It seems you have copied the data from Excel and there is a loss of data in doing that.
A= [0.0000 0.4341 -0.0000 -0.5910 -0.0352 2.0350 -0.0000 -0.9597 0.0000 -1.2164 -2.7826 -0.0000 0.3716 -0.0000 -0.0000 -0.0000 1.4557 0.0000 -0.0000 0.5599 -0.0000 -0.2463 -0.7001 0.0000];
%range
idx = A>=-0.5 & A<=0.5;
%sorted A values in the range
B = sort(A(idx))
avg = mean(B)
Note that the range containing maximum number of elements can vary according to the bin size and the start point.
For example -
%bin size 0.5, start point -3.5, the range [0 0.5] has the max no. of elements
bin=-3.5:0.5:3.5;
histogram(A,bin)
xticks(bin)
%bin size - 0.3, start point -3.2, range with max elements is [-0.2 0.1]
bin=-3.2:0.3:3;
histogram(A,bin)
xticks(bin)
