What is the difference between for and foreach

Question

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

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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

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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

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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  lt   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  lt  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  lt   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

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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

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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

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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 lt String gt  str   new ArrayList lt  gt              for int i 0  i lt 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 lt 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 lt String gt  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

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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

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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  lt   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

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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

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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 lt TestClass gt  lst   new List lt TestClass gt             for  int i   1  i  lt   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 lt 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

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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  lt   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

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One important thing related with foreach is that   foreach iteration variable cannot be updated or assign new value  in loop body   for example    List lt string gt  myStrlist   new List lt string gt       Sachin    Ganguly    Dravid                   foreach string item in myStrlist                                      item      cricket         Not Possible

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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

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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

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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  lt  100  amp  amp  j  lt  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

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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

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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

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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

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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

[c#] What is the difference between for and foreach?

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