Is there a difference between foreach and map?

问题

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?

回答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)

回答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.

回答3:

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.

回答4:

Array.protototype.map method & Array.protototype.forEach are both quite similar.

Run the following code: http://labs.codecademy.com/bw1/6#:workspace

var arr = [1, 2, 3, 4, 5];

arr.map(function(val, ind, arr){
    console.log("arr[" + ind + "]: " + Math.pow(val,2));
});

console.log();

arr.forEach(function(val, ind, arr){
    console.log("arr[" + ind + "]: " + Math.pow(val,2));
});

They give the exact ditto result.

arr[0]: 1
arr[1]: 4
arr[2]: 9
arr[3]: 16
arr[4]: 25

arr[0]: 1
arr[1]: 4
arr[2]: 9
arr[3]: 16
arr[4]: 25

But the twist comes when you run the following code:-

Here I've simply assigned the result of the return value from the map and forEach methods.

var arr = [1, 2, 3, 4, 5];

var ar1 = arr.map(function(val, ind, arr){
    console.log("arr[" + ind + "]: " + Math.pow(val,2));
    return val;
});

console.log();
console.log(ar1);
console.log();

var ar2 = arr.forEach(function(val, ind, arr){
    console.log("arr[" + ind + "]: " + Math.pow(val,2));
    return val;
});

console.log();
console.log(ar2);
console.log();

Now the result is something tricky!

arr[0]: 1
arr[1]: 4
arr[2]: 9
arr[3]: 16
arr[4]: 25

[ 1, 2, 3, 4, 5 ]

arr[0]: 1
arr[1]: 4
arr[2]: 9
arr[3]: 16
arr[4]: 25

undefined

Conclusion

Array.prototype.map returns an array but Array.prototype.forEach doesn't. So you can manipulate the returned array inside the callback function passed to the map method and then return it.

Array.prototype.forEach only walks through the given array so you can do your stuff while walking the array.

回答5:

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, or lazyness, to avoid calling some at all.

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

回答6:

Short answer: map and forEach are different. Also, informally speaking, map is a strict superset of forEach.

Long answer: First, let's come up with one line descriptions of forEach and map:

• forEach iterates over all elements, calling the supplied function on each.
• map iterates over all elements, calling the supplied function on each, and produces a transformed array by remembering the result of each function call.

In many languages, forEach is often called just each. The following discussion uses JavaScript only for reference. It could really be any other language.

Now, let's use each of these functions.

Using forEach:

Task 1: Write a function printSquares, which accepts an array of numbers arr, and prints the square of each element in it.

Solution 1:

var printSquares = function (arr) {
    arr.forEach(function (n) {
        console.log(n * n);
    });
};

Using map:

Task 2: Write a function selfDot, which accepts an array of numbers arr, and returns an array wherein each element is the square of the corresponding element in arr.

Aside: Here, in slang terms, we are trying to square the input array. Formally put, we are trying to compute it's dot product with itself.

Solution 2:

var selfDot = function (arr) {
    return arr.map(function (n) {
        return n * n;
    });
};

How is map a superset of forEach?

You can use map to solve both tasks, Task 1 and Task 2. However, you cannot use forEach to solve the Task 2.

In Solution 1, if you simply replace forEach by map, the solution will still be valid. In Solution 2 however, replacing map by forEach will break your previously working solution.

Implementing forEach in terms of map:

Another way of realizing map's superiority is to implement forEach in terms of map. As we are good programmers, we'll won't indulge in namespace pollution. We'll call our forEach, just each.

Array.prototype.each = function (func) {
    this.map(func);
};

Now, if you don't like the prototype nonsense, here you go:

var each = function (arr, func) {
    arr.map(func); // Or map(arr, func);
};

So, umm.. Why's does forEach even exist?

The answer is efficiency. If you are not interested in transforming an array into another array, why should you compute the transformed array? Only to dump it? Of course not! If you don't want a transformation, you shouldn't do a transformation.

So while map can be used to solve Task 1, it probably shouldn't. For each is the right candidate for that.

---

Original answer:

While I largely agree with @madlep 's answer, I'd like to point out that map() is a strict super-set of forEach().

Yes, map() is usually used to create a new array. However, it may also be used to change the current array.

Here's an example:

var a = [0, 1, 2, 3, 4], b = null;
b = a.map(function (x) { a[x] = 'What!!'; return x*x; });
console.log(b); // logs [0, 1, 4, 9, 16] 
console.log(a); // logs ["What!!", "What!!", "What!!", "What!!", "What!!"]

In the above example, a was conveniently set such that a[i] === i for i < a.length. Even so, it demonstrates the power of map().

Here's the official description of map(). Note that map() may even change the array on which it is called! Hail map().

Hope this helped.

---

Edited 10-Nov-2015: Added elaboration.

回答7:

Here is an example in Scala using lists: map returns list, foreach returns nothing.

def map(f: Int ⇒ Int): List[Int]
def foreach(f: Int ⇒ Unit): Unit

So map returns the list resulting from applying the function f to each list element:

scala> val list = List(1, 2, 3)
list: List[Int] = List(1, 2, 3)

scala> list map (x => x * 2)
res0: List[Int] = List(2, 4, 6)

Foreach just applies f to each element:

scala> var sum = 0
sum: Int = 0

scala> list foreach (sum += _)

scala> sum
res2: Int = 6 // res1 is empty

回答8:

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.

回答9:

ForEach tries to apply a function such as writing to db etc on each element of the RDD without returning anything back.

But the map() applies some function over the elements of rdd and returns the rdd. So when you run the below method it won't fail at line3 but while collecting the rdd after applying foreach it will fail and throw an error which says

File "<stdin>", line 5, in <module>

AttributeError: 'NoneType' object has no attribute 'collect'

nums = sc.parallelize([1,2,3,4,5,6,7,8,9,10])
num2 = nums.map(lambda x: x+2)
print ("num2",num2.collect())
num3 = nums.foreach(lambda x : x*x)
print ("num3",num3.collect())

来源：https://stackoverflow.com/questions/354909/is-there-a-difference-between-foreach-and-map