tryparse method for positive integer is not returning an error with inputs that are not integers

Question

I have the following code:

int orderQuantity;

Write("Number of Items: \t");
while(int.TryParse(ReadLine(), out orderQuantity) && orderQuantity > 0 == false)
{
     Write("Error: Number of Items must be a positive, whole number: \t");
     int.TryParse(ReadLine(), out orderQuantity);
}

The goal is that while the input is not a positive integer, continue to return an error. Once the input is a positive integer, continue on.

The problem is this logic only works with the negative numbers. When I try to make the code into while(!int.TryParse(ReadLine(), out orderQuantity) && orderQuantity > 0 == false)

I run into the code not recognizing integers nor negative numbers as errors.

Use while (!int.TryParse(ReadLine(), out orderQuantity) || orderQuantity <= 0) - "While the line couldn't be parsed as an int OR the parsed value was less than or equal to zero". And remove the parse inside the loop - it's being done in the while statement. — Matthew Watson
– Matthew Watson, Commented Oct 22, 2021 at 9:33

Tim Schmelter · Accepted Answer · 2021-10-22 09:57:17Z

2

Use:

while(!int.TryParse(ReadLine(), out orderQuantity) || orderQuantity <= 0)
{
    Write("Error: Number of Items must be a positive, whole number: \t");
}

So you have to handle the cases that it's not a valid integer and that it's valid but not greater than zero separately. Also remove the TryParse from the loop body.

answered Oct 22, 2021 at 9:57

Tim Schmelter

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1 Comment

G H Over a year ago

I'm getting the following errors: CS0019 Operator '||' cannot be applied to operands of type 'bool' and 'int' and CS0029 Cannot Implicitly convert type 'string' to 'int'. Both errors seem odd to me though. I must have messed up the syntax, so copying your solution exactly seemed to solve it.

iSR5 · Accepted Answer · 2021-10-22 10:01:04Z

1

you need to parenthesize the condition so this :

int.TryParse(ReadLine(), out orderQuantity) && orderQuantity > 0 == false

changed to be this :

(int.TryParse(ReadLine(), out orderQuantity) && orderQuantity > 0) == false

answered Oct 22, 2021 at 10:01

iSR5

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Comments

SomeBody · Accepted Answer · 2021-10-22 10:49:22Z

1

An extension method might help you to get code that is more expressive and thus more easy to read:

public static class StringExtensions
{
   public static bool IsPositiveInt32(this string input, out int value)
   {
      if(!int.TryParse(input, out value))
      {
         return false;
      }
      return value > 0;
   }
}

Usage will be

int orderQuantity;
Write("Number of Items: \t");
while(!ReadLine().IsPositiveInt32(out orderQuantity))
{
    Write("Error: Number of Items must be a positive, whole number: \t");
}

answered Oct 22, 2021 at 10:49

SomeBody

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Comments

UHM · Accepted Answer · 2021-10-22 09:45:22Z

0

while((!int.TryParse(ReadLine(), out orderQuantity)) || (orderQuantity <= 0))

answered Oct 22, 2021 at 9:45

UHM

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1 Comment

Gerhard Over a year ago

While this code snippet may solve the question, including an explanation will help people understand the reasons for your code suggestion.

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