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247 lines
3.6 KiB
Markdown
247 lines
3.6 KiB
Markdown
# Loops
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---
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# Overview
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While Loop
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For Loop
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Nested Loops
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Breaking Out of Loops
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# Loops
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Loops are useful when you need to repeat a block of code multiple times and to avoid code like this:
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Console.WriteLine(count);
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++count;
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Console.WriteLine(count);
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++count;
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Console.WriteLine(count);
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// Yuck! Nobody wants to see this.
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Loops can also be used for iterating through arrays and lists (more on those later)
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# while Loop
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_while_ loop keeps executing its code block as long as the condition in its statement is true:
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int count = 0;
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while(count < 4)
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{
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Console.WriteLine(count);
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++count;
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}
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// Outputs '0', '1', '2' and '3'
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# do..while Loop
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_do..while_ works the same way as while, but it executes the code at least once and checks the condition at the end:
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int count = 0;
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do
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{
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Console.WriteLine(count);
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++count;
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} while (count < 4);
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// Outputs '0', '1', '2' and '3'
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# for Loop
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_for _ loops are used when the number of iterations are predefined
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for loop executes in three inherent steps:
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for (/*initial value*/;/*condition*/;/*increment*/)
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{
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/* Code to be executed */
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}
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The initial value is set before the first iteration. The condition is checked before each iteration, and the increment is executed after each iteration.
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# for Loop (continued)
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Try to understand the following two examples:
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for (int i = 0; i < 4; ++i)
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{
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Console.WriteLine
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("Current: " + i);
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}
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/* Outputs
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Current: 0
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Current: 1
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Current: 2
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Current: 3
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*/
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for (int i = 8; i > 0; i -= 2)
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{
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Console.WriteLine
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("Current: " + i);
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}
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/* Outputs
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Current: 8
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Current: 6
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Current: 4
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Current: 2
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*/
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We start from zero, print the current value and add one until the value is no longer under four.
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We start from eight, print the current value and remove two until the value is no longer larger than zero.
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for (int i = 8; i > 0; i -= 2)
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{
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Console.WriteLine
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("Current: " + i);
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}
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for (int i = 0; i < 4; ++i)
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{
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Console.WriteLine
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("Current: " + i);
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}
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# Nested Loops
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Nothing prevents you from using a loop inside another loop:
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int columns = 3;
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int rows = 4;
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for (int i = 0; i < columns; ++i)
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{
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for (int j = 0; j < rows; ++j)
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{
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// Console.Write doesn't add a line break after printout
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Console.Write("(" + j.ToString() + "," + i.ToString() + ")");
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}
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Console.WriteLine("");
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}
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# Breaking Out of Loops
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To stop the execution of a loop completely, use the _break _ -keyword:
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int i = 0;
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while(true)
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{
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++i;
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if(i > 3)
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break;
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}
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// i is now 4
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To skip the current iteration, use the _continue _ -keyword
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int i = 0;
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while(i < 10)
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{
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++i;
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if (i % 2 == 0)
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continue;
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Console.WriteLine(i);
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}
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// Prints every odd number from 1 to 10
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# Exercise 1: The Main Loop
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At the heart of every dynamic program, there is something called the _event loop _ a.k.a the _main loop. _ Create a console application which keeps asking the user for an input, until the user inputs "quit".
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Expand the program so that if the user inputs "help", a dialog shows up which shows the command and explanation for the command for both quit and help commands.
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# Exercise 2: Prime Numbers
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Prime number is a number greater than 1 that is not a product of two small numbers, i.e. cannot be divided into a whole number. (Wikipedia: [https://en.wikipedia.org/wiki/Prime\_number](https://en.wikipedia.org/wiki/Prime_number) )
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Create a console application which prints all the prime numbers between 0 .. 40
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# Assignments
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[Assignments about this topic can be found here](https://gitea.buutti.com/education/academy-assignments/src/branch/master/C%23%20Basics/4.%20Loops)
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