Savatore, it depends on what you want to do. You need to tell us because that will affect the answer. For example if you have matrix A and matrix B and you want to set matrix B to 999 every place that matrix A is between 100 and 200, you'd do something like this
B(A>100 & A<200) = 999;
No for loop needed. In general, you do something like
logicalIndexes = (A == someCondition);
someMatrix(logicalIndexes) = someValue;
But if you wanted to do something like beep a tone the number of times that equals the value of A but only for all A that are less than 10, or something else that's more complicated, then you might want to use a loop.