about_For

Short description

Describes a language command you can use to run statements based on a conditional test.

Long description

The For statement (also known as a For loop) is a language construct you can use to create a loop that runs commands in a command block while a specified condition evaluates to $true.

A typical use of the For loop is to iterate an array of values and to operate on a subset of these values. In most cases, if you want to iterate all the values in an array, consider using a Foreach statement.

Syntax

The following shows the For statement syntax.

for (<Init>; <Condition>; <Repeat>)
{
    <Statement list>
}

The Init placeholder represents one or more commands that are run before the loop begins. You typically use the Init portion of the statement to create and initialize a variable with a starting value.

This variable will then be the basis for the condition to be tested in the next portion of the For statement.

The Condition placeholder represents the portion of the For statement that resolves to a $true or $false Boolean value. PowerShell evaluates the condition each time the For loop runs. If the statement is $true, the commands in the command block run, and the statement is evaluated again. If the condition is still $true, the commands in the Statement list run again. The loop is repeated until the condition becomes $false.

The Repeat placeholder represents one or more commands, separated by commas, that are executed each time the loop repeats. Typically, this is used to modify a variable that is tested inside the Condition part of the statement.

The Statement list placeholder represents a set of one or more commands that are run each time the loop is entered or repeated. The contents of the Statement list are surrounded by braces.

Support for multiple operations

The following syntaxes are supported for multiple assignment operations in the Init statement:

# Comma separated assignment expressions enclosed in parentheses.
for (($i = 0), ($j = 0); $i -lt 10; $i++)
{
    "`$i:$i"
    "`$j:$j"
}

# Sub-expression using the semicolon to separate statements.
for ($($i = 0;$j = 0); $i -lt 10; $i++)
{
    "`$i:$i"
    "`$j:$j"
}

The following syntaxes are supported for multiple assignment operations in the Repeat statement:

# Comma separated assignment expressions.
for (($i = 0), ($j = 0); $i -lt 10; $i++, $j++)
{
    "`$i:$i"
    "`$j:$j"
}

# Comma separated assignment expressions enclosed in parentheses.
for (($i = 0), ($j = 0); $i -lt 10; ($i++), ($j++))
{
    "`$i:$i"
    "`$j:$j"
}

# Sub-expression using the semicolon to separate statements.
for ($($i = 0;$j = 0); $i -lt 10; $($i++;$j++))
{
    "`$i:$i"
    "`$j:$j"
}

Note

Operations other than pre or post increment may not work with all syntaxes.

For multiple Conditions use logical operators as demonstrated by the following example.

for (($i = 0), ($j = 0); $i -lt 10 -and $j -lt 10; $i++,$j++)
{
    "`$i:$i"
    "`$j:$j"
}

For more information, see about_Logical_Operators.

Syntax examples

At a minimum, a For statement requires the parenthesis surrounding the Init, Condition, and Repeat part of the statement and a command surrounded by braces in the Statement list part of the statement.

Note that the upcoming examples intentionally show code outside the For statement. In later examples, code is integrated into the For statement.

For example, the following For statement continually displays the value of the $i variable until you manually break out of the command by pressing CTRL+C.

$i = 1
for (;;)
{
    Write-Host $i
}

You can add additional commands to the statement list so that the value of $i is incremented by 1 each time the loop is run, as the following example shows.

for (;;)
{
    $i++; Write-Host $i
}

Until you break out of the command by pressing CTRL+C, this statement will continually display the value of the $i variable as it is incremented by 1 each time the loop is run.

Rather than change the value of the variable in the statement list part of the For statement, you can use the Repeat portion of the For statement instead, as follows.

$i=1
for (;;$i++)
{
    Write-Host $i
}

This statement will still repeat indefinitely until you break out of the command by pressing CTRL+C.

You can terminate the For loop using a condition. You can place a condition using the Condition portion of the For statement. The For loop terminates when the condition evaluates to $false.

In the following example, the For loop runs while the value of $i is less than or equal to 10.

$i=1
for(;$i -le 10;$i++)
{
    Write-Host $i
}

Instead of creating and initializing the variable outside the For statement, you can perform this task inside the For loop by using the Init portion of the For statement.

for($i=1; $i -le 10; $i++){Write-Host $i}

You can use carriage returns instead of semicolons to delimit the Init, Condition, and Repeat portions of the For statement. The following example shows a For that uses this alternative syntax.

for ($i = 0
  $i -lt 10
  $i++){
  $i
}

This alternative form of the For statement works in PowerShell script files and at the PowerShell command prompt. However, it is easier to use the For statement syntax with semicolons when you enter interactive commands at the command prompt.

The For loop is more flexible than the Foreach loop because it allows you to increment values in an array or collection by using patterns. In the following example, the $i variable is incremented by 2 in the Repeat portion of the For statement.

for ($i = 0; $i -le 20; $i += 2)
{
    Write-Host $i
}

The For loop can also be written on one line as in the following example.

for ($i = 0; $i -lt 10; $i++) { Write-Host $i }

Functional example

The following example demonstrates how you can use a For loop to iterate over an array of files and rename them. The files in the work_items folder have their work item ID as the filename. The loop iterates through the files to ensure that the ID number is zero-padded to five digits.

First, the code retrieves the list of work item data files. They're all JSON files that use the format <work-item-type>-<work-item-number> for their name. With the file info objects saved to the $fileList variable, you can sort them by name and see that while items are grouped by type, the ordering of the items by ID is unexpected.

$fileList = Get-ChildItem -Path ./work_items
$fileList | Sort-Object -Descending -Property Name

bug-219.json
bug-41.json
bug-500.json
bug-697.json
bug-819.json
bug-840.json
feat-176.json
feat-367.json
feat-373.json
feat-434.json
feat-676.json
feat-690.json
feat-880.json
feat-944.json
maint-103.json
maint-367.json
maint-454.json
maint-49.json
maint-562.json
maint-579.json

To ensure that you can sort the work items alphanumerically, the work item numbers need to be zero-padded.

The code does this by first searching for the work item with the longest numerical suffix. It loops over the files using a for loop, using the index to access each file in the array. It compares each filename to a regular expression pattern to extract the work item number as a string instead of an integer. Then it compares the lengths of the work item numbers to find the longest number.

# Default the longest numeral count to 1, since it can't be smaller.
$longestNumeralCount = 1

# Regular expression to find the numerals in the filename - use a template
# to simplify updating the pattern as needed.
$patternTemplate = '-(?<WorkItemNumber>{{{0},{1}}})\.json'
$pattern         =  $patternTemplate -f $longestNumeralCount

# Iterate, checking the length of the work item number as a string.
for (
    $i = 0                 # Start at zero for first array item.
    $i -lt $fileList.Count # Stop on the last item in the array.
    $i++                   # Increment by one to step through the array.
) {
    if ($fileList[$i].Name -match $pattern) {
        $numeralCount = $Matches.WorkItemNumber.Length
        if ($numeralCount -gt $longestNumeralCount) {
            # Count is higher, check against it for remaining items.
            $longestNumeralCount = $numeralCount
            # Update the pattern to speed up the search, ignoring items
            # with a smaller numeral count using pattern matching.
            $pattern = $patternTemplate -f $longestNumeralCount
        }
    }
}

Now that you know the maximum numeral count for the work items, you can loop over the files to rename them as needed. The next snippet of code iterates over the file list again, padding them as needed. It uses another regular expression pattern to only process files with a numeral count smaller than the maximum.

# Regular expression to find the numerals in the filename, but only if the
# numeral count is smaller than the longest numeral count.
$pattern = $patternTemplate -f 1, ($longestNumeralCount - 1)
for (
    $i = 0                 # Start at zero for first array item.
    $i -lt $fileList.Count # Stop on the last item in the array.
    $i++                   # Increment by one to step through the array.
) {
    # Get the file from the array to process
    $file = $fileList[$i]

    # If the file doesn't need to be renamed, continue to the next file
    if ($file.Name -notmatch $pattern) {
        continue
    }

    # Get the work item number from the regular expression, create the
    # padded string from it, and define the new filename by replacing
    # the original number string with the padded number string.
    $workItemNumber = $Matches.WorkItemNumber
    $paddedNumber   = "{0:d$longestNumeralCount}" -f $workItemNumber
    $paddedName     = $file.Name -replace $workItemNumber, $paddedNumber

    # Rename the file with the padded work item number.
    $file | Rename-Item -NewName $paddedName
}

Now that the files are renamed, you can retrieve the list of files again and sort both the old and new files by name. The following snippet retrieves a the files again to save in a new array and compare with the initial set of objects. Then it sorts both arrays of files, saving the sorted arrays into the new variables $sortedOriginal and $sortedPadded. Finally, it uses a for loop to iterate over the arrays and output an object with the following properties:

• Index represents the current index in the sorted arrays.
• Original is the item in the sorted array of original filenamess at the current index.
• Padded is the item in the sorted array of padded filenames at the current index.

$paddedList = Get-ChildItem -path ./work_items

# Sort both file lists by name.
$sortedOriginal = $fileList    | Sort-Object -Property Name
$sortedPadded   = $renamedList | Sort-Object -Property Name

# Iterate over the arrays and output an object to simplify comparing how
# the arrays were sorted before and after padding the work item numbers.
for (
  $i = 0
  $i -lt $fileList.Count
  $i++
) {
    [pscustomobject] @{
        Index    = $i
        Original = $sortedOriginal[$i].Name
        Padded   = $sortedPadded[$i].Name
    }
}

Index Original       Padded
----- --------       ------
    0 bug-219.json   bug-00041.json
    1 bug-41.json    bug-00219.json
    2 bug-500.json   bug-00500.json
    3 bug-697.json   bug-00697.json
    4 bug-819.json   bug-00819.json
    5 bug-840.json   bug-00840.json
    6 feat-176.json  feat-00176.json
    7 feat-367.json  feat-00367.json
    8 feat-373.json  feat-00373.json
    9 feat-434.json  feat-00434.json
   10 feat-676.json  feat-00676.json
   11 feat-690.json  feat-00690.json
   12 feat-880.json  feat-00880.json
   13 feat-944.json  feat-00944.json
   14 maint-103.json maint-00049.json
   15 maint-367.json maint-00103.json
   16 maint-454.json maint-00367.json
   17 maint-49.json  maint-00454.json
   18 maint-562.json maint-00562.json
   19 maint-579.json maint-00579.json

In the output, the sorted work items after padding are in the expected order.

See also

• about_Comparison_Operators
• about_Foreach