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

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

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

Syntax

1for (initializer; condition; update_expression)
2{
3    code_block;
4}

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

Before the loop, initializer is executed exactly once. It is a good opportunity to declare and initialize loop variables.

Check whether to satisfy condition. If satisfied, run loop logic code_block. Otherwise, terminate the loop.
After code_block, execute update_expression. Update loop variable here for next condition check.
Go back to step two to judge condition again. Repeat this process until condition==False.

Practice

1string[] tasks = {"bedroom", "kitchen", "toilet"};
2// loop variable i presents the index of tasks array
3for (int i = 0; i < tasks.Length; ++i)
4{
5    Console.WriteLine("mission complete: clean {0}", tasks[i]);
6}
7/*output:
8mission complete: clean bedroom
9mission complete: clean kitchen
10mission complete: clean toilet */

Advanced Usage

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

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.

1// x starts from 0 and add 1 each time; y starts from 12 and subtract 2 each time
2// iteration when x is not greater than y and production less than 18
3for (int x = 0, y = 12; x <= y && x * y < 18; ++x, y -= 2)
4{
5    // calculate difference between y and x, then print out
6    int difference = y - x;
7    Console.WriteLine("x={0}, y={1}, difference={2}", x, y, difference);
8}
9/* output:
10x=0, y=12, difference=12
11x=1, y=10, difference=9
12x=2, y=8, difference=6 */

foreach

Syntax

1foreach (element in IEnumerable)
2{
3    code_block;
4}

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

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

Practice

1string[] tasks = {"bedroom", "kitchen", "toilet"};
2// foreach the element in string array tasks
3foreach (string task in tasks)
4{
5    Console.WriteLine("mission complete: clean {0}", task);
6}

Scope of Usages

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).

foreach
IEnumbered

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.

A Comprehensive Walkthrough of C# Iterative Statements Part 1 - while, do
A Comprehensive Walkthrough of C# Iterative Statements Part 2 - for, foreach
A Comprehensive Walkthrough of C# Iterative Statements Part 3 - advanced usages
A Comprehensive Walkthrough of C# Conditional Statements Part 1 - if, else
A Comprehensive Walkthrough of C# Conditional Statements Part 2 - switch
A Comprehensive Walkthrough of C# Jump Statements Part 1 - break, continue
A Comprehensive Walkthrough of C# Jump Statements Part 2 - goto