【答案】应助回帖

data=load('test.dat');
x = unique(data(:,1));
y1 = 49.5;
y2 = 79.5;
for i = 1:length(x)
    sum_I = sum(data(data(:,1)==x(i) & data(:,2)>=y1 & data(:,2)<=y2,:));
    disp([x(i),sum_I(3)])
end

    0.5000   -0.3586

    1.0000    1.4329

    1.5000    0.0431

    2.0000   -6.0450

    2.5000   -2.3909

    3.0000    2.7712