clear all; close all; clc

a =    [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20];

%  GESUCHTE AUSGABE
%  a = [0 0 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 0 0];

largeEnough = a >= 3;
smallEnough = a <= 18;

% bereich = find(largeEnough & smallEnough);
% a(~bereich) = [];

[r, c, v] = find(largeEnough & smallEnough);
a(1, 1:c-1) = 0;
a(1, c(end)+1:a(end)) = 0;