How to implement a vending machine? - winforms

Question

For a school project I have been asked to create a virtual vending machine in a windows form application using visual basic. I am around 80% complete but I have a few of problems that I have researched multiple times but still cannot find help.
Firstly, on the vending machine when I purchase an item if the credit left over (change) is less than the products originals price it will display an error message saying you do not have enough credit to buy the product. How do I make it so it only displays that message if the credit is not enough to buy the product before-hand and not straight away after the purchase?
Secondly, how would I go about making a system for the credit left over after the purchase, as at the moment I have only got a credit return button which the user would use to extract the remaining credit left over after the purchase, instead of a system for giving out change.
Finally, how would I make it so the user is not allowed to enter text into a text-box as the vending machine uses two text box's, one to display credit and the other to display other general messages such as to clarify that you have bought a certain product or that you do not have enough credit to purchase a certain product.
Here is my code so far:
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
RichTextBox1.Text = "You Have Selected Galaxy Caramel, Please Insert 60p"
If credit >= 60 Then RichTextBox1.Text = "Thank you for purchasing a Galaxy Caramel bar"
credit = credit - 60
TextBox1.Text = credit
If credit < 60 Then RichTextBox1.Text = "You do not have enough credit to purchase a Galaxy Caramel Bar"
End Sub
Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
RichTextBox1.Text = "You Have Selected Cadburys Milk Chocolate, Please Insert 75p"
If credit >= 75 Then RichTextBox1.Text = "Thank you for purchasing a Cadburys Milk Chocolate Bar"
credit = credit - 75
TextBox1.Text = credit
If credit < 75 Then RichTextBox1.Text = "You do not have enough credit to purchase a Cadburys Milk Chocolate Bar"
End Sub
Private Sub Button3_Click(sender As System.Object, e As System.EventArgs) Handles Button3.Click
RichTextBox1.Text = "You Have Selected Bounty, Please Insert 70p"
If credit >= 70 Then RichTextBox1.Text = "Thank you for purchasing a Bounty bar"
credit = credit - 70
TextBox1.Text = credit
If credit < 70 Then RichTextBox1.Text = "You do not have enough credit to purchase a Bounty Bar"
End Sub
Private Sub Button4_Click(sender As System.Object, e As System.EventArgs) Handles Button4.Click
RichTextBox1.Text = "You Have Selected Yorkie, Please Insert 60p"
If credit >= 60 Then RichTextBox1.Text = "Thank you for purchasing a Yorkie bar"
credit = credit - 60
TextBox1.Text = credit
If credit < 60 Then RichTextBox1.Text = "You do not have enough credit to purchase a Yorkie Bar"
End Sub
Private Sub Button8_Click(sender As System.Object, e As System.EventArgs) Handles Button8.Click
RichTextBox1.Text = "You Have Selected Doritos Tangy Cheese, Please Insert 85p"
If credit >= 85 Then RichTextBox1.Text = "Thank you for purchasing Doritos Tangy Cheese Crisps"
credit = credit - 85
TextBox1.Text = credit
If credit < 85 Then RichTextBox1.Text = "You do not have enough credit purchase Doritos Tangy Cheese Crisps"
End Sub
Private Sub Button7_Click(sender As System.Object, e As System.EventArgs) Handles Button7.Click
RichTextBox1.Text = "You Have Selected Doritos Cool Original, Please Insert 75p"
If credit >= 75 Then RichTextBox1.Text = "Thank you for purchasing Doritos Cool Original Crisps"
credit = credit - 75
TextBox1.Text = credit
If credit < 75 Then RichTextBox1.Text = "You do not have enough credit to purchase Doritos Cool Original Crisps"
End Sub
Private Sub Button6_Click(sender As System.Object, e As System.EventArgs) Handles Button6.Click
RichTextBox1.Text = "You Have Selected Walkers Cheese & Onion, Please Insert 70p"
If credit >= 70 Then RichTextBox1.Text = "Thank you for purchasing Walkers Cheese & Onion Crisps"
credit = credit - 70
TextBox1.Text = credit
If credit < 70 Then RichTextBox1.Text = "You do not have enough credit to purchase Walkers Cheese & Onion Crisps"
End Sub
Private Sub Button5_Click(sender As System.Object, e As System.EventArgs) Handles Button5.Click
RichTextBox1.Text = "You Have Selected Mccoy's Cheddar, Please Insert 80p"
If credit >= 80 Then RichTextBox1.Text = "Thank you for purchasing Mccoy's Cheddar Crisps"
credit = credit - 80
TextBox1.Text = credit
If credit < 80 Then RichTextBox1.Text = "You do not have enough credit to purchase Mccoy's Cheddar Crisps"
End Sub
Private Sub Button9_Click(sender As System.Object, e As System.EventArgs) Handles Button9.Click
RichTextBox1.Text = "You Have Selected Pepsi Max, Please Insert £1.10"
If credit >= 110 Then RichTextBox1.Text = "Thank you for purchasing Pepsi Max"
credit = credit - 110
TextBox1.Text = credit
If credit < 110 Then RichTextBox1.Text = "You do not have enough credit to purchase Pepsi Max"
End Sub
Private Sub Button12_Click(sender As System.Object, e As System.EventArgs) Handles Button12.Click
RichTextBox1.Text = "You Have Selected Mountain Dew, Please Insert 99p"
If credit >= 99 Then RichTextBox1.Text = "Thank you for purchasing Mountain Dew"
credit = credit - 99
TextBox1.Text = credit
If credit < 99 Then RichTextBox1.Text = "You do not have enough credit to purchase Mountain Dew"
End Sub
Private Sub Button10_Click(sender As System.Object, e As System.EventArgs) Handles Button10.Click
RichTextBox1.Text = "You Have Selected Fanta, Please Insert £1.05"
If credit >= 105 Then RichTextBox1.Text = "Thank you for purchasing Fanta"
credit = credit - 105
TextBox1.Text = credit
If credit < 105 Then RichTextBox1.Text = "You do not have enough credit to purchase Fanta"
End Sub
Private Sub Button11_Click(sender As System.Object, e As System.EventArgs) Handles Button11.Click
RichTextBox1.Text = "You Have Selected Dr.Pepper, Please Insert £1.20"
If credit >= 120 Then RichTextBox1.Text = "Thank you for purchasing Dr.Pepper"
credit = credit - 120
TextBox1.Text = credit
If credit < 120 Then RichTextBox1.Text = "You do not have enough credit to purchase Dr.Pepper"
End Sub
Private Sub Button13_Click(sender As System.Object, e As System.EventArgs) Handles Button13.Click
RichTextBox1.Text = "You Have Selected Buxton Mineral Water, Please Insert 90p"
If credit >= 90 Then RichTextBox1.Text = "Thank you for purchasing Buxton Mineral Water"
credit = credit - 90
TextBox1.Text = credit
If credit < 90 Then RichTextBox1.Text = "You do not have enough credit to purchase Buxton Mineral Water"
End Sub
Private Sub Button14_Click(sender As System.Object, e As System.EventArgs) Handles Button14.Click
credit = credit + 1
TextBox1.Text = credit
End Sub
Private Sub Button21_Click(sender As System.Object, e As System.EventArgs) Handles Button21.Click
credit = credit + 2
TextBox1.Text = credit
End Sub
Private Sub Button20_Click(sender As System.Object, e As System.EventArgs) Handles Button20.Click
credit = credit + 5
TextBox1.Text = credit
End Sub
Private Sub Button19_Click(sender As System.Object, e As System.EventArgs) Handles Button19.Click
credit = credit + 10
TextBox1.Text = credit
End Sub
Private Sub Button18_Click(sender As System.Object, e As System.EventArgs) Handles Button18.Click
credit = credit + 20
TextBox1.Text = credit
End Sub
Private Sub Button17_Click(sender As System.Object, e As System.EventArgs) Handles Button17.Click
credit = credit + 50
TextBox1.Text = credit
End Sub
Private Sub Button16_Click(sender As System.Object, e As System.EventArgs) Handles Button16.Click
credit = credit + 100
TextBox1.Text = credit
End Sub
Private Sub Button15_Click(sender As System.Object, e As System.EventArgs) Handles Button15.Click
credit = credit + 200
TextBox1.Text = credit
End Sub
Private Sub Button22_Click(sender As System.Object, e As System.EventArgs) Handles Button22.Click
credit = 0
TextBox1.Text = credit
RichTextBox1.Text = "Credit has been returned"
End Sub
Private Sub TextBox1_TextChanged(sender As System.Object, e As System.EventArgs) Handles TextBox1.TextChanged
If credit < 0 Then credit = 0
TextBox1.Text = credit
If credit >= 1000 Then credit = 1000
TextBox1.Text = credit
If credit = 1000 Then RichTextBox1.Text = "Maximum credit of £10"
End Sub
Private Sub Button23_Click(sender As System.Object, e As System.EventArgs) Handles Button23.Click
RichTextBox1.Text = "You Have Selected Monster Munch, Please Insert 90p"
If credit >= 90 Then RichTextBox1.Text = "Thank you for purchasing Monster Munch Crisps"
credit = credit - 90
TextBox1.Text = credit
If credit < 90 Then RichTextBox1.Text = "You do not have enough credit to purchase Monster Munch Crisps"
End Sub
End Class

Answer 1

How do I make it so it only displays that message if the credit is not
enough to buy the product before-hand and not straight away after the
purchase?
First off, you should always use an End If to close off your If statements, even if they only have one line in them. Not doing so leads to subtle bugs where you think a "block" of code is running when the If statement is true, but only the first line is affected and the rest always run.
For your specific code, you're so close. By adding an Else block, an End If, and reformatting the code, it becomes as simple as:
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
RichTextBox1.Text = "You Have Selected Galaxy Caramel, Please Insert 60p"
If credit >= 60 Then
RichTextBox1.Text = "Thank you for purchasing a Galaxy Caramel bar"
credit = credit - 60
TextBox1.Text = credit
Else
RichTextBox1.Text = "You do not have enough credit to purchase a Galaxy Caramel Bar"
End If
End Sub
Obviously you need to change all of the items to follow the pattern above.
Secondly, how would I go about making a system for the credit left
over after the purchase, as at the moment I have only got a credit
return button which the user would use to extract the remaining credit
left over after the purchase, instead of a system for giving out
change.
Not sure what you mean here. Can you explain in more detail what you want to happen?
Finally, how would I make it so the user is not allowed to enter text
into a text-box as the vending machine uses two text box's, one to
display credit and the other to display other general messages such as
to clarify that you have bought a certain product or that you do not
have enough credit to purchase a certain product.
Both the TextBox and the RichTextBox have a ReadOnly() property you can set to True so that they cannot be changed by the user. But since they are using the buttons only, and don't need to type at all, why not use a different control like Labels instead?

Answer 2

You can use a State Machine for this scenario. Head First Design Patterns has a good example of Vending Machine using State Design pattern. Even though the book gives samples in Java, you can still work out the concepts.
For calculating change, I wrote this block below. It takes in available coin inventory and change required and returns a boolean to indicate if all change can be returned. The method will attempt to return the maximum change possible with the given inventory.
/// <summary>
/// This method attempts to calculate required change from a given coin inventory
/// </summary>
/// <param name="coinInventory">Key is the denomination of coin and value is the available number </param>
/// <param name="changeRequired">The change to count and return</param>
/// <param name="updatedCoinInventory"></param>
/// <param name="coinsToReturn"></param>
/// <returns>True if changeRequired could be calculated, False if coinsToReturn represent less than changeRequired </returns>
public bool GetChange(IDictionary<int, int> coinInventory, int changeRequired,
ref IDictionary<int, int> updatedCoinInventory,
ref IDictionary<int, int> coinsToReturn,
ref int amountDue)
{
int changeOutstanding = changeRequired;
updatedCoinInventory = coinInventory.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
coinsToReturn = new Dictionary<int, int>();
foreach (KeyValuePair<int, int> coin in coinInventory)
{
if (changeOutstanding == 0) break;
int numberOfAvailableCoins = coin.Value;
while (changeOutstanding >= coin.Key && numberOfAvailableCoins >= 1)
{
if (!coinsToReturn.ContainsKey(coin.Key))
{
coinsToReturn.Add(coin.Key, 1);
}
else
{
++ coinsToReturn[coin.Key];
}
changeOutstanding = changeOutstanding - coin.Key;
-- updatedCoinInventory[coin.Key];
-- numberOfAvailableCoins;
}
}
amountDue = changeOutstanding;
return changeOutstanding == 0;
}
}
Hope this helps.