computers, variables and types engineering 1d04, teaching session 2
TRANSCRIPT
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Computers, Variables and Types
Engineering 1D04, Teaching Session 2
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© Copyright 2006 David Das, Ryan Lortie, Alan Wassyng 2
Typical Computer
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An Abstract View of Computers
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An Abstract View of Computers
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Programming Languages Algorithms are communicated to the computer
using a programming language Evolution of programming languages:
Machine language: 01000111000110100 Assembly code: mov, and, push, add, jmp, etc High level languages: FORTRAN, Pascal, Basic, C,
C++, Java, Visual C#, Visual Basic, Haskell, Ocaml, …
Conversion of high level language to machine code is via a compiler
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Memory “boxes”
Abstract View of Memory
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Introduction to Variables
When we create algorithms that solve problems, we typically model the problem we are solving - we create an abstraction of the real problem and then use properties of that abstract model to get a solution that (hopefully) solves the original problem.
We often create mathematical variables to represent physical entities.
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Mathematical Variablesvelocity of car is vstarts at velocity v0
t represents timeacceleration of car is a
t
v = v0 + at
so, at this point in time, what is the car’s velocity?
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Introduction to Program Variables
Computer programs implement algorithms and we need to calculate values that represent physical entities or simply data items. We therefore need to manipulate and store data.
A program variable is a named memory location that can be used to store data.
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myVariable
myStringBirthday65
Introduction to Program Variables
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Introduction to Program Variables
Variables can have different forms depending on the type of data you wish to store in them. Int variables store integer numbers. Float variables (double) store real numbers. Char variables store a single character. String variables store text (multiple
characters). Bool variables store true or false.
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int myInteger; double myDouble;
Introduction to Program Variables
Variables must be declared before they can be used.
Declaring a variable assigns a name to a previously unused memory location.
program segments will beshown in these “page” templates
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int myInteger; double myDouble;
myInteger
Introduction to Program Variables
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myInteger
int myInteger; double myDouble;
myDouble
Introduction to Program Variables
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int myInteger, myOtherInteger, myThirdInt;double myDouble;
Introduction to Program Variables
Multiple variables of the same type can be declared at the same time by separating them with a comma.
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Variables can be assigned a value with the assignment (=) operator.
myInteger = 5;myDouble = 4.5;myInteger = 25;
Variable Constant / Literal
Introduction to Program Variables
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myInteger
myDouble
myInteger = 5;myDouble = 4.5;myInteger = 25;se
quen
ce
5
Introduction to Program Variables
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myInteger
myDouble
sequ
ence
5
myInteger = 5;myDouble = 4.5;myInteger = 25;
4.5
Introduction to Program Variables
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myInteger
myDouble
sequ
ence
4.5
myInteger = 5;myDouble = 4.5;myInteger = 25;
25
The value at any location can change from time to time.
Introduction to Program Variables
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Introduction to Program Variables
Remember the definition of a variable: A named memory location used to store data.
The assignment operator (=) can be thought of as a left arrow, placing a value into a memory location. myInteger 5; (put 5 in the myInteger box)
But we don’t have on our keyboard, so we use the equals sign instead. myInteger = 5;
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Introduction to Program Variables
In our example, myInteger holds different values at different times. Sequence is important here! The current contents of a variable is what was
most recently assigned to it. Assignment always occurs from right to left
myInteger = 5;• Storing 5 into “myInteger” makes sense.
5 = myInteger;• Storing “myInteger” into the value of 5 makes no sense.
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myInteger = myOtherInteger;
What does this do?
Introduction to Program Variables
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myInteger = myOtherInteger;
Introduction to Program Variables
myInteger
123123
myOtherInteger
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myInteger = 20;
Introduction to Program Variables
When not being assigned to, a variable stands in for the value that it contains.
In this case, if myOtherInteger contained 20 then this statement would be equivalent to the following:
myInteger = myOtherInteger;
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myInteger = myInteger + 5;
What does this do?
Introduction to Program Variables
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Introduction to Program Variables
This adds 5 to the current contents of myInteger and stores the result back into myInteger. If myInteger used to contain 20, it now contains
25.
Repeating this statement will result in repeatedly adding 5 to myInteger.
myInteger = myInteger + 5;
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double a, b, c, d, e, f, g;
a = 4; b = 25; c = 30; d = 60; e = 10;
f = b;f = f + c;f = f + d;f = f + e;g = f / a;
What does this program do?
Example Program 1
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Example Program 1
It’s not intuitively obvious what this program does.
It can be improved easily by giving the variables useful names.
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double b, c, d, e, sum, average;int numTerms;
numTerms = 4;b = 25; c = 30; d = 60; e = 10;
sum = b;sum = sum + c;sum = sum + d;sum = sum + e;average = sum / numTerms;
Example Program 1
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Program Variables
There are a number of variable naming conventions.
In 1D04, the following convention will be adopted: All first words are lowercase. Subsequent words have their first letter
capitalized.• fred, theVariable, bobLovesMe
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Program Variables
Variables cannot start with a number.
Extremely short or exceptionally long variable names can be hard to work with. Use an appropriate length.
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Program Variables
There is still room for improvement in the code that calculates the average. What can you do to make it “better”?
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Program Variables
There is still room for improvement in the code that calculates the average. In this case, we can simply add the values of b, c, d and e together without doing it sequentially.
While we do this we need to bear in mind that: Rules for the mathematical order of operations
apply to operations on variables. We should use brackets anywhere our intention
might be unclear.
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double b, c, d, e, average;int numTerms;
b = 25; c = 30; d = 60; e = 10;numTerms = 4;
average = (b+c+d+e) / numTerms;
“Better” Version
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Program Variables
Lastly, variables can be initialized when they are declared.
Does that mean those variables will behave like constants?
No Their values can still be changed later in the
program.
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double b = 25, c = 30, d = 60, e = 10, average;
int numTerms = 4;
average = (b+c+d+e) / numTerms;
Initialized Version
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Program Variables
Notice the use of quotes in characters and strings.
The type of box that a literal is given is defined by its quotes.
Single quotes get a character box. ‘x’ ‘#’ Double quotes get a string box. “x” “1x3@” Double quotes are not single quotes typed
twice.
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char name1, name2, name3;string s1, s2;
name1 = 'B';name2 = 'abc';name3 = "abc";
s1 = "B";
s2 = "abc";
Examples:
66
name1 name2
works fine
s1
s2
box not big enough
66
97 98 99
name3
box not the right kind
works fine
works fine
Program Variables
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Booleans can contain only the values true or false.
They will become more useful as the course progresses.
Program Variables
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Types
The type of a variable provides very important information to the programmer and to the compiler
The number of students in a tutorial is an integer, not a real number or double – you cannot have 23.45 students in a class!
Some operations are only valid over certain types
Type conversions when mixing types
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Remember “Click Me” from lab 1?
void …(…){ textBox1.Text = "You Clicked 'Click Me'";}
Type Conversion
You Clicked 'Click Me'
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void …(…){ int myInteger = 10; textBox1.Text = myInteger;}
Type Conversion
What happens when we run the method with an integer?
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Type Conversion
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Type Conversion
The variables are in different size/kind of boxes.
We need to do an explicit conversion from integer to string.
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void …(…){ int myInteger = 10; textBox1.Text = Convert.ToString(myInteger);}
Example Program 2
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Type Conversion
The Convert methods are found in the System library.
In order to use them as demonstrated in this tutorial the following line must be at the beginning of your source file: using System;
Visual Studio does this by default for you.
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Example Program 3
It’s quite natural to want a numerical type along with a descriptive string in a textbox.
How do we do that?
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private void ClickMe_Click(object sender, EventArgs e){ int sum;
sum = 35 + 7; textBox1.Text = "The Answer to Life, The Universe, and Everything is ??";}
We want the value of sum!
Example Program 3
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private void ClickMe_Click(object sender, EventArgs e){ int sum;
sum = 35 + 7; textBox1.Text = "The Answer to Life, The Universe, and Everything is sum";}
Example Program 3
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private void button1_Click(object sender, EventArgs e){ int sum;
sum = 35 + 7; textBox1.Text = "The Answer to Life, The Universe, and Everything is sum";}
Example Program 3
Click Me
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Example Program 3
We need to convert sum into a string and concatenate it with “The Answer to …”.
Convert.ToString(intValue) produces a string consisting of characters representing the value of intValue.
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private void ClickMe_Click(object sender, EventArgs e){ int sum;
sum = 35 + 7; textBox1.Text = "The Answer to Life, The Universe, and Everything is " +
Convert.ToString(sum);}
Example Program 3
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private void button1_Click(object sender, EventArgs e){ int sum;
sum = 35 + 7; textBox1.Text = "The Answer to Life, The Universe, and Everything is " +
Convert.ToString(sum);}
Example Program 3
Click Me
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Example Program 3
Actually, this is a special case! We did not need to convert the integer to a string in this particular case.
Why? Because the concatenation operator (+) implicitly converts numerical values to strings if necessary.
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private void ClickMe_Click(object sender, EventArgs e){ int sum;
sum = 35 + 7; textBox1.Text = "The Answer to Life, The Universe, and Everything is "+ sum;}
Example Program 3
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Concatenation
Multiple things can be concatenated at once.
All must be either strings or implicitly convertible to strings by the concatenation operator.
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string myVeryLongString;int myAge = 22; string myBirthday = "January 3, 1984";
myVeryLongString = " I was born on " + myBirthday + ". I am now " + myAge;
Example Program 4
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string myVeryLongString;int myAge = 22; string myBirthday = "January 3, 1984";
myVeryLongString = " I was born on " + myBirthday + ". I am now " + myAge;
String
Variable of type stringString Integer which is implicitly
convertible to a string.
Example Program 4
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Inputting Data
Suppose we want to change the inputs used to calculate the answer to Life, the Universe, and Everything.
We could read two numbers from two textboxes, add them, and then write the answer to a textbox.
Click Me
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private void ClickMe_Click(object sender, EventArgs e)
{ int sum;
sum = inputBox1.Text + inputBox2.Text; outputBox.Text = "The Answer to Life, The Universe,
and Everything is " + sum; }
Example Program 5
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Inputting Data
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Inputting Data
Textboxes deal only with Strings.
We need our inputs to be integers.
An explicit conversion is required.
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private void ClickMe_Click(object sender, EventArgs e)
{ int sum;
sum = Convert.ToInt32(inputBox1.Text) + Convert.ToInt32(inputBox2.Text); outputBox.Text = "The Answer to Life, The Universe,
and Everything is " + sum; }
Example Program 6
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private void ClickMe_Click(object sender, EventArgs e)
{ int sum;
sum = Convert.ToInt32(inputBox1.Text) + Convert.ToInt32(inputBox2.Text); outputBox.Text = "The Answer to Life, The Universe,
and Everything is " + sum; }
Example Program 6
Click Me
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Points to Ponder
Program variables Variable typing Type conversions