What is the major difference between for and foreach loops?
In which scenarios can we use for and not foreach and vice versa.
Would it be possible to show with a simple program?
Both seem the same to me. I can't differentiate them.
a for loop is a construct that says "perform this operation n. times".
a foreach loop is a construct that says "perform this operation against each value/object in this IEnumerable"
You can use foreach if the object you want to iterate over implements the IEnumerable interface. You need to use for if you can access the object only by index.
I'll tryto answer this in a more general approach:
foreach is used to iterate over each element of a given set or list (anything implementing IEnumerable) in a predefined manner. You can't influence the exact order (other than skipping entries or canceling the whole loop), as that's determined by the container.
foreach (String line in document) { // iterate through all elements of "document" as String objects
Console.Write(line); // print the line
}
for is just another way to write a loop that has code executed before entering the loop and once after every iteration. It's usually used to loop through code a given number of times. Contrary to foreach here you're able to influence the current position.
for (int i = 0, j = 0; i < 100 && j < 10; ++i) { // set i and j to 0, then loop as long as i is less than 100 or j is less than 10 and increase i after each iteration
if (i % 8 == 0) { // skip all numbers that can be divided by 8 and count them in j
++j
continue;
}
Console.Write(i);
}
Console.Write(j);
If possible and applicable, always use foreach rather than for (assuming there's some array index). Depending on internal data organisation, foreach can be a lot faster than using for with an index (esp. when using linked lists).
Everybody gave you the right answer with regard to foreach, i.e. it's a way to loop through the elements of something implementing IEnumerable.
On the other side, for is much more flexible than what is shown in the other answers. In fact, for is used to executes a block of statements for as long as a specified condition is true.
From Microsoft documentation:
for (initialization; test; increment)
statement
initialization
Required. An expression. This expression is executed only once, before the loop is executed.
test
Required. A Boolean expression. If test is true, statement is executed. If test if false, the loop is terminated.
increment
Required. An expression. The increment expression is executed at the end of every pass through the loop.
statement
Optional. Statement to be executed if test is true. Can be a compound statement.
This means that you can use it in many different ways. Classic school examples are the sum of the numbers from 1 to 10:
int sum = 0;
for (int i = 0; i <= 10; i++)
sum = sum + i;
But you can use it to sum the numbers in an Array, too:
int[] anArr = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
int sum = 0;
for (int i = 0; i < anArr.Length; i++)
sum = sum + anArr[i];
(this could have been done with a foreach, too):
int[] anArr = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
int sum = 0;
foreach (int anInt in anArr)
sum = sum + anInt;
But you can use it for the sum of the even numbers from 1 to 10:
int sum = 0;
for (int i = 0; i <= 10; i = i + 2)
sum = sum + i;
And you can even invent some crazy thing like this one:
int i = 65;
for (string s = string.Empty; s != "ABC"; s = s + Convert.ToChar(i++).ToString()) ;
Console.WriteLine(s);
for loop:
1) need to specify the loop bounds( minimum or maximum).
2) executes a statement or a block of statements repeatedly
until a specified expression evaluates to false.
Ex1:-
int K = 0;
for (int x = 1; x <= 9; x++){
k = k + x ;
}
foreach statement:
1)do not need to specify the loop bounds minimum or maximum.
2)repeats a group of embedded statements for
a)each element in an array
or b) an object collection.
Ex2:-
int k = 0;
int[] tempArr = new int[] { 0, 2, 3, 8, 17 };
foreach (int i in tempArr){
k = k + i ;
}
foreach is almost equivalent to :
var enumerator = list.GetEnumerator();
var element;
while(enumerator.MoveNext()){
element = enumerator.Current;
}
and in order to implemetn a "foreach" compliant pattern, this need to provide a class that have a method GetEnumerator() which returns an object that have a MoveNext() method, a Reset() method and a Current property.
Indeed, you do not need to implement neither IEnumerable nor IEnumerator.
Some derived points:
foreach does not need to know the collection length so allows to iterate through a "stream" or a kind of "elements producer".
foreach calls virtual methods on the iterator (the most of the time) so can perform less well than for.
It depends on what you are doing, and what you need.
If you are iterating through a collection of items, and do not care about the index values then foreach is more convenient, easier to write and safer: you can't get the number of items wrong.
If you need to process every second item in a collection for example, or process them ion the reverse order, then a for loop is the only practical way.
The biggest differences are that a foreach loop processes an instance of each element in a collection in turn, while a for loop can work with any data and is not restricted to collection elements alone. This means that a for loop can modify a collection - which is illegal and will cause an error in a foreach loop.
For more detail, see MSDN : foreach and for
Difference Between For and For Each Loop in C#
For Loops executes a block of code until an expression returns false while ForEach loop executed a block of code through the items in object collections.
For loop can execute with object collections or without any object collections while ForEach loop can execute with object collections only.
The for loop is a normal loop construct which can be used for multiple purposes where as foreach is designed to work only on Collections or IEnumerables object.
foreach is useful if you have a array or other IEnumerable Collection of data. but for can be used for access elements of an array that can be accessed by their index.
A for loop is useful when you have an indication or determination, in advance, of how many times you want a loop to run. As an example, if you need to perform a process for each day of the week, you know you want 7 loops.
A foreach loop is when you want to repeat a process for all pieces of a collection or array, but it is not important specifically how many times the loop runs. As an example, you are formatting a list of favorite books for users. Every user may have a different number of books, or none, and we don't really care how many it is, we just want the loop to act on all of them.
The for loop executes a statement or a block of statements repeatedly until a specified expression evaluates to false.
There is a need to specify the loop bounds (minimum or maximum). Following is a code example of a simple for loop that starts 0 till <= 5.
we look at foreach in detail. What looks like a simple loop on the outside is actually a complex data structure called an enumerator:
An enumerator is a data structure with a Current property, a MoveNext method, and a Reset method. The Current property holds the value of the current element, and every call to MoveNext advances the enumerator to the next item in the sequence.
Enumerators are great because they can handle any iterative data structure. In fact, they are so powerful that all of LINQ is built on top of enumerators.
But the disadvantage of enumerators is that they require calls to Current and MoveNext for every element in the sequence. All those method calls add up, especially in mission-critical code.
Conversely, the for-loop only has to call get_Item for every element in the list. That’s one method call less than the foreach-loop, and the difference really shows.
So when should you use a foreach-loop, and when should you use a for-loop?
Here’s what you need to do:
When you’re using LINQ, use foreach
When you’re working with very large computed sequences of values, use foreach
When performance isn’t an issue, use foreach
But if you want top performance, use a for-loop instead
The major difference between the for and foreach loop in c# we understand by its working:
The for loop:
The for loop's variable always be integer only.
The For Loop executes the statement or block of statements repeatedly until specified expression evaluates to false.
In for loop we have to specify the loop's boundary ( maximum or minimum).-------->We can say this is the limitation of the for loop.
The foreach loop:
In the case of the foreach loop the variable of the loop while be same as the type of values under the array.
The Foreach statement repeats a group of embedded statements for each element in an array or an object collection.
In foreach loop, You do not need to specify the loop bounds minimum or maximum.--->
here we can say that this is the advantage of the for each loop.
I prefer the FOR loop in terms of performance. FOREACH is a little slow when you go with more number of items.
If you perform more business logic with the instance then FOREACH performs faster.
Demonstration:
I created a list of 10000000 instances and looping with FOR and FOREACH.
Time took to loop:
FOREACH -> 53.852ms
FOR -> 28.9232ms
Below is the sample code.
class Program
{
static void Main(string[] args)
{
List<TestClass> lst = new List<TestClass>();
for (int i = 1; i <= 10000000; i++)
{
TestClass obj = new TestClass() {
ID = i,
Name = "Name" + i.ToString()
};
lst.Add(obj);
}
DateTime start = DateTime.Now;
foreach (var obj in lst)
{
//obj.ID = obj.ID + 1;
//obj.Name = obj.Name + "1";
}
DateTime end = DateTime.Now;
var first = end.Subtract(start).TotalMilliseconds;
start = DateTime.Now;
for (int j = 0; j<lst.Count;j++)
{
//lst[j].ID = lst[j].ID + 1;
//lst[j].Name = lst[j].Name + "1";
}
end = DateTime.Now;
var second = end.Subtract(start).TotalMilliseconds;
}
}
public class TestClass
{
public long ID { get; set; }
public string Name { get; set; }
}
If I uncomment the code inside the loop:
Then, time took to loop:
FOREACH -> 2564.1405ms
FOR -> 2753.0017ms
Conclusion
If you do more business logic with the instance, then FOREACH is recommended.
If you are not doing much logic with the instance, then FOR is recommended.
Many answers are already there, I just need to identify one difference which is not there.
for loop is fail-safe while foreach loop is fail-fast.
Fail-fast iteration throws ConcurrentModificationException if iteration and modification are done at the same time in object.
However, fail-safe iteration keeps the operation safe from failing even if the iteration goes in infinite loop.
public class ConcurrentModification {
public static void main(String[] args) {
List<String> str = new ArrayList<>();
for(int i=0; i<1000; i++){
str.add(String.valueOf(i));
}
/**
* this for loop is fail-safe. It goes into infinite loop but does not fail.
*/
for(int i=0; i<str.size(); i++){
System.out.println(str.get(i));
str.add(i+ " " + "10");
}
/**
* throws ConcurrentModificationexception
for(String st: str){
System.out.println(st);
str.add("10");
}
*/
/* throws ConcurrentModificationException
Iterator<String> itr = str.iterator();
while(itr.hasNext()) {
System.out.println(itr.next());
str.add("10");
}*/
}
}
Hope this helps to understand the difference between for and foreach loop through different angle.
I found a good blog to go through the differences between fail-safe and fail-fast, if anyone interested:
You can use the foreach for an simple array like
int[] test = { 0, 1, 2, 3, ...};
And you can use the for when you have a 2D array
int[][] test = {{1,2,3,4},
{5,2,6,5,8}};
foreach syntax is quick and easy. for syntax is a little more complex, but is also more flexible.
foreach is useful when iterating all of the items in a collection. for is useful when iterating overall or a subset of items.
The foreach iteration variable which provides each collection item, is READ-ONLY, so we can't modify the items as they are iterated. Using the for syntax, we can modify the items as needed.
Bottom line- use foreach to quickly iterate all of the items in a collection. Use for to iterate a subset of the items of the collection or to modify the items as they are iterated.
simple difference between for and foreach
for loop is working with values. it must have condition then increment and intialization also. you have to knowledge about 'how many times loop repeated'.
foreach is working with objects and enumaretors. no need to knowledge how many times loop repeated.
The foreach statement repeats a group of embedded statements for each element in an array or an object collection that implements the System.Collections.IEnumerable or System.Collections.Generic.IEnumerable interface. The foreach statement is used to iterate through the collection to get the information that you want, but can not be used to add or remove items from the source collection to avoid unpredictable side effects. If you need to add or remove items from the source collection, use a for loop.
One important thing related with foreach is that , foreach iteration variable cannot be updated(or assign new value) in loop body.
for example :
List<string> myStrlist = new List<string>() { "Sachin", "Ganguly", "Dravid" };
foreach(string item in myStrlist)
{
item += " cricket"; // ***Not Possible***
}
Related
I have a huge list of strings _wordList List<string> containing about 100,000 values. The problem I'm having is that I also require multiple nested loops within this. The nested loop is also a list but with a structure containing only variables, containing about 0-100 values depending on what happens
for (int y = 0; y < _wordList.Count; y++)
{
string word = _wordList[y];
for(int x = 0; x < _secondWordList.Count; x++)
{
if (!word.Contains(_secondWordList[x].Word) || word == _secondWordList[x].Word)
continue;
// do other stuff
}
}
Here is part of the code, I won't post all of it since most of it will be irrelevant but within the second loop I have about 2 other short loops, the whole function completes in 350-600ms. What would the best way to optimize the loops? The word.Contains also have an impact of about 100-150ms on performance.
It seems that you're looking for a text search, and in this case, you can benefit from projects like LuceneNet.
If the call to string.Contains() didn't exist and you were looking for exact matches I was to suggest that you swap the list for hashset as it'll give you a great performance boost in your case. Like below.
static void Main(string[] args)
{
var wordList = new HashSet<string>(); //Assuming Initialized
var secondWordList = new List<X>(); //Assuming Initialized
for (var c = 0; c < secondWordList.Count; c++)
{
if(wordList.Contains(secondWordList[c].Word))
continue;
// do other stuff
}
}
With this, you're going to iterate on the smaller list and look for the value in the HashSet which has a complexity of O(1), which Means that it'll be extremely fast.
I have a list of objects. This object has a field called val. This value shouldn't be smaller than zero but there are such objects in this list. I want to replace those less-than-zero values with zero. The easiest solution is
foreach(Obj item in list)
{
if (item.val < 0)
{
item.val = 0;
}
}
But I want to do this using LINQ. The important thing is I do not want a list of updated elements. I want the same list just with the necessary values replaced. Thanks in advance.
As I read the comments I realized what I wanted to do is less efficient and pointless. LINQ is for querying and creating new collections rather than updating collections. A possible solution I came across was this
list.Select(c => { if (c.val < 0 ) c.val= 0; return c;}).ToList();
But my initial foreach solution is more efficient than this. So dont make the same mistake I do and complicate things.
you can try this one, which is faster because of parallelism
Parallel.ForEach(list, item =>
{
item.val = item.val < 0 ? 0 : item.val;
});
The Parallel ForEach in C# provides a parallel version of the standard, sequential Foreach loop. In standard Foreach loop, each iteration processes a single item from the collection and will process all the items one by one only. However, the Parallel Foreach method executes multiple iterations at the same time on different processors or processor cores. This may open the possibility of synchronization problems. So, the loop is ideally suited to processes where each iteration is independent of the others
More Details - LINK
loop 'for' is faster than 'foreach' so you can use this one
for (int i = 0; i < list.Count; i++)
{
if(list[i].val <= 0)
{
list[i].val = 0;
}
}
In C# a question has been bugging me for a while and its what is that actual major difference between a While and For Loop. Is it just purely readability ie; everything you can essentially do in a for loop can be done in a while loop , just in different places. So take these examples:
int num = 3;
while (num < 10)
{
Console.WriteLine(num);
num++;
}
vs
for (int x = 3; x < 10; x++)
{
Console.WriteLine(x);
}
Both code loops produce the same outcome and is the only difference between the two that the for loop forces you to declare a new variable and also set the iteration value each loop cycle at the start? Maybe I'm missing something else in terms of any major differences but it would good if someone can set me straight regarding this. Thanks.
is the only difference between the two that the for loop forces you to declare a new variable and also set the iteration value each loop cycle at the start?
The for loop forces you to nothing. You can omit any of the 3 elements. The smallest form is
for(;;) // forever
{
DoSomething();
}
But you can use the 3 elements to write more concise code:
for(initializer; loop-condition; update-expression)
{
controlled-statements;
}
is equivalent to:
{
initializer;
while(loop-condition)
{
controlled-statements;
continue_target: // goto here to emulate continue
update-expression;
}
}
Note the outer {} braces, in for(int i = 0; ...; ...) the i is local to the for-loop. A clear benefit.
But the major difference in usage is when you call continue; inside the loop, much easier to get the logic wrong in a while-loop than in a for-loop.
Yes, they're exactly the same in the background (in Assembly, that is).
Usually, it is more common to use the for loop when the number of repeats is known and we're not going to change our counter(index).
while loop also has the advantage of a sentinel loop which is easier for example when taking inputs from a user until something specific is given.
Fundamentally, the differences are:
For loop knows in advance how many times it will loop, whereas a while loop doesn’t know.
For loop has an initialization step whereas a while loop doesn’t
For loop uses a “step value” or increment/decrement step, whereas a while loop doesn’t.
Here is an example of a Java for loop looping from 0 to 99 and printing out the numbers. Notice how the loop is initialized and incremented as part of the for loop structure.
for(int i=0; i<100; i++) {
System.out.println(i);
}
Here is an example of a Java while loop printing out all the elements in an integer array. Notice how the loop variable is initialized before the loop and is incremented inside the loop body.
int [] intArray = {1, 3, 5, 7, 9};
int i=0;
while(i<intArray.length) {
System.out.println(intArray[i++]);
}
When you are sure what will be the end value or how much the loop should execute(until what value), use 'for' loop, otherwise use 'while' loop.
So I've been trying to turn this codes for into a for each, but I keep running into problems, and I'm not all that sure how to do it, the only thing I've done that will work is a small code but it repeats several times, I'm still really new to C# and using for() so I'm not really understanding what to do. Sorry for being so new to this, and I appreciate all the help I can get!
double[] numbers = new double[9.7, 2.2, 4.3, 1.7, 5.6, 3.6, 4.5, 1.3, 4.6, 3.0];
static double ComputeSum(double[] array)
{
double sum = 0;
int intCounter;
for (intCounter = 0; intCounter < array.Length; intCounter++)
{
sum += array[intCounter];
}
return sum;
}
static double ComputeAverage(double[] array)
{
return ComputeSum(array) / array.Length;
}
You got your specific question answered, but not the more general question of "how do I turn a for loop into a foreach?
Let's look at your loop, with some better naming. (Don't name things according to their type in C#. Name things according to their meaning.)
for (index = 0; index< array.Length; index++)
{
sum += array[index];
}
The most important thing is that you understand the idea that this loop represents. A surface level reading is "for every valid index in this array, obtain the element value associated with that index and add it to the sum". But we can make that a bit more abstract. We could also say that the loop means "for every element in the collection, add it to the sum".
That is the key difference between the for loop and the foreach loop. The for loop concentrates on the index, and you fetch the value via the index. The foreach loop concentrates on the value itself, and doesn't consider the index at all.
Knowing that we can now see how to translate a for into a foreach. First locate the bit that is the value, and make a variable for it:
for (index = 0; index < array.Length; index++)
{
var item = array[index];
sum += item;
}
Now we see that the loop body can be expressed almost entirely in terms of an operation on the item, not on the index. We then translate that into a foreach:
foreach (var item in array)
{
sum += item;
}
Everything dealing with the index gets deleted. If you find you cannot delete everything dealing with the index, then you probably should not be using a foreach.
Since foreach loop presents elements of the collection one-by-one to your code, all you need to do is
foreach (double num in Numbers)
{
sump += num;
}
Console.WriteLine("{0}", sump);
You could also use Sum extension method to eliminate loop constructs altogether:
var sump = Numbers.Sum(); // <<== The loop is hidden inside the Sum() method
Console.WriteLine("{0}", sump);
I'm trying to understand a book from Don Gosselin on ASP.NET Programming with Visual C#. To solve it I just simply make it to work by adhering to while loops: one while loop is to assign a number to an array element, the other while loop is to display that array. Total array count displays 1 through 100. This should have worked but didn't. Visual Studio 2013 debugger for some reason assigns count = 100, that's why it's failing.
<%
int count = 0;
int[] numbers = new int[100];
while (count <= 100)
{
numbers[count] = count;
++count;
}
while (count <= 100)
{
Response.Write(numbers[count] + "<br />");
++count;
}
%>
You should set count to 0 after first while loop:
int count = 0;
int[] numbers = new int[100];
while (count <= 100)
{
numbers[count] = count;
++count;
}
count = 0;
while (count <= 100)
{
Response.Write(numbers[count] + "<br />");
++count;
}
You need to reset the count to 0 before you attempt the next while statement. Currently, the first loop ends when it reaches a count equal to 101. WHen you proceed to the next while, the count is 101 so the loop automatically ends. Just set count = 0; before the second while loop.
This seems like a very convoluted and unrealistic way of using while loops and arrays. In order to understand it better, it may be worth thinking about it per step.
var i = 0;
while (i < 100)
{
Response.Write(++i + "<br />");
}
The first important distinction is between i++ and ++i. The former utilises the value, and then increments by one; the latter, increments the number and then utilises the value.
In C#, you should really be working with Collections, rather than Arrays. Arrays are zero-indexed, and are renowned for causing serious errors, including exposing potential exploits. Being statically allocated, there is no failsafe when attempting to access indicies outside of the bounds of the Array. Collections, on the other hand, are (for the most part) one-indexed, dynamically allocated, and provide fallbacks when accessing indicies. The most commonly used Collection is a List.
var i = 1;
var list = new List<int>();
while (i <= 100)
{
list.Add(i++);
}
For the second while loop, it's not really suitable to use a while loop here, for any practical example. The excercise is forcing while loops where they are not needed. In this instance, the aim is to iterate through each element in the array (List) and dump its contents to the screen. Because we want to perform an action for each element, a while loop may cause issues. If the array has less than 100 elements, the program will crash, if the array has more than 100 elements, we'll miss some of them. By using a foreach loop, instead of a while, we can eliminate these potential errors.
foreach (var num in list)
{
Response.Write(num + "<br />");
}
Now, I realise that the excercise is about while loops, however, it is teaching you to use them in the wrong way. A much better way - and how you'll most often use them - is to perform an action until a particular condition is met, rather than for simple iteration. By this, I mean, a condition is set to false, then inside the while loop, we manipulate a variable, test the condition, and if it's still false, we go round again. The most common example of this is to work out factorials of numbers.
var num = 5;
var factorial = 1;
while (counter > 1)
{
factorial *= num--;
}
Response.Write(String.Format("{0}! = {1}", input, factorial));
The other main way in which while loops are used is to force an infinite loop, unless a break condition is met. I'll show a very arbitrary use of this here, but a real world example would be the loop() method in Arduino C coding, or a HTTP Listener that constantly repeats the same procedures, until stopped.
var stop = 13;
Response.Write("Pick a number between 1 and 100...<br /><br />");
while (true)
{
var num = new Random().Next(1, 101);
Response.Write(num + " ..... ");
if (num == stop) break;
Response.Write("You got lucky!<br />");
}
Response.Write("Unlucky for you!);
The best way to learn these things is to practice them. Pick a task and find out just how many ways there are to complete it. There is one last important distinction to mention though. a while loop tests the condition at the beginning of the loop. A do while loop, tests the condition at the end.
while(false)
{
// This code will never be run.
}
Compared to:
do
{
// This code will be run once only.
}
while(false)
As a final thought, here's how I'd write the original code (using a LINQ foreach loop):
var numbers = new List<int>();
for (var count = 1; count <= 100; count++)
{
numbers.Add(count);
}
numbers.ForEach(num => Response.Write(num + "<br />")));