list manipulation - Replace values which obey certain criteria

I'd like to replace all values in a list which obey one or more criteria with another value.

Example: Replace all values>30 by $30$.

data={{3,35},{2,7}}

Afterwards it should be:

{{3,30},{2,7}}

Example #2:

aa= {300., 150., 100., 76.8421, 64.0909, 55.567, 49.1935, 44.2262, 
    40.2247, 36.9355, 34.1667, 31.8138, 29.7887, 28.0087, 26.4537, 
    25.0612, 23.8186, 22.7059, 21.6854, 20.7663, 19.9265, 19.1519, 
    18.4323, 17.7739, 17.1672, 16.6007, 16.0648, 15.5605, 15.098}

If a value of aa is greater than 100 I want to replace it with an arbitrary value, e.g. 50.

Answer

Up front you have a choice between pattern-based and numeric manipulation of an array. Pattern-based is more general; numeric is usually fastest when applicable.

a = {{21, 95, 50}, {39, 32, 76}, {9, 12, 75}};

Examples of pattern based methods:

a /. n_Integer /; n > 30 -> 30

a /. n_?NumericQ /; n > 30 -> 30


Replace[a, n_?(#>30&) -> 30, {2}]

Examples of numeric methods:

Clip[a, {-∞, 30}]

(a - 30) UnitStep[30 - a] + 30

Other, less desirable methods:

If[# > 30, 30, #, #] & //@ a


Map[#~Min~30 &, a, {-1}]

---

Fast numeric methods for the second example:

Clip[aa, {-∞, 100}, {0, 50.}]

(1 - #) 50. + aa # & @ UnitStep[100 - aa]