How to call powershell function properly?

Question

I wrote a function to convert the input month parameter to a certain format. such as if I passed 04 to the function and the function will return the "_APR_". the function I wrote is like below:

function GetEnMonth()
{
param([string] $month)
switch ($month)
{
    ($_ -eq "01"){$result = "_JAN_"}
    ($_ -eq "02"){$result = "_FEB_"}
    ($_ -eq "03"){$result = "_MAR_"}
    ($_ -eq "04"){$result = "_APR_"}
    ($_ -eq "05"){$result = "_MAY_"}
    ($_ -eq "06"){$result = "_JUN_"}
    ($_ -eq "07"){$result = "_JUL_"}
    ($_ -eq "08"){$result = "_AUG_"}
    ($_ -eq "09"){$result = "_SEP_"}
    ($_ -eq "10"){$result = "_OCT_"}
    ($_ -eq "11"){$result = "_NOV_"}
    ($_ -eq "12"){$result = "_DEC_"}
    default {$result ="_No_Result_"}
}
return [string]$result;
}

Then I use below command to execute the function to get the result:

$mYear = $today.substring(0,4) 
$mMonth =$today.substring(4,2)
$mDate = $today.substring(6,2)
$monthInEn = GetEnMonth $mMonth

well, the result is always "_No_Result_", why? below is the exception:

 **** Exception type : System.Management.Automation.RuntimeException
 **** Exception message : You cannot call a method on a null-valued expression.

Could anyone give me an answer for this? I have searched Google a lot but don't find useful solutions.

this parameter is mannually operated. we pass previous date to it, suce as 20120409. — CharlieShi
– CharlieShi, Commented Apr 10, 2012 at 10:02

jon Z · Accepted Answer · 2012-04-10 10:05:36Z

3

This should work:

function GetEnMonth
{
param([string] $month)
switch ($month)
{
    "01"{$result = "_JAN_"}
    "02"{$result = "_FEB_"}
    "03"{$result = "_MAR_"}
    "04"{$result = "_APR_"}
    "05"{$result = "_MAY_"}
    "06"{$result = "_JUN_"}
    "07"{$result = "_JUL_"}
    "08"{$result = "_AUG_"}
    "09"{$result = "_SEP_"}
    "10"{$result = "_OCT_"}
    "11"{$result = "_NOV_"}
    "12"{$result = "_DEC_"}
    default {$result ="_No_Result_"}
}
return [string]$result;
}

answered Apr 10, 2012 at 10:05

jon Z

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Comments

Shay Levy · Accepted Answer · 2012-04-10 12:48:19Z

3

Here's my take on this. I changed the parameter type to Int, with this you'll be able to get process such as "03" or 3. I also added break statements to make the switch work faster.

function GetEnMonth
{
    param([int] $month)

    switch ($month)
    {
        1 {"_JAN_"; break}
        2 {"_FEB_"; break}
        3 {"_MAR_"; break}
        4 {"_APR_"; break}
        5 {"_MAY_"; break}
        6 {"_JUN_"; break}
        7 {"_JUL_"; break}
        8 {"_AUG_"; break}
        9 {"_SEP_"; break}
        10 {"_OCT_"; break}
        11 {"_NOV_"; break}
        12 {"_DEC_"; break}
        default {"_No_Result_"}
    }
}

answered Apr 10, 2012 at 12:48

Shay Levy

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Comments

CB. · Accepted Answer · 2012-04-10 10:07:54Z

2

Your switch statment is wrong try this:

function GetEnMonth()
{
param([string] $month)
switch ($month)
{
    "01" {$result = "_JAN_"}
    "02" {$result = "_FEB_"}
    "03" {$result = "_MAR_"}
    "04" {$result = "_APR_"}
    "05" {$result = "_MAY_"}
    "06" {$result = "_JUN_"}
    "07" {$result = "_JUL_"}
    "08" {$result = "_AUG_"}
    "09" {$result = "_SEP_"}
    "10" {$result = "_OCT_"}
    "11" {$result = "_NOV_"}
    "12" {$result = "_DEC_"}
    default {$result ="_No_Result_"}
}
return [string]$result;
}

answered Apr 10, 2012 at 10:07

CB.

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5 Comments

CharlieShi Over a year ago

well, Could you pls tell me $_ means what?

CB. Over a year ago

This is the variable for the current value in the pipe line: es: 1..10 | Foreach-object { write-host $_ } prints numbers from 1 to 10 in the screen. $_ is also the current argument to the current script block executed

CharlieShi Over a year ago

think that it's like the $(this) in jquery when using each function, right?

CB. Over a year ago

try get-help about_automatic_variables to know more about automatics variables in powershell.

Rynant Over a year ago

@CharlieShi Your example will work if you change ($_ -eq "XX") to {$_ -eq "XX"} The $_ refers to the current item from the $month variable at the top of the switch statement. Switch statements can take entire arrays as arguments. $_ will refer to each element as one element at a time is passed through the switch.

JPBlanc · Accepted Answer · 2012-04-10 11:38:26Z

2

This is not an answer, but another way to do the same in one line and with a culture param:

$month = 3
$smonth=[string]::format([System.Globalization.CultureInfo]"en-US", "_{0:MMM}_",[datetime]("$month/01"))
$smonth
_MAR_

answered Apr 10, 2012 at 11:38

JPBlanc

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