13.what is the difference between for and foreach exec system

This is because you are looping over a collection of objects, and for each object in the collection, you execute a specific statement. As an example, you have a list of names and you need to loop over these names without the need for an evaluation phase or to use any counter not index-based loop.

Case in point, the following code is a foreach loop clause that loops over a string located in a list of strings in C :. After describing for loops and foreach loops, we will show how these operations are implemented within SSIS. There are many methods to implement loops within SSIS within different scopes:.

In this section, we will not focus on how to implement loops using scripting in SSIS. Instead, we will give an overview of the containers mentioned above. It has three phases: Initialization, Evaluation and Counter Increment.

Variables must be added within the SSIS package, and to use them, you must add the character as the prefix. To catch the current item while looping over a collection, we must add an SSIS package and map this variable to the item within the SSIS Foreach Loop Container variable mappings tab page by specifying the item index within the current row and the variable name.

By using the SSIS Foreach Loop Container, we can get the current item using variable mappings, but we cannot know the iteration number. To do that, we must add an expression task or a script task with an incremental variable. Your email address will not be published. Key Differences Between while and do-while Loop The while loop checks the condition at the starting of the loop and if the condition is satisfied statement inside the loop, is executed.

As against, in the do-while loop, the condition is checked after the execution of all statements in the body of the loop. If the condition in a while loop is false, not a single statement inside the loop is executed.

The while loop is also known as the entry-controlled and pre-checking loop because in this loop, the body of the loop is executed prior to checking the condition. Conversely, the alternate name for the do-while loop is the exit-controlled and post-checking loop, the reason behind this is that the checking of the loop condition is followed by the execution of the body of the loop. The syntax of a do-while loop includes a semi-colon to terminate the loop.

On the contrary, there is no use of the semi-colon in the while loop syntax. Comments I loved this page very nicely presented and easy answers with key points covered in it……. Pascal is more clean and clear: Repeat …. 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. 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. Everybody gave you the right answer with regard to foreach, i. 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.

An expression. This expression is executed only once, before the loop is executed. A Boolean expression. If test is true, statement is executed. If test if false, the loop is terminated. The increment expression is executed at the end of every pass through the loop. This means that you can use it in many different ways. Classic school examples are the sum of the numbers from 1 to 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. 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.

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. 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. The following example shows several less common usages of the initializer and iterator sections: assigning a value to an external variable in the initializer section, invoking a method in both the initializer and the iterator sections, and changing the values of two variables in the iterator section:. All the sections of the for statement are optional.

For example, the following code defines the infinite for loop:. The foreach statement executes a statement or a block of statements for each element in an instance of the type that implements the System. IEnumerable or System. The foreach statement isn't limited to those types. You can use it with an instance of any type that satisfies the following conditions:.

The following example uses the foreach statement with an instance of the System. Beginning with C 7. If the foreach statement is applied to null , a NullReferenceException is thrown. If the source collection of the foreach statement is empty, the body of the foreach statement isn't executed and skipped.

Beginning with C 8. Each iteration of the loop may be suspended while the next element is retrieved asynchronously. The following example shows how to use the await foreach statement:.

You can also use the await foreach statement with an instance of any type that satisfies the following conditions:. By default, stream elements are processed in the captured context. If you want to disable capturing of the context, use the TaskAsyncEnumerableExtensions.