Mastering the For Loop: Fixed Iteration

Why Do We Need Loops? The Concept of Repetition

Imagine you want to find the highest price of the last 50 candlesticks. With the tools you have now, you'd need to write code to check the high of the first bar, then the second, then the third, all the way to 50. It would be a nightmare to write and even worse to read.

A loop solves this problem by providing a structure to repeat a set of instructions. A for loop is ideal when you know exactly how many repetitions you need. For example:

• "Repeat this 50 times."
• "Count from 1 to 100."
• "Look at each of the last 10 bars."

The Anatomy of a for Loop

The for loop might look a bit intimidating at first, but it's very logical. It consists of three parts, all contained within parentheses, that control its execution.

The general structure is:
for (initialization; condition; iteration)
{
// Code to be repeated goes here
}

Let's break down these three essential parts:

1. Initialization: A statement executed only once when the loop begins. This is where you typically declare and initialize a counter variable. For example: int i = 0; (Start counting at 0).
2. Condition: An expression evaluated before each loop cycle. If the condition is true, the code inside the loop's curly braces {} is executed. If it becomes false, the loop terminates. For example: i < 10; (Keep looping as long as i is less than 10).
3. Iteration: A statement executed at the end of each loop cycle. This is where you update your counter variable, usually by incrementing it. For example: i++ (Add 1 to i).

The following video, "LEARN MQL5 TUTORIAL BASICS - 9 HOW TO USE THE FOR LOOP", provides a clear, step-by-step walkthrough of creating a basic for loop that counts upwards.

For a written reference that reinforces this structure, you can consult the official MQL5 documentation.

Understanding the Execution Flow

It's crucial to understand the exact order of operations in a for loop to predict its behavior and avoid mistakes.

Here's the sequence:

1. Initialization runs once.
2. The Condition is checked.
   • If true, the code inside the {} runs.
   • If false, the loop stops, and the program continues with the code after the loop.
3. The Iteration step runs.
4. Go back to step 2.

This cycle—Condition, Code, Iteration—repeats until the condition becomes false.

The article "From Basic to Intermediate: FOR Statement" has a great flowchart that visualizes this process perfectly.

Practical Exercise: Creating a Counter EA

Let's put this into practice. We'll create a simple Expert Advisor that uses a for loop to print messages to the Experts journal.

1. Open MetaEditor.
2. Create a new Expert Advisor named LoopCounter.
3. Delete the default template and replace it with this code:

//+------------------------------------------------------------------+
//|                                                  LoopCounter.mq5 |
//+------------------------------------------------------------------+



#property copyright "Copyright 2024, Your Name"
#property version   "1.00"

//--- This function runs when the EA is first started on the chart
void OnInit()
{
  // A simple loop that counts from 1 up to 10
  Print("--- Starting Upward Count ---");
  for(int i = 1; i <= 10; i++)
  {
    Print("The current count is: ", i);
  }
  Print("--- Upward Count Finished ---");
}

//--- The OnTick function is not needed for this example
void OnTick()
{
}

Your Task:

1. Compile and Run: Compile the code (F7). Open the Strategy Tester in MetaTrader 5 (Ctrl+R). Select the LoopCounter.ex5 EA, choose any symbol/period, and click "Start".
2. Check the Journal: Go to the "Journal" tab at the bottom of the Strategy Tester window. You should see the output of your for loop, counting from 1 to 10.
3. Experiment:
   • Count Down: Modify the for loop in OnInit() to count down from 20 to 15. You'll need to change the initialization, condition, and iteration. (Hint: Initialize i to 20, check if i >= 15, and use i-- for the iteration.)
   • Count in Steps: Modify the loop to count from 0 to 100 in steps of 10 (0, 10, 20...). (Hint: i = i + 10 is one way to do this.)

The video you watched earlier also demonstrates how to make these exact changes.

Making it Relevant: Loops in Trading

Simple counting is useful, but how does this apply to building a trading bot? The most common use of for loops in trading is to iterate over historical data like prices, volumes, or indicator values.

The video "MQL5 Programming Tutorial 1.07 - Loops and Lists" contains an excellent, practical example. It uses a for loop to get the volume of the last 5 bars.

This example gives you a glimpse of the power you've just unlocked. The counter variable i in for(int i=0; ...) is not just a number; it becomes an index that you can use to pinpoint a specific piece of historical data. i=0 refers to the current, developing bar, i=1 refers to the most recently closed bar, i=2 is the one before that, and so on. This is the mechanism you will use to build nearly every trading strategy.

Conclusion

You have now learned how to control repetition in your programs using the for loop. This is a massive leap forward. Instead of writing code that runs once, you can now write code that systematically works through sets of data, performs calculations, and automates tasks that would be impossible to do by hand.

Key Takeaways:

• Loops are used to execute a block of code repeatedly.
• The for loop is best used when you know the number of iterations in advance.
• A for loop is defined by three parts: initialization (runs once at the start), condition (checked before each run), and iteration (runs at the end of each run).
• In trading, the for loop's counter is commonly used as an index to access historical price bars, indicator values, or other time-series data.

In this lesson, we focused on loops that run a pre-defined number of times. But what if you want a loop to run until a specific event happens, and you don't know when that will be? For example, "keep checking the price every second until it crosses above $100." For these situations, we need a different kind of loop. In our next lesson, we will explore the while loop, which continues as long as a condition remains true.