Read-Host in While loop with if statement

Question

This should be really simple, but I cannot make it work. I'm new to the PowerShell so my mistake is probably obvious. I'm trying to ask a user for an input using a while loop, and prompting the user to try again if what was entered was not numeric. In bash it would look like this:

while read -p "What is the first number? " first; do
if [[ $first = *[[:digit:]]* ]]; then
break   # if $first is numeric, break loop
else
echo "$first is not numeric. Enter a numeric value. "
fi
done

Here's what I have for the PS:

While ($first = (Read-Host "What is the first number?") -as [double]) {
if ($first -eq $null) { Write-Host "Please enter a numeric value." }
else { break }
}

In the example above, I can break the loop fine when a number is entered, but I can't see the Write-Host statement if I type a string.

the while test is run before the loop is run. that means your if test will prevent the "then" result from ever running. [grin] — Lee_Dailey
– Lee_Dailey, Commented Nov 19, 2018 at 1:23

Joel Coehoorn · Accepted Answer · 2018-11-19 22:12:57Z

4

Looking at the While line:

While ($first = (Read-Host "What is the first number?") -as [double])

This will only enter/continue the loop body when the input is already a double, because it rolls up the cast to double as part of the loop condition. You want to enter the loop body on any input, and only check if it's a double afterwards. At least, that's what the bash code does:

While ($first = (Read-Host "What is the first number?")) {
    if ( ($first -as [double]) -eq $null) { Write-Host "Please enter a numeric value." }
    else { break }
}

Or you could continue using the cast as part of the condition by negating the whole expression, and thus avoid the need for the awkward break:

While (-not ($first = (Read-Host "What is the first number?") -as [double])) {
    Write-Host "Please enter a numeric value." 
}

edited Nov 19, 2018 at 22:12

answered Nov 19, 2018 at 0:18

Joel Coehoorn

418k114 gold badges582 silver badges820 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Hubert Léveillé Gauvin Over a year ago

Perfect. I think it should be [double] and not double in the first solution though.

Joel Coehoorn Over a year ago

@HubertLéveilléGauvin Yes, that was a copy/pasta error.

Drew · Accepted Answer · 2018-11-19 00:19:52Z

4

You could use this to keep prompting for a valid number input as you are looking for.

Do {
    "Please enter a numeric value." | Out-Host
    $first = (Read-Host "What is the first number?") -as [double]
} While($null -eq $first)

answered Nov 19, 2018 at 0:19

Drew

4,0682 gold badges13 silver badges29 bronze badges

Collectives™ on Stack Overflow

Read-Host in While loop with if statement

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related