Generating a Pseudo-random sequence of plus/minus 1 integers

Question

Can anybody help me create a simple pseudo-random sequence of +-1 integers with length 1000 using Matlab?

I.e. a sequence such as

-1 -1 1 1 -1 -1 1 -1 -         


        

Answer 1

Thanks for these many helpful answers. I figure this topic might be general enough it may well deserve a comparison.

In my setup ( Windows8.4 x64 i74820k cpu and with R2014a) the fastest version is consistently:

x=2*round(rand(L,1))-1;

Being half an order of magnitude faster than the slowest solution. Hope this helps.

comparison: figure comparing execution times for pseudo-random sign generation

code:

L=[];
for expon=0:6
    for mant=1:9
    L=cat(1,L,mant*power(10,expon));
    end
end
clear expon mant

t1=zeros(length(L),1);
x=2*round(rand(L(1),1))-1;
for li=1:length(L)
    tic,
    x=2*round(rand(L(li),1))-1;
    t1(li)=toc;
end

t2=zeros(length(L),1);
x=(rand(L(1),1)>0.5)*2-1;
for li=1:length(L)
    tic,
    x=(rand(L(li),1)>0.5)*2-1;
    t2(li)=toc;
end

t3=zeros(length(L),1);
x=(randi([0,1],L(1),1)>0.5)*2-1;
for li=1:length(L)
    tic,
    x=(randi([0,1],L(li),1)>0.5)*2-1;
    t3(li)=toc;
end

t4=zeros(length(L),1);
x=rand(L(1),1);ind=x>=0.5;x(ind)=1;x(~ind)=-1;
for li=1:length(L)
    tic,
    x=rand(L(li),1);
    ind=x>=0.5;
    x(ind)=1;
    x(~ind)=-1;
    t4(li)=toc;
end

t5=zeros(length(L),1);
x=sign(randn(L(1),1));
for li=1:length(L)
    tic,
    x=sign(randn(L(li),1));
    x(x==0)=1;
    t5(li)=toc;
end

t6=zeros(length(L),1);
vec = [-1 1];
x=vec(randi(numel(vec),L(1),1));
for li=1:length(L)
    tic,
    x=vec(randi(numel(vec),L(li),1));
    t6(li)=toc;
end

t7=zeros(length(L),1);
x=2*randi(2,L(1),1)-3;
for li=1:length(L)
    tic,
    x=2*randi(2,L(li),1)-3;
    t7(li)=toc;
end

t8=zeros(length(L),1);
x=randsample([-1 1],L(1),true);
for li=1:length(L)
    tic,
    x=randsample([-1 1],L(li),true);
    t8(li)=toc;
end

clear x vec ind li

figure,
loglog(L,[t1 t2 t3 t4 t5 t6 t7 t8],'.-','linewidth',2)
grid on 
grid minor
title('Generating pseudo-random sequence +1/-1')
ylabel('Exec. Time [s]')
xlabel('Output Vector Length')
T{1}='x=2*round(rand(L(1),1))-1';
T{2}='x=(rand(L(1),1)>0.5)*2-1';
T{3}='x=(randi([0,1],L(1),1)>0.5)*2-1';
T{4}='x=rand(L(1),1);ind=x>=0.5;x(ind)=1;x(~ind)=-1';
T{5}='x=sign(randn(L(1),1))';
T{6}='vec=[-1 1];x=vec(randi(numel(vec),L(1),1))';
T{7}='x=2*randi(2,L(1),1)-3';
T{8}='x=randsample([-1 1],L(1),true)';
legend(T,'location','northwest')