for Loop in C#: Build a Sales Grid Step by Step (.NET 10)

Four stores, six months, one grid: that’s what our Phoenix regional manager asked for. A for loop in C# repeats code a set number of times with a counter: for (int i = 0; i < n; i++) { ... }.

In this tutorial, I’ll show you, one small step at a time, how the for loop in C# counts forward, backward and by steps, nests for a table, and uses break and continue to build that grid on .NET 10.

Here’s what we’re building. Each row is a store, each column is a month in thousands of dollars, and the last column is the store’s total:

Before You Start the for Loop Build

You need the .NET 10 SDK and an editor. I run every step as a single-file app with dotnet run File.cs; if that’s new to you, here’s how to create a console application in Visual Studio Code.

All steps share this sample data (sales in thousands of USD, January to June 2026):

Store	Jan	Feb	Mar	Apr	May	Jun
Austin, TX	42	38	51	47	53	58
Denver, CO	29	33	0	36	31	34
Phoenix, AZ	55	49	58	61	64	70
Tampa, FL	31	35	30	40	38	44

Denver’s March is 0 because the store closed for a remodel. We’ll use that gap in Step 5.

Step 1: Write a Basic for Loop (Initializer, Condition, Iterator)

A for header has three sections separated by semicolons. Let’s walk the months forward:

string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];

//   initializer; condition;         iterator
for (int m = 0;   m < months.Length; m++)
{
    Console.WriteLine($"Month {m}: {months[m]}");
}
Console.WriteLine($"The loop ran {months.Length} times");

Output:

Month 0: Jan
Month 1: Feb
Month 2: Mar
Month 3: Apr
Month 4: May
Month 5: Jun
The loop ran 6 times

Here’s the order C# runs the pieces in, per the Microsoft reference:

1. Initializer int m = 0 runs once, before the first pass.
2. Condition m < months.Length is checked before every pass; false ends the loop.
3. The body runs.
4. The iterator m++ runs, then it’s back to the condition.

m only exists inside the loop, so you can’t read it after the closing brace. My guide to arrays in C# covers Length and indexes in more depth.

Checkpoint: Month 0: Jan through Month 5: Jun, then The loop ran 6 times.

Step 2: Loop Backward with a Decrementing Counter

The manager wants the newest month first. Start at the last index, keep going while m >= 0, and count down:

string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];

for (int m = months.Length - 1; m >= 0; m--)
    Console.Write($"{months[m]} ");
Console.WriteLine("<- newest first");

Output:

Jun May Apr Mar Feb Jan <- newest first

The last index is Length - 1, not Length. If you only need the array flipped, there’s a shortcut to reverse an array in C# without writing the loop.

Checkpoint: Jun May Apr Mar Feb Jan <- newest first.

Step 3: Step by 2 or 3 in a for Loop

The iterator can be any assignment, so m += 3 jumps a whole quarter at a time. Same Austin row, now as quarter totals:

string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];
int[] austin = [42, 38, 51, 47, 53, 58];   // $ thousands

for (int m = 0; m < austin.Length; m += 3)
{
    int q = austin[m] + austin[m + 1] + austin[m + 2];
    Console.WriteLine($"Q{m / 3 + 1} {months[m]}-{months[m + 2]}: ${q}K");
}

Console.Write("Every 2nd month:");
for (int m = 1; m < months.Length; m += 2)
    Console.Write($" {months[m]}");
Console.WriteLine();

Output:

Q1 Jan-Mar: $131K
Q2 Apr-Jun: $158K
Every 2nd month: Feb Apr Jun

The start value and the step together decide which items you touch.

Checkpoint: Q1 Jan-Mar: $131K, Q2 Apr-Jun: $158K, then Feb Apr Jun.

Step 4: Nest for Loops to Print the Grid

Now the full table. The outer loop walks the rows (stores); for each row, the inner loop walks the columns (months):

string[] stores = ["Austin", "Denver", "Phoenix", "Tampa"];
string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];
int[,] sales = { { 42, 38, 51, 47, 53, 58 },
                 { 29, 33,  0, 36, 31, 34 },
                 { 55, 49, 58, 61, 64, 70 },
                 { 31, 35, 30, 40, 38, 44 } };   // $ thousands

for (int s = 0; s < sales.GetLength(0); s++)    // stores
{
    Console.Write($"{stores[s],-8}");
    for (int m = 0; m < sales.GetLength(1); m++)  // months
        Console.Write($"{sales[s, m],4}");
    Console.WriteLine();
}

Output:

Austin    42  38  51  47  53  58
Denver    29  33   0  36  31  34
Phoenix   55  49  58  61  64  70
Tampa     31  35  30  40  38  44

On a two-dimensional array, Length is the total cell count (24 here), so use GetLength(0) and GetLength(1) for each dimension. The ,4 in the interpolation right-aligns each number; more options are in format numbers with commas and decimals in C#.

Checkpoint: four rows of six numbers, with Phoenix’s row ending in 70.

Step 5: Use break and continue in a for Loop

Two questions from the manager: what did Denver average in the months it was open, and when did Phoenix first top $60K?

string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];
int[] denver = [29, 33, 0, 36, 31, 34];
int[] phoenix = [55, 49, 58, 61, 64, 70];

int total = 0, open = 0;
for (int m = 0; m < denver.Length; m++)
{
    if (denver[m] == 0) continue;   // closed for remodel
    total += denver[m]; open++;
}
Console.WriteLine($"Denver: ${total}K over {open} open months");

for (int m = 0; m < phoenix.Length; m++)
{
    if (phoenix[m] < 60) continue;
    Console.WriteLine($"Phoenix hit $60K in {months[m]}"); break;
}

Output:

Denver: $163K over 5 open months
Phoenix hit $60K in Apr

continue jumps straight to the iterator, so the closed month isn’t counted. break leaves the loop at once, and May and June are never read.

Note: if every month were closed, open would be 0, so check it before you divide. That’s the division by zero exception in C# waiting to happen.

Checkpoint: Denver: $163K over 5 open months and Phoenix hit $60K in Apr.

Step 6: Declare Multiple Variables in the for Loop Initializer

To compare each early month with its mirror late month, use two counters: one moving forward, one moving back.

string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];
int[] phoenix = [55, 49, 58, 61, 64, 70];

for (int first = 0, last = months.Length - 1; first < last; first++, last--)
{
    int change = phoenix[last] - phoenix[first];
    Console.WriteLine($"{months[first]} vs {months[last]}: {change:+0;-0}K");
}

Output:

Jan vs Jun: +15K
Feb vs May: +15K
Mar vs Apr: +3K

You can declare several variables of the same type with commas, and the iterator can update both: first++, last--. It’s the same two-index pattern I use to swap two numbers in C# inside an array.

Checkpoint: Jan vs Jun: +15K, Feb vs May: +15K, Mar vs Apr: +3K.

Test the Finished for Loop Sales Grid

Here’s the full program from the top screenshot. After a header line, the nested loops print each store and its total:

string[] stores = ["Austin", "Denver", "Phoenix", "Tampa"];
string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];
int[,] sales = { { 42, 38, 51, 47, 53, 58 },
                 { 29, 33,  0, 36, 31, 34 },
                 { 55, 49, 58, 61, 64, 70 },
                 { 31, 35, 30, 40, 38, 44 } };   // $ thousands
Console.WriteLine("Store    Jan  Feb  Mar  Apr  May  Jun Total");
for (int s = 0; s < stores.Length; s++)
{
    int total = 0;
    Console.Write($"{stores[s],-7}");
    for (int m = 0; m < months.Length; m++)
    {
        Console.Write($"{sales[s, m],5}");
        total += sales[s, m];
    }
    Console.WriteLine($"{total,6}");
}

Output:

Store    Jan  Feb  Mar  Apr  May  Jun Total
Austin    42   38   51   47   53   58   289
Denver    29   33    0   36   31   34   163
Phoenix   55   49   58   61   64   70   357
Tampa     31   35   30   40   38   44   218

total is declared inside the outer loop, so it resets to 0 for every store. Phoenix leads with $357K, and Denver’s $163K reflects the closed month.

for vs foreach in C#: Which Should You Use?

Both loops can walk the months array. I pick by what I need from the loop; the foreach loop in C# builds the same kind of report without an index.

	for	foreach
Index available	Yes, the counter	No (use Index() on .NET 9+)
Backward or step by N	Yes	No
Write to array items	Yes, months[m] = ...	No, loop variable is read-only
Works on	Arrays, lists (use Count)	Any enumerable
Off-by-one risk	Yes	No

Why Does My for Loop Throw IndexOutOfRangeException?

Almost always it’s <= where you meant <. Arrays are zero-based, so a six-item array has indexes 0 to 5:

string[] months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"];

for (int m = 0; m <= months.Length; m++)   // <= is the bug
    Console.WriteLine(months[m]);

Output (unhandled exception):

Jan
Feb
Mar
Apr
May
Jun
Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at Program.<Main>$(String[] args) in C:\DotNetDemo\ForLoopDemo\OffByOne.cs:line 4

Important: write the condition as i < array.Length going forward and start at array.Length - 1 going backward.

Troubleshooting: Infinite for Loops

All three sections are optional, so for (;;) runs until something inside calls break. Here it projects when Phoenix reaches $80K at 5% growth a month:

decimal forecast = 70m;   // Phoenix June sales, $ thousands
int monthsAhead = 0;
for (;;)
{
    if (forecast >= 80m) break;
    forecast *= 1.05m;     // 5% growth a month
    monthsAhead++;
}
Console.WriteLine($"$80K reached in {monthsAhead} months");

Output:

$80K reached in 3 months

It works, but when the exit depends on a value instead of a count, I’d write a while loop in C#. An accidental infinite loop usually means the iterator moves away from the condition, like i-- with i < n.

Where to Take It Next with for Loops

• Add a totals row per month: swap the loops so months are outer, then sort an array in C# to rank the stores.
• Fill the grid with test data using the trick to generate random numbers in a range.
• Practice with the classic C# program to check a prime number, which is a for loop with an early break.

If you’re comparing loop types, these are worth reading next:

• The do-while loop in C#, which always runs at least once
• Dynamic arrays in C# when the size isn’t known up front
• Switch case in C# for labeling months and regions
• Tuples in C# for returning a store and its total together

Frequently Asked Questions