Is there a difference between foreach and map?

Question

Ok this is more of a computer science question, than a question based on a particular language, but is there a difference between a map operation and a foreach operation? Or are they simply different names for the same thing?

Answer 1

Different.

foreach iterates over a list and applies some operation with side effects to each list member (such as saving each one to the database for example)

map iterates over a list, transforms each member of that list, and returns another list of the same size with the transformed members (such as converting a list of strings to uppercase)

Answer 2

The important difference between them is that map accumulates all of the results into a collection, whereas foreach returns nothing. map is usually used when you want to transform a collection of elements with a function, whereas foreach simply executes an action for each element.

Answer 3

If you're talking about Javascript in particular, the difference is that map is a loop function while forEach is an iterator.

Use map when you want to apply an operation to each member of the list and get the results back as a new list, without affecting the original list.

Use forEach when you want to do something on the basis of each element of the list. You might be adding things to the page, for example. Essentially, it's great for when you want "side effects".

Other differences: forEach returns nothing (since it is really a control flow function), and the passed-in function gets references to the index and the whole list, whereas map returns the new list and only passes in the current element.

Answer 4

the most 'visible' difference is that map accumulates the result in a new collection, while foreach is done only for the execution itself.

but there are a couple of extra assumptions: since the 'purpose' of map is the new list of values, it doesn't really matters the order of execution. in fact, some execution environments generate parallel code, or even introduce some memoizing to avoid calling for repeated values.

foreach, on the other hand, is called specifically for the side effects; therefore the order is important, and usually can't be parallelised.

Answer 5

In short, foreach is for applying an operation on each element of a collection of elements, whereas map is for transforming one collection into another.

There are two significant differences between foreach and map.

1. foreach has no conceptual restrictions on the operation it applies, other than perhaps accept an element as argument. That is, the operation may do nothing, may have a side-effect, may return a value or may not return a value. All foreach cares about is to iterate over a collection of elements, and apply the operation on each element.

   map, on the other hand, does have a restriction on the operation: it expects the operation to return an element, and probably also accept an element as argument. The map operation iterates over a collection of elements, applying the operation on each element, and finally storing the result of each invocation of the operation into another collection. In other words, the map transforms one collection into another.

2. foreach works with a single collection of elements. This is the input collection.

   map works with two collections of elements: the input collection and the output collection.

It is not a mistake to relate the two algorithms: in fact, you may view the two hierarchically, where map is a specialization of foreach. That is, you could use foreach and have the operation transform its argument and insert it into another collection. So, the foreach algorithm is an abstraction, a generalization, of the map algorithm. In fact, because foreach has no restriction on its operation we can safely say that foreach is the simplest looping mechanism out there, and it can do anything a loop can do. map, as well as other more specialized algorithms, is there for expressiveness: if you wish to map (or transform) one collection into another, your intention is clearer if you use map than if you use foreach.

We can extend this discussion further, and consider the copy algorithm: a loop which clones a collection. This algorithm too is a specialization of the foreach algorithm. You could define an operation that, given an element, will insert that same element into another collection. If you use foreach with that operation you in effect performed the copy algorithm, albeit with reduced clarity, expressiveness or explicitness. Let's take it even further: We can say that map is a specialization of copy, itself a specialization of foreach. map may change any of the elements it iterates over. If map doesn't change any of the elements then it merely copied the elements, and using copy would express the intent more clearly.

The foreach algorithm itself may or may not have a return value, depending on the language. In C++, for example, foreach returns the operation it originally received. The idea is that the operation might have a state, and you may want that operation back to inspect how it evolved over the elements. map, too, may or may not return a value. In C++ transform (the equivalent for map here) happens to return an iterator to the end of the output container (collection). In Ruby, the return value of map is the output sequence (collection). So, the return value of the algorithms is really an implementation detail; their effect may or may not be what they return.