# Applying Functions to Parts of Expressions

If you have a list of elements, it is often important to be able to apply a function separately to each of the elements. You can do this in the Wolfram Language using Map.

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Map[f,{a,b,…}] | apply f to each element in a list, giving |

Applying a function to each element in a list.

What Map[f,expr] effectively does is to "wrap" the function f around each element of the expression expr. You can use Map on any expression, not just a list.

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Map[f,expr] applies f to the first level of parts in expr. You can use MapAll[f,expr] to apply f to *all* the parts of expr.

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*all*levels in . If you look carefully at this expression, you will see an wrapped around every part.

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In general, you can use level specifications as described in "Levels in Expressions" to tell Map to which parts of an expression to apply your function.

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Map[f,expr] or f/@expr | apply f to the first‐level parts of expr |

MapAll[f,expr] or f//@expr | apply f to all parts of expr |

Map[f,expr,lev] | apply f to each part of expr at levels specified by lev |

Ways to apply a function to different parts of expressions.

Level specifications allow you to tell Map to which levels of parts in an expression you want a function applied. With MapAt, however, you can instead give an explicit list of parts where you want a function applied. You specify each part by giving its indices, as discussed in "Parts of Expressions".

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MapAt[f,expr,{part_{1},part_{2},…}] | apply f to specified parts of expr |

Applying a function to specific parts of an expression.

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MapIndexed[f,expr] | apply f to the elements of an expression, giving the part specification of each element as a second argument to f |

MapIndexed[f,expr,lev] | apply f to parts at specified levels, giving the list of indices for each part as a second argument to f |

Applying a function to parts and their indices.

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Map allows you to apply a function of one argument to parts of an expression. Sometimes, however, you may instead want to apply a function of several arguments to corresponding parts of several different expressions. You can do this using MapThread.

MapThread[f,{expr_{1},expr_{2},…}] | apply f to corresponding elements in each of the |

MapThread[f,{expr_{1},expr_{2},…},lev] | apply f to parts of the at the specified level |

Applying a function to several expressions at once.

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Functions like Map allow you to create expressions with parts modified. Sometimes you simply want to go through an expression, and apply a particular function to some parts of it, without building a new expression. A typical case is when the function you apply has certain “side effects”, such as making assignments, or generating output.

Scan[f,expr] | evaluate f applied to each element of expr in turn |

Scan[f,expr,lev] | evaluate f applied to parts of expr on levels specified by lev |

Evaluating functions on parts of expressions.

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