how to implement spectrogram in matlab?

I am trying to plot the spectrogram of the following signal with following code

% Signal 2
fs = 40;              % Sampling frequency 

t2 = 0:( 1/fs ):6;    % Time vector 

S2 = [ sin( 2*pi*5*t2( t2<=2 ) ), sin( 2*pi*10*t2( 2<t2<4 ) ),sin( 2*pi*15*t2( t2>4 ) ) ];

% codes for spectrogram 
X = S2 + 2*randn(size(t2)); % Defining the Entire Data Vector for Spectogram
NFFT = 2^nextpow2(402); 
window = 100;
spectrogram(X,window,window/2,NFFT,fs);

I am not getting the right spectrogram plot. Can someone tell me where is the problem with the code?

Answer 1

[Walter Roberson]

2<t2<4 is parsed as ((2<t2)<4). The 2<t2 part returns 0 (false) or 1 (true) and then <4 part compares that 0 or 1 to <4, which is always true. The fix is:

S2 = [ sin( 2*pi*5*t2( t2<=2 ) ), sin( 2*pi*10*t2( 2<t2 & t2<4 ) ),sin( 2*pi*15*t2( t2>4 ) ) ];

By the way: is there a reason that you want the sample at t2 == 4 exactly to be omitted ?