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Recnac

A Comprehensive Walkthrough of C# Iterative Statements Part 2 - for, foreach

Recnac

  • Jun 14, 2019
  • 9 Min read
  • 37 Views
  • Jun 14, 2019
  • 9 Min read
  • 37 Views
Programming Languages
C#

Introduction

This is the second part of a three-part series on C# Iterative Statements. In the first part, we delved into while/do while loop. Check it out in case you missed it.

In this guide, we will focus on another important loop: for/foreach loop.

As this will be a guide for beginners, I try my best to use vivid visualizations and examples to help you understand.

  1. First, I will start with an interesting scenario.
  2. Then I will show you the code templates and flowcharts of iterative statements.

`for` Loop

Scenario

Imagine this scenario:

This time, we develop a cleaning robot. We will give it a task list, like {"bedroom", "kitchen", "toilet"}, and it should clean them all.

for and foreach loop are good choices to solve this problem.

for

Syntax

By convention, we learn the syntax of for statement first:

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for (initializer; condition; update_expression)
{
    code_block;
}
csharp

The for statement is composed of initializer, condition, update_expression, code_block:

  • code_block represents the logic executed in the loop. It could include multiple statements, even another loop block.
  • initializer is the logic you want to execute before entering the loop. Usually, it is for the initialization of loop variables.
  • condition is a boolean expression. It decides whether it should continue looping or exit loop.
  • update_expression defines update expressions in every loop. It is executed after code_block. Like initializer, it is for the update of loop variables as usual.

Flowchart

Please take a look at the for loop flowchart. It shows flow control intuitively.

iterative statements for flowchart

Through the above flowchart, we can figure out the execution process of for loop:

  1. Before the loop, initializer is executed exactly once. It is a good opportunity to declare and initialize loop variables.
  1. Check whether to satisfy condition. If satisfied, run loop logic code_block. Otherwise, terminate the loop.
  2. After code_block, execute update_expression. Update loop variable here for next condition check.
  3. Go back to step two to judge condition again. Repeat this process until condition==False.

Practice

Since we have learned the mechanism of for loop, it is time to solve an example problem:

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string[] tasks = {"bedroom", "kitchen", "toilet"};
// loop variable i presents the index of tasks array
for (int i = 0; i < tasks.Length; ++i)
{
    Console.WriteLine("mission complete: clean {0}", tasks[i]);
}
/*output:
mission complete: clean bedroom
mission complete: clean kitchen
mission complete: clean toilet */
csharp

Advanced Usage

Each of these four sections - initializer, condition, update_expression, and code_block - could be optional. Also, they could include multiple statements:

  • code_block uses ; to separate statements.
  • initializer and update_expression use , to separate statements.
  • condition use || or && to connect boolean expressions.

Let's practice multiple statements in all of the sections from this complex example:

You’re given two numbers: x and y. x starts from 0, add 1 each time; y starts from 12, subtract 2 each time. Calculate the difference between y and x repetitively when x is not greater than y and production is less than 18.

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// x starts from 0 and add 1 each time; y starts from 12 and subtract 2 each time
// iteration when x is not greater than y and production less than 18
for (int x = 0, y = 12; x <= y && x * y < 18; ++x, y -= 2)
{
    // calculate difference between y and x, then print out
    int difference = y - x;
    Console.WriteLine("x={0}, y={1}, difference={2}", x, y, difference);
}
/* output:
x=0, y=12, difference=12
x=1, y=10, difference=9
x=2, y=8, difference=6 */
csharp

foreach

Syntax

For the scenario about accessing elements in a collection, foreach is the best solution.

The syntax of foreach:

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foreach (element in IEnumerable)
{
    code_block;
}
csharp
  • element represents the current element in a collection.
  • IEmuerable represents a collection-like data structure which implements the interface IEnumerable. We will talk about this later.
  • code_block is similar to others. It represents the logic executed in the loop.

Flowchart

iterative statements foreach flowchart

The foreach loop flowchart visualizes the flow control process:

  1. Check whether IEnumberable has any elements left.
  2. If it has, assign to element and process in code_block. Otherwise, exit loop.
  3. Repeat this process until IEnumberable is exhausted.

Practice

For the above scenario, foreach the element in string array tasks:

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string[] tasks = {"bedroom", "kitchen", "toilet"};
// foreach the element in string array tasks
foreach (string task in tasks)
{
    Console.WriteLine("mission complete: clean {0}", task);
}
csharp

Scope of Usages

foreach supports iterating an element in all kinds of IEnumerable. So what is IEnumberable?

An easily understandable explanation is that IEnumberable is a collection-like data structure, such as int[], List, Dictionary, or DataRow.

A more formal illustration is that foreach covers the type which implements the interface IEnumberable or

IEnumberable. This type returns an element by GetEnumerator(); an element should implement two essential features to support foreach:

  • Implement Current property to help foreach access the current element.
  • Implement bool MoveNext() to check whether we can get the next element (return false when reaching the end).

We are not going to expand here. I have attached official documents in case you want to dive into it:

for vs foreach

  • Similar to while vs do while, you can consider foreach as the extension of for. It is typically used when you focus on processing an element of a collection in a loop.
  • For element iteration case, for deals with the index of collection and accesses an element by the index indirectly. So, if we want to access an element of a collection in order, and care nothing about the index, foreach is the best practice.
  • for loop is widely used without any restriction, but foreach has some limitations:
    • It is unable to modify the collection when iterating (modifying the member of element is permitted).
    • It does not track the index, so it does not support index operations in a loop.
    • The iteration order can only be forward. It does not support to traversing in reverse or random order.
    • You can not add an extra condition to filter elements.

Conclusion

In this part, we have learned the other iterative statements: for and foreach. We analyzed the syntax and flowchart of for/foreach and practiced them with examples. Furthermore, we made a comparison of them.

In the last part, we will compare them and summarize the best practices. In addition, we will explore advanced usages.

As this is the end, I have drawn a mind map to help you organize and review the knowledge in this series.

Iterative Statements Loop mind map

This guide is one of a series of C# Flow Control guides:

Hope you enjoyed it. If you have any questions, you’re welcome to contact me at [email protected]

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