Have you ever wondered what the formula for that thing that looks like a sine wave, but is really just half circles? Well, take a look below for the Matlab code of how to generate and plot such a signal with a particular frequency. If you need help translating to another programming or graphing language, just let me know.

Also, a handy feature of this code is the ability to only plot the first N periods. Take a look!

Fs = 100;                     % Sampling frequency
T = 1/Fs;
L = 1000;                     % Number of samples
t = (0:L-1)*T;                % Time vector

fs = 1;                       % Frequency of signal.
A = 1;                        % Amplitude of signal.
% Half-circle waveform:
r = 1/(fs*4);
y = (A/r)*((2*(mod(t,r*4) == mod(t,2*r)) - 1).*sqrt(r^2 - (mod(t,2*r)-r).^2));

% Regular sine waveform:
y2 = A*sin(2*pi*fs*t);

% Plot the waveforms.
numPeriods = 3;
tEnd = round(numPeriods/(T*fs));
plot(t(1:tEnd),y(1:tEnd),t(1:tEnd),y2(1:tEnd))

This is some simple code I cooked up to convert a unix timestamp into a human readable date. The code will work up to the year 2100, when it will incorrectly add a leap day. Anyway, I couldn’t find this anywhere on the Internet, so maybe someone else will find this useful.

void dateAndTimeFromTimestamp(int i)
{
   int days    = i/(24*3600);

   // How many 4-year segments do you have (going 365, 365, 366, 365 days)
   int year    = days/(365*4 + 1);
       year    = 1970 + year*4;

   // How much is left over when you divide days into 4-year segments
       days    = days%(365*4 + 1);
   if(days >= 365 && days < 2*365)
   {
      days -= 365;
      year += 1;
   }
   else if(days >= 2*365 && days < 3*365 + 1)
   {
      days -= 2*365;
      year += 2;
   }
   else
   {
      days -= (3*365 + 1);
      year += 3;
   }

   int seconds = i%(24*3600);
   int hours   = seconds/3600;
       seconds = seconds%3600;
   int minutes = seconds/60;
       seconds = seconds%60;

   int month;
   for(month=1; month <= 12; month++)
   {
	// 30 days has September, April, June, and November
	if((month == 4 || month == 6 || month == 9 || month == 11) && days >= 30)
		days -= 30;

	// Except for February, which has 28 days
	// (the 29 days on leap year already accounted for above)
	else if(month == 2 && days >= 28)
		days -= 28;

	// All the rest have 31
	else if(days >= 31)
		days -= 31;

        else
                break;
   }
   days++;

   printf("%d-%d-%d %d:%d:%d",month,days,year,hours,minutes,seconds);
}