I have a problem : suppose I have a matrix

A =

    -1     2    -3
    -4     5    -6
    -7     8    -9

I convert it into a column matrix

B = A(:)

B =

    -1
    -4
    -7
     2
     5
     8
    -3
    -6
    -9

Suppose I want to force the first column elements to lie within a particular range (-range1 : range1) , second column elements within (-range2 : range2) & third column elements within (-range3:range3). I tried doing that by implementing this code :

range1 = 0;
range2 = -5;
range3 = 0;

B(B(1:3,1)<range1)=10;
B(B(4:6,1)>range2)=0;
B(B(7:9,1)<range3)=20;

The answer I get is this :

B =

    20
    20
    20
     2
     5
     8
    -3
    -6
    -9

Whereas the correct answer I should get is this :

B =

    10
    10
    10
     0
     0
     0
    20
    20
    20

What I am doing wrong ? Please help.

What is wrong:

Look closely at your command:

>> B( B(7:9, 1) < range3 ) = 20;

And now let's do it step by step

You are conditioning on the last three elements B( 7:9, 1 ) which are -3, -6 and -9.
Therefore you end up with

>> B(7:9, 1) < range3
ans =
  true
  true
  true

You have a logical indexing of three elements. Using these logical indices to access B, which has 9 elements result with an access to the first three elements out of nine.
Thus, all your commands only modifes the first three elements of B without affecting the rest of B.

A possible fix:

You can actively define the range you are working on, for example, the second column:

>> aRange = 4:6;
>> B( aRange( B(aRange, 1) > range2 ) ) = 0

See how the three-vector logical indexing B(aRange, 1) > range2 ) now index aRange (which has 3 elements) and not B (which has 9 elements) directly.