While Loops: Repeating Actions

The while Loop: Repeating Based on a Condition

Unlike a for loop which is built around a counter, a while loop is simpler in its structure. It continuously executes a block of code as long as a specific logical expression evaluates to true.

The basic syntax is straightforward:

while (condition)
{
  // Code to be repeated goes here
}

The program checks the condition before each potential run. If it's true, the code inside the curly braces {} is executed. If it's false, the loop is skipped entirely, and the program moves on.

The following flowchart provides a clear visual of this logic. The program enters the loop, checks the condition, and if the answer is "Yes," it performs the action and loops back to check the condition again. If the answer is "No," it exits the loop and proceeds.

Let's start with a basic written explanation.

A Simple Counter with a while Loop

The best way to understand the while loop is to see it in action. Let's build a simple counter, similar to what we did with the for loop, but using while instead.

The video below, "LEARN MQL5 TUTORIAL BASICS - 8 HOW TO USE THE WHILE LOOP," will walk you through creating a simple script that uses a while loop. The example in the video is slightly different (it uses a large number for a delay), but the core principles are identical.

The Critical Danger: Infinite Loops

With for loops, the iteration step is built into the syntax, so they usually terminate as planned. With while loops, you are responsible for ensuring the condition eventually becomes false. If you forget, you create an infinite loop.

An infinite loop is a loop that never ends. In MQL5, this will cause your script or Expert Advisor to freeze, stop responding, and consume all available processing power. It will not be able to process new ticks, manage trades, or do anything else. Learning to avoid this is one of the most important lessons in programming.

The cause is almost always the same: the variable being checked in the loop's condition is never changed inside the loop body.

The video you just started watching has an excellent demonstration of exactly what happens when you make this mistake.

This is the golden rule of while loops: Something inside the loop must work towards making the condition false.

The MQL5 documentation also warns about a more subtle cause of infinite loops, particularly when counting down.

Practical Exercise: Build and Break a while Loop

Let's apply this knowledge. We'll create a simple script that counts and then intentionally break it to see the effect of an infinite loop firsthand.

1. Open MetaEditor and create a new script named WhileTest. (Go to File > New > MQL5 Program > Script).
2. Replace the default code with the following:

//+------------------------------------------------------------------+
//|                                                    WhileTest.mq5 |
//+------------------------------------------------------------------+



#property copyright "Your Name"
#property version   "1.00"

//--- This function runs when the script is started
void OnStart()
{
  Print("Start of script");
  
  int counter = 1;
  
  // --- YOUR TASK: Add a while loop here ---
  // It should print the value of 'counter' as long as it is less than 3 (i.e., 1 and 2)
  // Remember to increment the counter inside the loop!

  
  Print("Loop has finished");
  Alert("Loop has finished");
}

Your Tasks:

1. Implement the Loop: In the space indicated, write a while loop that runs as long as counter is less than 3. Inside the loop, use Print(counter); and then increment the counter.
2. Compile and Run: Compile the code (F7). In MetaTrader 5, find the WhileTest script in the "Navigator" window under "Scripts". Drag it onto any chart.
3. Check the Output: Go to the "Experts" tab in the "Toolbox" window (Ctrl+T). If you did it correctly, the log should show "Start of script," then "1," then "2," and finally "Loop has finished." An alert box will also pop up. The output should look similar to the log in this image:

4. Create an Infinite Loop: Now, comment out or delete the line inside your loop that increments the counter (counter++; or counter = counter + 1;). Compile the code again. Before you run it, be aware that MetaTrader might become unresponsive. Run the script. Notice how the "Loop has finished" message never appears and the platform freezes. You will likely need to close and restart MetaTrader 5. This experience, while disruptive, is the best way to understand the importance of loop termination.

Trading Application: while vs. if

So, when would you use a while loop in a trading bot? A common scenario is when you need to ensure an action is completed, but a for loop isn't suitable.

A fantastic, non-obvious example is opening multiple trades at once. Imagine your strategy requires you to open 5 positions immediately. If you use an if statement like if (PositionsTotal() < 5), it will only open one trade per tick. The OnTick() function will run, open one trade, and then finish. On the next tick, it will open another, and so on.

However, a while loop behaves differently. The while (PositionsTotal() < 5) loop will execute repeatedly within the same tick until 5 positions are open. It does not wait for the next tick.

This article provides a great walkthrough comparing these two behaviors.

This ability to execute code repeatedly within a single event makes the while loop a tool for aggressive, rapid actions, but one that must be used with extreme care to avoid freezing your trading terminal.

Conclusion

In this lesson, you've added the while loop to your MQL5 toolkit. Unlike the for loop, which runs a set number of times, the while loop provides the flexibility to repeat actions based on a dynamic condition.

Key Takeaways:

• A while loop executes a block of code repeatedly as long as its condition is true.
• The condition is checked before each iteration. If the condition is false from the start, the loop will never run.
• The most critical aspect of a while loop is ensuring the condition will eventually become false. Failure to do so results in an infinite loop, which will freeze your program.
• In trading, a while loop inside OnTick() executes its entire cycle within a single tick, allowing for rapid-fire actions that an if statement cannot perform.

You have now completed the "Logic and Control Flow" module. You can declare variables, perform operations, make decisions with if-else, and create loops with for and while. These are the absolute fundamental building blocks of any program.

In our next module, we will begin organizing our code more effectively. Instead of writing everything in one long sequence, we'll learn how to create reusable blocks of code called functions, starting with an exploration of the OnStart() event handler you've already been using.