Formatting a percentage
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  1. #1
    Join Date
    Feb 2013
    Posts
    23

    Formatting a percentage

    I have a form that I am working on, and have a quick question:

    The form has the user enter a value for Interest Rate, like 12 for 12%. I need it to format anything entered in that box so that it displays as 12.00%. The problem is, if I just use something like txtInterestRate.Text = interestRateYearly.ToString ("p"); and the user enters 12 it displays 1,200.00%.

    I am trying to use String.Format instead, and this is what I have so far:
    Code:
    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;
    
    namespace FutureValue
    {
        // This is the starting point for exercise 9-3 from
        // "Murach's C# 2010" by Joel Murach
        // (c) 2010 by Mike Murach & Associates, Inc. 
        // www.murach.com
    
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }
    
            private void btnCalculate_Click(object sender, System.EventArgs e)
            {
                try
                {
                    if (DataIsValid())
                    {
                        // TODO: use Strip method here, where necessary
                        decimal monthlyInvestment = Convert.ToDecimal(Strip(txtMonthlyInvestment.Text));
                        decimal interestRateYearly =
                            Convert.ToDecimal(Strip(txtInterestRate.Text));
                        int years =
                            Convert.ToInt32(Strip(txtYears.Text));
    
                        int months = years * 12;
                        decimal interestRateMonthly = interestRateYearly / 12 / 100;
                        decimal futureValue = CalculateFutureValue(
                            monthlyInvestment, interestRateMonthly, months);
    
                        txtMonthlyInvestment.Text = monthlyInvestment.ToString("c");
                        string.Format ("Value: {0:P2}.", txtInterestRate.Text);
                        txtFutureValue.Text = futureValue.ToString("c");
                        txtMonthlyInvestment.Focus();
                    }
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message + "\n\n" +
                        ex.GetType().ToString() + "\n" +
                        ex.StackTrace, "Exception");
                }
            }
    
            public bool DataIsValid()
            {
                return
                    // Validate the Monthly Investment text box
                    IsPresent(txtMonthlyInvestment, "Monthly Investment") &&
                    IsDecimal(txtMonthlyInvestment, "Monthly Investment") &&
                    IsWithinRange(txtMonthlyInvestment, "Monthly Investment", 1, 1000) &&
    
                    // Validate the Yearly Interest Rate text box
                    IsPresent(txtInterestRate, "Yearly Interest Rate") &&
                    IsDecimal(txtInterestRate, "Yearly Interest Rate") &&
                    IsWithinRange(txtInterestRate, "Yearly Interest Rate", 1, 20) &&
    
                    // Validate the Number of Years text box
                    IsPresent(txtYears, "Number of Years") &&
                    IsInt32(txtYears, "Number of Years") &&
                    IsWithinRange(txtYears, "Number of Years", 1, 40);
            }
    
            public bool IsPresent(TextBox textBox, string name)
            {
                if (textBox.Text == "")
                {
                    MessageBox.Show(name + " is a required field.", "Entry Error");
                    textBox.Focus();
                    return false;
                }
                return true;
            }
    
            // TODO: improve this IsDecimal method
            public bool IsDecimal(TextBox textBox, string name)
            {
                string s = textBox.Text;
                int decimalCount = 0;
                bool validDecimal = true;
                foreach (char c in s)
                {
                    if (!(
                        c == '0' || c == '1' || c == '2' || c == '3' ||
                        c == '4' || c == '5' || c == '6' || c == '7' ||
                        c == '8' || c == '9' || c == '.' || c == '$' ||
                        c == '%' || c == ',' || c == ' '
                        ))
                    {
                        validDecimal = false;
                        break;
                    }
                    if (c == '.')
                    {
                        decimalCount++;
                    }
                }
                if (validDecimal && decimalCount <= 1)
                {
                    return true;
                }
                else
                {
                    MessageBox.Show(name + " must be a decimal value.", "Entry Error");
                    textBox.Focus();
                    return false;
                }
            }
    
            // TODO: add Strip method
            public string Strip(string s)
            {
                foreach (char c in s)
                {
                    if (c == '$' || c == '%' || c == ',' || c == ' ')
                    {
                        int i = s.IndexOf(c);
                        s = s.Remove(i, 1);
                    }
                }
                return s;
            }
    
            // TODO: improve this IsInt32 method
            public bool IsInt32(TextBox textBox, string name)
            {
                string s = textBox.Text;
                bool validDecimal = true;
                foreach (char c in s)
                {
                    if (!(
                       c == '0' || c == '1' || c == '2' || c == '3' ||
                       c == '4' || c == '5' || c == '6' || c == '7' ||
                       c == '8' || c == '9' || c == '$' || c == '%' ||
                       c == ',' || c == ' '
                       ))
                    {
                        validDecimal = false;
                        break;
                    }
                }
                if (validDecimal == true)
                {
                    return true;
                } 
                else
                {
                    MessageBox.Show(name + " must be an integer.", "Entry Error");
                    textBox.Focus();
                    return false;
                }
            }
            public bool IsWithinRange(TextBox textBox, string name,
                decimal min, decimal max)
            {
                // TODO: add Strip method where necessary
                string s = textBox.Text;
                decimal number = Convert.ToDecimal(Strip(s));
                if (number < min || number > max)
                {
                    MessageBox.Show(name + " must be between " + min +
                        " and " + max + ".", "Entry Error");
                    textBox.Focus();
                    return false;
                }
                return true;
            }
    
            private decimal CalculateFutureValue(decimal monthlyInvestment,
                decimal interestRateMonthly, int months)
            {
                decimal futureValue = 0m;
                for (int i = 0; i < months; i++)
                {
                    futureValue = (futureValue + monthlyInvestment)
                        * (1 + interestRateMonthly);
                }
                return futureValue;
            }
    
            private void btnExit_Click(object sender, System.EventArgs e)
            {
                this.Close();
            }
    
        }
    }
    This doesn't change the value at all, so I know I am doing something wrong. My book is not helping so far. Any ideas what I am doing wrong? The book example is:

    string balance1 = String.Format("{0:P2}", 12);

    This won't work for me as the interest rate is a decimal and not a string. I appreciate any help on this.

  2. #2
    Join Date
    Feb 2013
    Posts
    23

    Re: Formatting a percentage

    Tried this:

    string txtInterestRate.Text = String.Format ("{0:P2}.00%", txtInterestRate.Text

    It works if I enter something like 12, but if someone enters 12.5 it will display 12.5.00%
    Still working it out, thanks for looking.

  3. #3
    Join Date
    Jan 2006
    Location
    Chicago, IL
    Posts
    14,986

    Re: Formatting a percentage

    Create a new variable TYPE DOUBLE and store the value there from the textbox. Then, multiply or divide by 100 to get the desired result.
    Then, use your FORMAT to get it to print.
    David

    CodeGuru Article: Bound Controls are Evil-VB6
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    CodeGuru Reviewer
    2006 Dell CSP
    2006, 2007 & 2008 MVP Visual Basic
    If your question has been answered satisfactorily, and it has been helpful, then, please, Rate this Post!

  4. #4
    Join Date
    Feb 2013
    Posts
    23

    Re: Formatting a percentage

    I did get this working, thanks for your advice. Here's how I did it in the end:

    Code:
    decimal interestConversion = interestRateMonthly * 12;
    txtInterestRate.Text = string.Format("{0:P2}", interestConversion);

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