writing this code in C# by the way, and used LINQPad to try it all out. Once I had it working I put into the class library I was working on.

private readonly Dictionary Prefixes = new Dictionary(){
    {'P', 1e15},
    {'T', 1e12},
    {'G', 1e9},
    {'M', 1e6},
    {'k', 1e3},
    {'h', 1e2},
    {'d', 1e-1},
    {'c', 1e-2},
    {'m', 1e-3},
    {'u', 1e-6},
    {'n', 1e-9},
    {'p', 1e-12},
    {'f', 1e-15},
};

Then I wrote a method that will take in the input string and convert it to a double. The first thing the method does is check to see if it is a plain number that Double.Parse() can take care of, it does this check using a regex. If the Regex matched then it simply calls the Double.Parse() method and returns the result.

If the regex fails then it check using two more regexes if the number looks like it contains SI prefixes. If it does then we find out what prefix is used then remove the prefix and convert the number.

I am not very good with regular expressions so there may be better ways of writing them than this. I have tested this code quite a bit with different types of input and it seems pretty solid. It will throw a FormatException if anything goes wrong.

// These are the regexes used by the method. These are initialised in a constructor.
Regex plain_number_regex = new Regex(@"^[+-]?(?=[\.\d])\d*(\.\d+)?$"); // For .2222, 0.222, 2.32
Regex si_number_a_regex = new Regex(@"^[+-]?[\d]+[PTGMkcmunpf]?[\d]*$"); // for 2k, 2k2
Regex si_number_b_regex = new Regex(@"^[+-]?[\d]+(\.\d+)?[PTGMkcmunpf]?$"); // For 1.2k
 
public double ParseInputStringSI(string input)
{
    // Test to see if it is a plain number with no SI prefixes
    if (plain_number_regex.IsMatch(input)) {
        return Double.Parse(input);
    }
 
    // Test to see if it is a number with an SI prefix.
    if(si_number_a_regex.IsMatch(input) || si_number_b_regex.IsMatch(input) ) {
        // Find where in the string the prefix is and what
        // kind of prefix it is.
        var input_prefix = from p in Prefixes.Keys
                           where input.IndexOf(p) > 0
                           select input[input.IndexOf(p)];
 
        // Make sure the above query worked. There should be
        // no reason for it to fail because the Regex checks
        // the prefix characters.
        if (input_prefix.Count() == 0) {
            throw new FormatException("Invalid Input");
        }
 
        // Get the multiplier for the prefix
        var multiplier = Prefixes[input_prefix.First()];
 
        // Ether replace the prefix with a decimal point or
    // remove it entierly. Depends on the format of the
    // input.
    string inputp;
    if( si_number_a_regex.IsMatch(input) ) {
            inputp = Regex.Replace(input, @"[PTGMkhdcmunpf]", ".");
    } else {
        inputp = Regex.Replace(input, @"[PTGMkhdcmunpf]", "");
    }
 
        // Attempt the conversion, multiply it then return it.
        var tmp = Double.Parse(inputp);
        return tmp * multiplier;
    } else {
        throw new FormatException("Input String is Invalid");
    }
}

Getting a List of the Available COM Ports in C#

It’s really nice to show the user a list of the COM ports they actually have on their machines. All too often I have seen software that makes you type in the COM port name. Even worse are the applications that force you to select from a list of COM ports, usually COM1 to COM5, without the option of typing in a different one!

Below is some really simple code that generates a list of the available COM ports and inserts the list into a drop-down selection control in a WinForms application.

string[] ports = SerialPort.GetPortNames();
if (ports.Length > 0) {
    Array.Sort(ports);
    COMPort.Items.AddRange(ports);
    COMPort.Text = ports[0];
} else {
    COMPort.Text = "Unable to Detect COM ports";
}

Creating an IO Assignment Header File

When developing embedded firmware I like to define macros that provide aliases for the IO port registers that I need. The name of each of the macros will correspond to the net name on the schematic to make it easy to check for errors. I put all of these macros into a single header file that I can include, I have sometimes seen these called board support packages. They can sometime be entire libraries that abstract details of the hardware. In my case they are just simple header files as that is all I need for now.

Recently I decided to automate some of the work of creating these files as it can be very time consuming. This is the Awk script I ended up with.

#!/usr/bin/awk -f
{
    if ($0 == "") next;
 
    if ($2 == "PORTA") { port = "A"; }
    if ($2 == "PORTB") { port = "B"; }
    if ($2 == "PORTC") { port = "C"; }
    if ($2 == "PORTD") { port = "D"; }
    if ($2 == "PORTE") { port = "E"; }
    if ($2 == "PORTF") { port = "F"; }
    if ($2 == "PORTG") { port = "G"; }
 
    print "#define " $1 "_PIN " $3;
    print "#define " $1 "_TRIS TRIS" port "bits.TRIS" port $3;
    print "#define " $1 "_LAT LAT" port "bits.LAT" port $3;
    print "#define " $1 "_PORT PORT" port "bits.R" port $3;
    print "";
}

All I need to do is write a space separated file in which each line contains the name I want, the port it is on and the number of the pin that it is attached to. Then this simple script generates the C code. This was for a PIC24F series micro, I have a slightly different script for a project involving a PIC32 which I may post later.

This makes creating BSP header files really easy, especially if you need to modify the pin assignments!

Profiling Python Applications

I’ve been working on a project that’s written in Python, it continuously communicates with some external industrial equipment. It will poll the status of this equipment 4 times per second and also sends commands to them when requested to. My job this week was to raise the update rate to 5Hz…. I needed to make sure I had enough time to do this!

I decided before doing anything I should profile the code to find out how much time the main loop needs to run and what methods take the longest time. That way I’d know if the code can support 5Hz and if not what I can do about it.

Once again the Python standard library comes to the rescue, the cProfile module will monitor the execution of your program and generate a report. Below is an example of how to use it.

import cProfile
cProfile.run("main()")

The next thing I did is write a simple bit of code that will print to stdout the current update rate of my application every second. It’s pretty much a Python port of the JavaScript library stats.js.

from __future__ import division
import time

class stats(object):
    def __init__(self):
        self.msMin = 1000
        self.msMax = 0
        self.msTime = 0
        self.fpsMin = 1000
        self.fpsMax = 0
        self.fps = 0;
        self.updates = 0
        self.startTime = int((time.time()+0.5)*1000)
        self.prevTime = int((time.time()+0.5)*1000)

    def begin(self):
        """Calling the method signifies the start of a frame
        """
        self.startTime = int((time.time()+0.5)*1000)

    def end(self):
        """Calling this method signifies the end of a frame
        """
        now = int((time.time()+0.5)*1000)

        self.msTime = now-self.startTime
        self.msMax = max(self.msMax, self.msTime)
        self.msMin = min(self.msMin, self.msTime)

        #print "ms: %i (%i - %i)" % (self.msTime, self.msMin, self.msMax)

        self.updates = self.updates + 1

        if now > (self.prevTime + 1000.0):
            self.fps = round((self.updates*1000.0)/float(now-self.prevTime))
            self.fpsMax = max(self.fpsMax, self.fps)
            self.fpsMin = min(self.fpsMin, self.fps)

            print "stats: %i fps (%.i fps - %i fps)" % (self.fps, self.fpsMin, self.fpsMax)

            self.prevTime = now
            self.updates = 0

        return now

        def update(self):
            self.startTime = self.end()

Simply add a call to begin() to the start of your loop and a corresponding call to end()  at the end of your loop. Now I get a nice counter that tells me how the code is performing. As I work on the code I can see how this affects the main loop performance.

Finding a File Descriptor Leak

My current project is a control system that runs on a small embedded PC running a Linux OS. I had a problem during development where I had to open/close two serial ports alternatively because they shared an interrupt. Doing this suddenly caused the software to crash after running for a few minutes. The problem turned out to be that I was leaking file descriptors!

I wrote a simple shell script that prints the the total file descriptors open on the serial ports. This helped me make sure my bug fix worked correctly.

#!/bin/sh
 
TIME=1
 
while [ 1 -eq 1 ]; do
    clear
    lsof | awk '''
BEGIN { ttyS0 = ttyS1 = ttyS3 = ttyS4 = 0; }
/\/dev\/ttyS4/ { ttyS4++; }
/\/dev\/ttyS3/ { ttyS3++; }
/\/dev\/ttyS0/ { ttyS0++; }
/\/dev\/ttyS1/ { ttyS1++; }
END {
    print " Descriptor    Counter ";
    print " ttyS4       " ttyS4;
    print " ttyS3       " ttyS3;
    print " ttyS1       " ttyS1;
    print " ttyS0       " ttyS0;
}
'''
    sleep $TIME
done

In the end I manually control the serial port using the sys module instead of using pySerial. I still have no idea why pyserial started leaking resources.

Plotting a Text File with Matplotlib and IPython

Earlier I was converting a Scilab simulation into C and I had the code emit a load of text files containing the data because I didn’t want to do any plotting or UI stuff in C. But I still wanted to plot the data so I can quickly check everything’s working. I also wanted to do some post processing of the data too… well that’s Python really shines in my opinion. I fired up IPython and used Numpy and Matplotlib.

 import numpy as np
 import matplotlib.pyplot as pyplot
 pyplot.plot( np.loadtxt("data/somedatafile.dat") )
 pyplot.show()

Done! In four lines of code I have my plot. I love IPython, numpy and matplotlib, they allow you to get things done really fast. I also love that IPython even auto-completes file paths, so very handy :-).

Formatting Numbers using SI Prefixes

Well it’s time for another post. A couple of weeks ago I was working on an interface program for a rig at work. We were adding some extra features to a controller board and I thought while I was working on that I might as well make some changes to the interface program as well.  The feature I wanted to add to the program was to make it format some of the numbers that are displayed in the UI using SI prefixes and with units of measure.

So after a quick look on the web to see if I can find some example code on how to do this, I decided to have a go writing the code on my own without looking at other stuff too much. This is what I came up with, It’s a function written in C# that can be used to convert floating point numbers to formatted strings. So the number 0.102 will be printed as “102m”, and it can append units to the end like this – “102mA”.

public static string FormatStringEng(double input, string units, string format)
{
    string prefix = "";
    double value = 0.0;

    if (input >= 1e+12 && input < 1e+15) {
        prefix = "T"; value = (input / 1e+12);    // Tera (1e+12)
    } else if (input >= 1e+9 && input < 1e+12) {
        prefix = "G"; value = (input / 1e+9);     // Giga (1e+9
    } else if (input >= 1e+6 && input < 1e+9) {
        prefix = "M"; value = (input / 1e+6);     // Mega (1e+6)
    } else if (input >= 1e+3 && input < 1e+6) {
        prefix = "k"; value = (input / 1e+3);     // Kilo (1e+3)
    } else if ((input >= 1) && (input < 1e+6)) {
        prefix = ""; value = input;               // Unity
    } else if (input >= 1e-3 && input < 1) {
        prefix = "m"; value = (input * 1e+3);     // Milli (1e-3)
    } else if (input >= 1e-6 && input < 1e-3) {
        prefix = "μ"; value = (input * 1e+6);     // Micro (1e-6)
    } else if (input >= 1e-9 && input < 1e-6) {
        prefix = "n"; value = (input * 1e+9);     // Nano (1e-9)
    } else if (input >= 1e-12 && input < 1e-9) {
        prefix = "p"; value = (input * 1e+12);    // Pico (1e-12)
    } else if (input >= 1e-15 && input < 1e-12) {
        prefix = "f"; value = (input * 1e+15);    // Fempto (1e-15)
    }

    return value.ToString() + prefix + units;
}

This code is working quite well so far. Haven’t had any problems with it… yet anyway. That’s it for now, see ya!