{"id":514,"date":"2020-10-14T17:24:30","date_gmt":"2020-10-14T17:24:30","guid":{"rendered":"https:\/\/marshallbrain.com\/wordpress\/?page_id=514"},"modified":"2020-10-14T17:24:30","modified_gmt":"2020-10-14T17:24:30","slug":"python2","status":"publish","type":"page","link":"https:\/\/marshallbrain.com\/python2","title":{"rendered":"Marshall Brain&#8217;s Quick and Easy Python Tutorials &#8211; Chapter 2"},"content":{"rendered":"\n<p>by&nbsp;<a href=\"http:\/\/www.marshallbrain.com\/\">Marshall Brain<\/a><\/p>\n\n\n\n<p>In the first tutorial we learned how to write the simplest computer programs using the Python programming language. We learned how to create&nbsp;<em>output<\/em>&nbsp;from a computer program. In this turotial we will learn about variables and&nbsp;<em>input<\/em>. With&nbsp;<em>input<\/em>, we can ask the user questions and use the answers to control our programs.<\/p>\n\n\n\n<p>Here is the simplest possible example. In Chapter 1 we wrote a program that printed &#8220;Hello World!&#8221; 20 times. What if we want to let the user control how many times &#8220;Hello World!&#8221; gets printed? It would look like this:<\/p>\n\n\n\n<iframe loading=\"lazy\" src=\"https:\/\/trinket.io\/embed\/python\/8e3ccc3241\" width=\"100%\" height=\"400\" frameborder=\"0\" marginwidth=\"0\" marginheight=\"0\" allowfullscreen><\/iframe>\n\n\n\n<p>When you run this program, it will ask you to enter a number. If you enter 10, it will print &#8220;Hello World!&#8221; 10 times. If you enter 3, it will print it 3 times. And so on. Enter any number you like.<\/p>\n\n\n\n<p>What is happening in the code? In the first line, &#8220;i&#8221; is called a variable. We have created it, and we can name it almost anything we like (except for special Python words, like &#8220;for&#8221; or &#8220;print&#8221;). A variable is a place where we can store a value we want to remember. In this case, we want to get input from the use and remember it in the place called &#8220;i&#8221;. If we want (and in bigger programs we definitely want to do this to avoid confusion), we could use a longer, more descriptive names. In stead of &#8220;i&#8221; we could have called our variable &#8220;NumberEnteredByUser&#8221; if we want to be super descriptive. In this case, &#8220;i&#8221; is easier to type so we will use it.<\/p>\n\n\n\n<p>The &#8220;input&#8221; command in Python prompts the user with a question (we can type anything we like for the question) and then waits for the user to type something on the keyboard. The user types a number and presses enter.<\/p>\n\n\n\n<p>Then the for loop uses the user&#8217;s input and prints &#8220;Hello World!&#8221; the right number of times.<\/p>\n\n\n\n<p>What is the little piece of code that says, &#8220;int(i)&#8221;? This is called &#8220;type casting&#8221;, and we will learn more about it in later tutorials. For Python to run this code, we need to make sure it has converted the variable i to an integer. The code &#8220;int(i)&#8221; does that.<\/p>\n\n\n\n<p>What if we want to get two pieces of input instead of 1? That&#8217;s easy, as shown here:<\/p>\n\n\n\n<iframe loading=\"lazy\" src=\"https:\/\/trinket.io\/embed\/python\/7ee70f366c\" width=\"100%\" height=\"400\" frameborder=\"0\" marginwidth=\"0\" marginheight=\"0\" allowfullscreen><\/iframe>\n\n\n\n<p>We simply declare a second variable j for the second input value, and declare k to hold the sum. The third line of code tells the computer to add i and j together and place the result into the variable k.<\/p>\n\n\n\n<p>What if we want to print all 3 variables? We change the print line to print all three, like this:<\/p>\n\n\n\n<iframe loading=\"lazy\" src=\"https:\/\/trinket.io\/embed\/python\/bf93616188\" width=\"100%\" height=\"400\" frameborder=\"0\" marginwidth=\"0\" marginheight=\"0\" allowfullscreen><\/iframe>\n\n\n\n<p>We have three &#8220;%d&#8221; place holders in the print statement, and then three variables that we want to put in those placeholders. We typecast i and j, but why didn&#8217;t we need to typecast k? It&#8217;s because Python &#8220;knows&#8221; that k is an integer based on the value we put into it &#8211; the sum of two integers. You can typecast k to int if you want &#8211; that won&#8217;t hurt anything. But it is not required like it is with i and j.<\/p>\n\n\n\n<p>So then you might ask, &#8220;why is it required for i and j?&#8221; It&#8217;s because, when the user typed in a number in the input statements, Python accepted the number as a string of characters. Python would accept a string of characters &#8220;abc&#8221; as well, or anything else the user types. We are hoping the user&#8217;s string contains one or more valid digits, and then the &#8220;int(i)&#8221; statement converts that string of digits into an actual integer number for the addition statement. What if the user enters &#8220;abc&#8221; instead of a valid number like &#8220;5&#8221;? Try it!<\/p>\n\n\n\n<p>This, by the way, is what &#8220;learning to program a computer&#8221; is about. To write a computers program in the Python language, you have to learn and then follow the rules of the Python language. Typecasting is one of those things that seems a little weird at first, but then you get used to it and it becomes second nature &#8211; you do it automatically.<\/p>\n\n\n\n<p>Now that you can accept user input and create output for the user, you can do all sorts of things with a computer. One thing scientists and engineers do with computers is simulations. They simulate things like weather patterns, ocean currents, traffic flow, air flow through an engine, stresses on a building, etc. In the next tutorial, we will create the simplest possible simulation&#8230;<\/p>\n\n\n\n<p class=\"has-text-align-center\"><a href=\"http:\/\/marshallbrain.com\/python1.htm\">&lt;&lt;&lt; Go to Tutorial 1<\/a>\u00a0\u00a0 | \u00a0\u00a0<a href=\"http:\/\/marshallbrain.com\/python.htm\">Intro<\/a>\u00a0\u00a0 | \u00a0\u00a0<a href=\"http:\/\/marshallbrain.com\/python3.htm\">Go to Tutorial 3 >>><\/a><\/p>\n\n\n\n<h2 class=\"has-text-align-center wp-block-heading\">Python Table of Contents<\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"https:\/\/marshallbrain.com\/python1\">Tutorial 1 &#8211; Getting started &#8211; creating output from a computer program<\/a><\/li><li><a href=\"https:\/\/marshallbrain.com\/python2\">Tutorial 2 &#8211; Adding input to a computer program<\/a><\/li><li><a href=\"https:\/\/marshallbrain.com\/python3\">Tutorial 3 &#8211; The coin flip simulator and random numbers<\/a><\/li><li><a href=\"https:\/\/marshallbrain.com\/python4\">Tutorial 4 &#8211; Functions and comments<\/a><\/li><li><a href=\"http:\/\/marshallbrain.com\/python5.htm\">Tutorial 5 &#8211; The<\/a><a href=\"https:\/\/marshallbrain.com\/python5.htm\"> <\/a><a href=\"http:\/\/marshallbrain.com\/python5\">dice simulator &#8211; lists in Python<\/a><\/li><\/ul>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"https:\/\/marshallbrain.com\/kids-programming\">Teaching your kids how to write computer programs<\/a><\/li><\/ul>\n\n\n\n<p><\/p>\n","protected":false},"excerpt":{"rendered":"<p>by&nbsp;Marshall Brain In the first tutorial we learned how to write the simplest computer programs using the Python programming language. We learned how to create&nbsp;output&nbsp;from a computer program. In this turotial we will learn about variables and&nbsp;input. With&nbsp;input, we can ask the user questions and use the answers to control our programs. Here is the &hellip; <a href=\"https:\/\/marshallbrain.com\/python2\" class=\"more-link\">Continue reading <span class=\"screen-reader-text\">Marshall Brain&#8217;s Quick and Easy Python Tutorials &#8211; Chapter 2<\/span> <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-514","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/pages\/514","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/comments?post=514"}],"version-history":[{"count":1,"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/pages\/514\/revisions"}],"predecessor-version":[{"id":515,"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/pages\/514\/revisions\/515"}],"wp:attachment":[{"href":"https:\/\/marshallbrain.com\/wp-json\/wp\/v2\/media?parent=514"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}