Is it possible to make a loop to run another loop?

Hello! I have a large matrix that I need to work with. Now I select a separate vector and work with it in a loop

Xvector=Xmatrix(:,1);
for j=1:100 % 100 - 100 is the length of the column
    x=mean(mean(Xvector(a(j):a(j+1)));) % a - the values ​​I'm working with
end

now i want to do so

for i=1:1000 % 1000 is the length of the lines
    Xvector(:,i)=Xmatrix(:,i);
for j=1:100 % 100 - 100 is the length of the column
    x=mean(mean(Xvector(a(j):a(j+1)));) % a - the values ​​I'm working with
end
end

only the last value (last column) counts to me.

how can I fix this? ps I understand the cycle will be long, but manually entering everything will take a very long time

[Walter Roberson]

for i=1:1000 % 1000 is the length of the lines
  Xvector(:,i)=Xmatrix(:,i);
j = 100 % 100 - 100 is the length of the column

x(i) = mean(mean(Xvector(a(j):a(j+1)));) % a - the values I'm working with
end

[Lev Mihailov]

a=[1:4:100]; 
Index exceeds the number of array elements (25).

Error in p (line 47)
x(i) = mean(mean(Xvector(a(j):a(j+1)))) % a - the values I'm working with