Map f onto the part at position 2:

Map f onto multiple parts:

Map f onto a more deeply nested part:

Map f onto the second element of all top-level parts (the "second column"):

Map f onto an association:

Use Key to specify position:

For string keys, Key is not needed:

Use negative position in an association:

Use the operator form of MapAt:

MapAt can be used on expressions with any head:

Key and integer part specifications in associations are different:

The function can be mapped onto the heads as well:

Map can be used on sparse arrays:

Reset an element in a matrix:

Reverse the sign of an element in a matrix:

Enumerate cases with f mapped onto successive elements:

Reverse the color of a cell at a random position at each step:

MapAt can use lists of parts returned by Position:

These are the subexpressions returned by Extract:

MapAt applies f repeatedly if a part is mentioned repeatedly:

MapAt[f,expr,{}] does not map f because the list of positions is empty:

MapAt[f,expr,{{}}] maps f to position {}, which corresponds to the whole expression expr: