الأسبوع 6 · الوحدة ١ الموضوع الثاني: مشروع محوّل الأعداد بلغة VB.NET: النافذة وأدوات التحكم والخصائص والكود · 3 دروس
3
Ready conversion functions: testing with known answers
ICT Secondary 3 Term 1, Unit 1, Topic 2: pp. 27–36
GoalUse Convert.ToInt32 and Convert.ToString, test the converter with answers you can check by hand, and name the inputs that make it stop.
New words
ready function · a function that comes with the language, so we do not write the steps ourselvestest case · an input together with the answer we expectexception · an error that stops the program when something unexpected happensTry … Catch · a block that tries some code and, if an exception happens, runs a safe message instead
Explanation
Two ready functions do the whole job. Convert.ToInt32(text, base) reads text written in base 2, 10 or 16 and gives a whole number. Convert.ToString(number, base) writes a number in a base. Hexadecimal comes out in lower case, so 255 becomes ff; adding .ToUpper() gives FF, the way we wrote it by hand in Weeks 4 and 5. How do we know that the program is right? We test it with known answers, numbers that we converted by hand: a test case is an input with the answer we expect. Then we also test bad inputs: an empty box, a wrong symbol such as 2 in a binary number, a minus sign in a binary or hexadecimal number, or a decimal number above 2,147,483,647. Each of these stops the program with an exception. Wrapping the two lines in Try … Catch lets the program show a polite message instead.
Function
Example
Gives
Convert.ToInt32(text, base)
Convert.ToInt32("3C", 16)
60 (a number)
Convert.ToString(number, base)
Convert.ToString(60, 16)
3c (text, hexadecimal in lower case)
Worked examples
Typed
From
To
Message
With .ToUpper()
111100
Binary
Decimal
60
60
60
Decimal
Hexadecimal
3c
3C
3C
Hexadecimal
Binary
111100
111100
255
Decimal
Hexadecimal
ff
FF
Four tests with answers that we can check by hand. They belong together: 60 is 111100 in binary and 3C in hexadecimal.
Typed
From
What happens
(empty box)
any
the program stops
12
Binary
the program stops: 2 is not a binary symbol
-101
Binary
the program stops: a minus sign is accepted only in decimal
3000000000
Decimal
the program stops: too big for a whole number of this kind
FFFFFFFF
Hexadecimal
no error, but the message is -1, a strange result
These bad inputs were run on the plain handler.
Vb
1Try
2 number = Convert.ToInt32(typed, fromBase)
3 MessageBox.Show(Convert.ToString(number, toBase).ToUpper())
4Catch ex As Exception
5 MessageBox.Show("Please type a valid number for the system you chose.")
6End Try
Output
D
Please type a valid number for the system you chose.
The same two lines inside Try … Catch, with capitals. The first run types 1101 with Binary to Hexadecimal; the second types 12 with Binary to Decimal.
Class activity
Make a test table on paper with five rows for the converter. Each row has a typed text, the From and To systems and the answer you expect from hand conversion. At least one row must be a bad input; say which message the Try … Catch version should show for it.
Practice
1
What does Convert.ToString(255, 16) give?
ff, in lower case; .ToUpper() turns it into FF.
2
The plain handler gets the text 12 with Binary ticked in From. What happens?
It stops with an error, because 2 is not a binary symbol.
3
Why does a test table use answers worked out by hand?
They give an independent answer to compare with the program’s message.
4
Name two inputs that stop the plain converter.
For example an empty box, a binary text with a 2 in it, or a decimal number above 2,147,483,647.
