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<span>while</span>
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<h1>while</h1>
<p>The GameMaker Language has a number of ways that you can perform <i>loops</i>, one of the most important is the <tt>while</tt> loop. This loop function has the form:</p>
<p class="code">while (<expression>)<br/> {<br/> <statement>;<br/> <statement>;<br/> ...<br/> }</p>
<p>Here you have a <a class="tooltip" title="In programming, a statement is a single line of code written legally in a programming language that expresses an action to be carried out. A statement might have internal components of its own, including expressions, operators and functions. An example of a statement is A = B + 5. A GameMaker Studio 2 program is nothing but a sequence of one or more statements that together perform a task (like move the player). ">statement</a> that is iterated over again and again based on the results of the evaluation of an <a class="tooltip" title="An expression is a combination of one or more constants, variables, operators, and/or functions that are interpreted according to particular rules of precedence and association to return another value. A simple expression would be (5 + 5), which returns 10.">expression</a>, ie: with
a <span class="inline">while</span> loop, as long as the expression evaluates to <span class="inline">true</span>, the statement (which can also be a code block of multiple statements with curly brackets <tt>{}</tt>) is executed. Below you can find
an example of a typical way to use "while":</p>
<p class="code">while (place_meeting(x, y, obj_Wall))<br/> {
<br/> x -= 1;<br/> }
</p>
<p>The above code is checking for a collision between the calling instance and a "wall" instance, and it will perform multiple <a class="tooltip" title="An iteration is a single pass through a set of operations in your project code. One form of iteration in computer programming is via loops. A loop will repeat a certain segment of code until a condition is met and it can proceed further. Each time the computer runs a loop, it is known as an iteration. In simple terms, iteration is the process of repeating a particular snippet of code over and over again to perform a certain action.">iteration</a><span class="glossextra">s</span> while one is
occurring - moving the instance left by one pixel - until the instance is no longer in collision.</p>
<p><strong>When should you use a <span class="inline">while</span> loop?</strong> It should be used any time you want to repeat one or more statements, but dont actually know -or care - how many times it has to repeat, keeping in mind that if the initial
evaluation is <span class="inline">false</span>, the statements may not even be run.</p>
<p>Please not that you should <strong>be very careful with your <tt>while</tt> loops</strong>! You can easily make <i>infinite</i> loops, in which case your game will hang and not react to any user input anymore and need to be force closed. For example:</p>
<p class="code">while (!place_free(x, y))<br/> {
<br/> x = random(rrom_width);<br/> y = random(room_height);<br/> }
</p>
<p>Now, the above code may work fine, but it may also cause an infinite loop if the instance is unable to find an empty position to move to, and this will cause the game to hang. If you find yourself in a position where this kind of thing is a possibility,
then you should either use a different non-blocking loop kind, or use an additoinal variable check in the expression (you can use multiple expressions along with the <a href="../Expressions_And_Operators.htm"><span class="inline">and</span> (&<span class="inline">&</span>)</a>,
<a href="../Expressions_And_Operators.htm"><span class="inline">or</span> (<span class="inline">||</span>)</a> and <a href="../Expressions_And_Operators.htm"><span class="inline">xor</span> (<span class="inline">^^</span>)</a> <a class="tooltip" title="An operator is a term used to denote something which can manipulate different operands. In the expression A + B - C, + and - are the operators. Examples of different operators are + (addition), - (subtrtaction), = (equals), != (not equal) and >= (greater than or equal to), but there are many more.">operator</a><span class="glossextra">s</span> for the check):</p>
<p class="code">var _check = 0;<br/> while ((!place_free(x, y)) && (_check < 100))<br/> {
<br/> x = random(room_width);<br/> y = random(room_height);<br/> _check += 1;<br/> }
<br/> if _check >= 100<br/> {<br/> // code failed, so deal with it<br/> }</p>
<p>Alternatively you can use the <span class="inline"><a href="break.htm">break</a></span> statement to break out of the loop, for example, the following code will generate 100 random numbers then continue, even though the <span class="inline">while</span> evaluation is <em>always</em> going to be <span class="inline">true</span>:</p>
<p class="code">var i = 0;<br/> while (true)<br/> {
<br/> x[i] = random(room_width);<br/> y[i] = random(room_height);<br/> if ((i++) >= 100)<br/> {<br/> break;<br/> }<br/> }
</p>
<p>You may also use the <a href="continue.htm">continue</a> statement in a <span class="inline">while</span> loop. Using this will end the current loop iteration and restart the loop again on a new iteration, for example:</p>
<p class="code">var file = file_text_open_read("Game_Data.txt");<br/> var _num = 0;<br/> while (!file_text_eof(file))<br/> {<br/> var _s = file_text_readln(file);<br/> if (_s == "")<br/> {<br/> continue;<br/> }<br/> str[num++] = _s;<br/> }<br/> file_text_close(file);
</p>
<p>This code above will open a file and read a line from it each loop iteration until the end of the file contents are reached. If the line being read is an empty string, the current loop iteration is ended using the <span class="inline">continue</span> statement and a new iteration will be started, otherwise the string is added into an <a href="../Arrays.htm">array</a> and the array position incremented.</p>
<p>For more examples of loop functions please see the sections on <a href="repeat.htm"><tt>repeat</tt></a>, <a href="do___until.htm"><tt>do... until</tt></a>, and <a href="for.htm"><tt>for</tt></a>.</p>
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