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For Loop

For Loop

The for loop executes a block of code until a specified condition is false. Although a for loop looks like a while loop, developers should use them properly. Use while loops when the number of iterations are variable, otherwise use for loops. A common use of for loops are array iterations.¹

Syntax

for ((Initial variable); (condition); (step)) 
{
	(code)
}

The C# for loop consists of three expressions and some code. When creating a for loop, it is important to understand that each of the 3 expressions that makeup the for loop are all optional expressions.

Description

Initial Variable: your starting state, eg int i = 0;
Condition: While this condition is true the code will continue to run, eg i <= 5;
Step: The increment, or decrement, of the initial variable, eg i++ or i -= 2;.

Example

int[] array = { 1, 2, 3, 4, 5 };
for (int i = 0; i < array.Length; i++)
{
	Console.WriteLine("Item on index {0} is {1}", i, array[i]);
}

Output:

> Item on index 0 is 1
> Item on index 1 is 2
> Item on index 2 is 3
> Item on index 3 is 4
> Item on index 4 is 5

Multiple Initial Variables and Steps

You are not limited to initializing only one variable in a for loop. However, you will be limited to what type any subsequent variables can be as they can only be what was initially specified. You can get around this type restriction by specifying their values outside the loop, but we'll get to that.

For example, if I have a for loop that starts off with int i and I want to add a j to that for loop for some extra processing, j can only be an int and no other type.

Let's look at an example with 2 initialized variables in the for loop using also 2 separate steps, where i will be increasing by 1 with i++ and j will be increasing by 2 using j += 2.

for (int i = 0, j = 1; i < 100; i++, j += 2) {
  Console.Write("i is: {0}, j is {1}", i, j);
  Console.WriteLine("");
}

And some of this example's sample output from the beginning:

i is: 0, j is 1
i is: 1, j is 3
i is: 2, j is 5
i is: 3, j is 7
i is: 4, j is 9

Multiple Conditions

Since the condition in a for loop is a boolean expression, you can use boolean logic on that condition to create more open conditions or stricter conditions based on more than one variable.

Let's look back at the previous for loop, but I'm going to put in one more condition. I want to stop this for loop from iterating if i reaches 100 or j becomes 100. I'm going to do this by using && (and) logic to say as long as i is less than 100 and j is less than 100, keep stepping through this loop.

for (int i = 0, j = 1; i < 100 && j < 100; i++, j += 2) {
  Console.Write("i is: {0}, j is {1}", i, j);
  Console.WriteLine("");
}

And some of this code's sample output from the end:

i is: 45, j is 91
i is: 46, j is 93
i is: 47, j is 95
i is: 48, j is 97
i is: 49, j is 99

Complex For Loops

As we just saw, you can add more initial variables, conditions, and steps to a for loop. But , you can also take away every single one of those, as well. Leaving off any of these, though, comes with a bit of extra complexity.

Even though this type of for loop isn't recommended for production code, you may come across this wild bit of code in your digital travels, so it is important to be aware of it.

A for loop with no condition acts as an infinite loop. If you leave off the initialized variables and the steps of the for loop, you can still break out of the for loop with a reachable condition, so long as it is present in the middle of the for loop. When there is no condition present, there is no way for the for loop to know when it needs to stop unless you explicitly tell it to.

Let's look at a simple for loop that's been restructured as an infinite loop that we explicitly tell to stop:

//variable must be initialized outside of the loop
int i = 0;
//No expressions inside for loop
for (; ;) {
	//Acts as the condition for the for loop
	if (i < 10) {
		Console.WriteLine(i);
	}
	else {
		//There must be some type of halting code here to get out of the for loop
		//in this case "break" literally breaks out of the for loop and
		//continues execution as a standard for loop would normally do
		break;

		//try replacing "break" above with "return" and see what happens
	}

	//You must have this stepping process happening somewhere inside the for loop
	//I put it at the end of this loop so that it follows the procedure of a standard for loop
	i++;
}
Console.WriteLine("Game over!");

And the output for the above code is:

0
1
2
3
4
5
6
7
8
9
Game over!

Notice that the break keyword allowed us to literally break out of the for loop and the continue execution of the program.

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