6.36 Selecting and replace elements from matrix

Given a matrix, say which has Indeterminate and we want to change all these entries in the matrix by zero.

mat = {{-1., -1., Indeterminate, -1., -1.}, 
       {-1., -1., Indeterminate, -1., -1.}, 
       {Indeterminate, Indeterminate, Indeterminate, Indeterminate, Indeterminate}, 
       {-1., -1., Indeterminate, -1., -1.}, 
       {-1., -1., Indeterminate, -1., -1.}} 
 
p = Position[mat, Indeterminate] 
mat = ReplacePart[mat, p -> 0] 
 
Out[169]= {{-1., -1., 0, -1., -1.}, 
           {-1., -1., 0, -1., -1.}, 
           {0, 0, 0, 0, 0}, 
           {-1., -1., 0, -1., -1.}, 
           {-1., -1., 0, -1., -1.}}
 

another example, given a matrix of values, replace those values which are less than \(0.5\) by NULL

n = 5; 
a = Table[RandomReal[], {n}, {n}]; 
p = Position[a, x_ /; x < 0.5]; 
a = ReplacePart[a, p -> Null] 
 
Out[173]= {{Null, Null, Null, 0.6781657418995635, 0.7290662037036753}, 
           {Null, 0.7084980071179792, Null, Null, 0.5811489862295911}, 
           {Null, Null, 0.8467863882617719, Null, 0.8891915946646993}, 
           {0.8173279058333203, 0.7272894246356278, Null, Null, 0.8665880423275274}, 
           {Null, Null, 0.662026816962838, 0.5982839657423036, 0.6603967280952212}}