Download Chapter 3 Loops And Decisions

`Lesson 02: Control Statements (If, For, Do etc)
The If Statement:
o Edit your source code file as follows, build and execute.
using System;
class test
{
public static void Main()
{
int x = 2;
if (x > 1)
Console.WriteLine("yes");
Console.ReadLine();
}
}
ie if x is bigger
than 1 which
of course it is
since it's 2, do
the following.
o Modify the code as follows:
int x = 2
;
if ( x == 2)
Console.WriteLine("yes");
Note that when we
are assigning we
use one equal sign
but when we are
testing (comparing)
we use two equal
signs.
If we wish more than one thing to be performed after an if then we must enclose them in braces
as follows (This is generally true for a group of C# statements – enclose them in braces.)
int x = 2
;
if (x == 2)
{
Console.WriteLine("yes");
Console.WriteLine("of course");
Enclosing braces.
These ensure that all
of the code contained
within these braces is
executed if the
condition (x = 2)is
true – which it is.
}
The opening and
closing braces
delimit a" block".
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Type if and then immediately
TAB TAB to the snippet - with
enclosing braces etc.
1
Else
o Try this:
using System;
class test
{
public static void Main()
{
int x = 2
Shortcut: type
if and then tab, tab
else and then tab, tab.
;
if (x == 2)
Console.WriteLine("yes");
else
Console.WriteLine("no") ;
Console.ReadLine();
}
}
o Change
if (x == 2)
to
if (x == 1)
and the else should be executed.
;
More than one if statement has the effect of AND
{
int x = 2
if (x == 2)
if (x > 1)
For AND, both
these conditions
must be satisfied.
;
Using else if has the effect of (exclusive) OR:
public static void Main()
{
int x = 2;
if (x > 2)
Console.WriteLine("x>2");
else if (x > 1)
Console.WriteLine("x>1");
else if (x > 0)
Console.WriteLine("x>0");
Console.ReadLine();
}
else if is like an if.
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Note also that if one else if (or
an if)is encountered which is true,
all other else if‘s are then not
even considered.
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Problems in class.
1. We have previously written a program to swap two numbers.
int i = 3;
int j = 2;
o Modify that code to swap the numbers if the first number is bigger than the second number –
which of course it is in the above case
We want the smaller number printed first.
ie if we originally had instead:
int i = 2;
int j = 3;
then your code should not swap them.
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Logical Operators
Page 99-100 C# 21 days.
OR:
public class test
{
static void Main()
{
int i = 2;
if (i == 1 || i == 2)
Console.WriteLine("yes");
Console.ReadLine();
or
i can be
either 1 or 2.
}
}
AND:
using System;
public class test
{
static void Main()
{
int i = 1, j = 2;
if (i == 1 && j == 2)
Console.WriteLine("yes");
Console.ReadLine();
and
i must be 1 and
j must be 2.
}
}
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The Switch (Looks good but perhaps only good for “a menu system”.C# in 21 Days page 123
break; is compulsory
…is an alternative to using else if.
using System;
class test
{
public static void Main()
{
int x = 5 ;
Type switch
and then
immediately
TAB TAB to
get the snippet
- with
enclosing
braces etc.
switch (x)
{
case 4:
Console.WriteLine("four")
break;
case 5:
Console.WriteLine("five")
break;
case 6:
Console.WriteLine("six")
break;
}
Console.ReadLine();
;
at the end of every case
block else it will “fall
through”.
Thus if one case is
found to be true, any
following case
statements are not
considered.
(This behaviour is the
same as else if.)
;
;
}
}
o Try this with i = 4 and then i = 5.
case 4:
case 5:
Console.WriteLine("four/five")
break;
case 4:
case 5:
break ;
; catch both 4 or 5.
will
If no case expression matches the switch value, then control is transferred to the statement(s)
that follow the optional default label. If there is no default label, control is transferred outside the
switch.
default:
Console.WriteLine("x doesnt match any value");
break;
Note when using a switch eg switch (x) we can only test one condition c.f. the if which can
test more than one
eg see page 7 : if (i == 1 && j == 2)
This is something of a disadvantage when using switch.
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True and False
Console.WriteLine(1==2)
;
Casting of bool
Casting to and from bool is not permitted
1. From int to bool:
int i = 1
b = (bool) i;
is not allowed. (It wont even compile)
Nor is
2. From bool to int:
bool b = true;
int i = (int) b;
true has value of 1 and
false has value of 0.
We can use however
int i = Convert.ToInt32(b);
Console.WriteLine(i);
The for Loop
C# in 21 Days page (bottom of) p134
using System;
class test
{
public static void Main()
{
int i
;
for (i=0; i<4; i++)
Console.WriteLine(i)
;
Console.ReadLine()
}
The int declaration could alternatively
be placed here.
for (int i=0;i<4;i++)
Note that in this case, the i will only be
“known” inside the loop.
;
}
We could also have:
for (i=4; i>=0 ; i-- )
for (i=0; i<=4 ; i+=2)
counts backwards
increments by 2
Type for and then immediately
TAB TAB to get the snippet with enclosing braces etc.
To execute a series of statements in a for loop, use enclosing braces (as we did for the if).
for (i=0;i<4;i++)
{
Console.WriteLine(i)
;
Console.WriteLine("looping")
}
Console.ReadLine()
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;
;
6
Ex 2. Write separate programs that will produce output like this below – using a nested loop ie a
loop inside a loop.
Hint 1: To print 4 numbers
across using a space in between
use
Console.Write("{0} ", i);
space
Hint 2: Use a loop inside a loop.
and this..
Hint : The inside loop counter
“upper limit” will depend on the
outer loop.
and this..
and this..
continue
using System;
class test
{
public static void Main()
{
int i
;
for (i=0; i<4; i++)
{
if(i==2) continue
;
Console.WriteLine(i) ;
}
Console.ReadLine()
;
}
}
continue causes
control to pass to the
end of this loop
ignoring statement/s in
between - if there are
any - there is only one
here - and then back
to the next i which will
be 3.
break (which we have previously met with switch) causes termination of the loop.
public static void Main()
{
int i
for (i=0; i<4; i++)
{
if(i==2) break
Console.WriteLine(i)
}
Console.ReadLine()
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;
;
;
;
7
while
C# in 21 Days page 128
static void Main()
{
int i = 0;
while (i < 4)
{
Console.WriteLine(i);
i++;
}
Console.ReadLine();
}
i < 4 evaluates to
either True or False.
If False ie if i = 5,
it will stop.
A while loop can also be terminated with a break, goto or a return.
Type while and then
immediately TAB TAB
to the snippet - with
enclosing braces etc.
do while
Type do and then
immediately TAB TAB
to the snippet - with
enclosing braces etc.
static void Main()
{
int i = 0;
do
{
Console.WriteLine(i);
i++;
}
while (i < 4);
Console.ReadLine();
}
The while and the do….while are almost identical. The only difference is that the do
while evaluates the condition at the end ie it always goes through at least once and
doesn’t find out if it shouldn’t have too late.
goto transfers control unconditionally. It is usually used with a label.
public static void Main()
{
int i;
for (i = 0; i < 4; i++)
{
if (i == 2) goto mylabel ;
Console.WriteLine(i)
mylabel:
}
Console.ReadLine()
}
;
A label is used
;
with a colon.
;
Further reference: The Microsoft Help online.
http://msdn.microsoft.com/en-us/library/370s1zax.aspx
(Consult this reference generally.)
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When do we use a for loop and when do we use a while? When we know in advance how
many loops are required, use a for but when we are waiting for something to happen use a
while.
We have previously seen that a for loops for a pre-determined number of times whereas the
while is used when we don’t know when the condition to stop will be encountered.
Run-Time is when the program is actually running.
eg
Design-Time is when you are actually writing the code.
using System;
public class test
{
static void Main()
{
Empty string.
string st = "";
while (st != "s")
{
st = Console.ReadLine();
Console.WriteLine(st);
}
}
Not equal.
The program loops and echoes the
letter to the screen until an “s” is
pressed. (And then Enter is pressed.)
}
The Conditional Operator
(Looks good but not actually used much.)
using System;
public class test
{
static void Main()
{
int i = 1
int j
If the condition is true, which it is in this case,
the first alternative (the 3) is assigned to j. If
false, the 4 would be assigned to j.
;
;
j = (i < 2) ? 3 : 4 ;
Console.WriteLine(j);
Console.ReadLine()
;
}
}
Read to the end of C# in 21 Days Sams Day 4.
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Debugging
o
o Continue
to press F11.
You will see
the program
looping
around the
for loop.
Click Build, Start Debug, Step Into (or press F11).
o If this window is not visible, choose
Debug Windows, Locals
o Note the values of the variables in the Locals
window.
o
By holding the mouse steady on a variable, the value will appear.
o To set a break point, click in the margin of the line.
This is only possible whilst debugging! To remove it
click it again to toggle it off. If the Run button is
pressed, the program will run and stop at this line.
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Binary Numbers
How many ice-creams can you order with one hand?
5
right ?
wrong 32!
Consider that a finger (and thumb) can be in 2 states raised or not-raised and that we start with
the thumb as 1 and multiply by 2 for the other fingers as shown
If the thumb and 1st finger were raised this could represent 3 (1+2) as shown below.
If all fingers were raised, this would represent 31 (1+2+4+8+16).
If no fingers were raised, this would of course represent 0- this would indeed have to be a valid
ice-cream order!.
Therefore the total no. of orders possible is 32.
We could indicate 5 as follows
We could further represent a finger raised by 1 and a finger not-raised by 0.
5 would then be represented by:
00101
- this is called binary
(actually the leading zeros can be omitted.)
The respective 1’s and 0‘s are called bits
(8 bits makes one byte)
Exercise
101
is the binary representation of ………
1101 is the binary representation of ………
1000 is the binary representation of ………
7
12
23
is the decimal representation of ……………
is the decimal representation of ……………
is the decimal representation of ……………
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Hexadecimal
Whereas decimal numbers are to base 10, hexadecimal numbers are to base 16.
Numbers in hexadecimal from 1 to 9 are the same – 0 to 9. But numbers from 10 to 15 are given
the letters A to F
Decimal
1
2
.
.
9
10
11
12
13
14
15
Hexadecimal
1
2
.
.
9
A
B
C
D
E
F
Hex numbers in C# are preceded with 0x eg 0xAD23F3.
o Try this
using System;
class test
{
public static void Main()
{
int h
;
int h
;
h = 12
;
Console.WriteLine ("{0:X}",h) ;
h = 0xC;
Console.WriteLine(h)
;
Console.ReadLine()
}
;
}
will convert decimal to hex.
page 392
To evaluate to 2 digit hex number eg 2A
Whereas of course decimals increase by a factor of 10 from right to left, hex placeholders
increase by a factor of 16 from right to left.
…
256
16
1
So hex 2A can be evaluated:
…
256
16
2
1
A
i.e. 2 times 16 plus 10 times 1 i.e. decimal 42.
Exercise: what is the decimal equivalent of the hex number 22A ?
What algorithm would you use to convert a number from decimal to hex e.g. convert decimal 42
to hex.
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Why use hex? Hexadecimal is a shorthand way of writing binary
For example, decimal 42 is the equivalent of the hex 2A.
o Write the 2 and the A separately in binary:
2
A
8
4
2
1
8
4
2
1
…
…
…
…
…
…
…
…
o Now write 42 as binary:
42
128
64
32
16
8
4
2
1
…
…
…
…
…
…
…
…
They are the same!
This is always true – since all of the numbers of the higher hex digit are 16 times those of the
lower.
Exercise 2
1. Write the code for a switch statement that switches on the variable name. If the
name is Robert, print a message that says Hi Bob. If the name is Richard,
print a message that says Hi Rich. If the name is Barbara, print a message that
says Hi Barb. If the name is Kalee, print a message that says Kalee!. On any
other name, print a message that says Hi x, where x is the person's name.
2. Write a program to roll a six-sided die 100 times. Print the number of times each of
the sides of the die was rolled.
You may wish to use:
Random rnd = new Random(); // This only needs to be done once.
int n = rnd.Next(1, 7);
to generate random numbers between 1 and 6.
Page 144 C# in 21 Days
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3. Write a program that calculates how much money you’ll end up with if you invest an
amount of money at a fixed interest rate, compounded yearly.
Have the user furnish the initial amount, the number of years, and the yearly interest rate in
percent. Some interaction with the program might look like this:
Enter initial amount: 3000
Enter number of years: 10
Enter interest rate (percent per year): 5.5
At the end of 10 years, you will have 5124.43 dollars. At the end of the first year you have 3000 +
(3000 * 0.055), which is 3165.
At the end of the second year you have 3165 + (3165 * 0.055), which is 3339.08.
Do this as many times as there are years. A for loop makes the calculation easy.
You may alternatively want to use the
compound interest formula. A(1+r)n.
ie A * Math.Pow(1+r,n). Don’t forget
that pow is expecting a Double.
Hint: When developing your program which requires keyboard input eg
double amt ;
ConsoleReadLine(amt);
then “hard code” the input temporarily eg amt= 10.00; temporarily until you program works .
Then finally reinstate ConsoleReadLine(amt);
(When sending course work to me pls also use the “hard-coded” version!)
4. using the formula for the normal curve from the previous chapter, plot the normal curve using
values from -3 to 3 with an increment of say 0.5
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14
5. “Plot” these values:
Here is the solution for a sine curve! Don’t worry if you can’t understand all of it for the moment!
Adapt it to produce your normal curve (see below).
o This is a function (not covered yet.)
The value val (eg val = 0) is passed to it
using System;
class SineWaveToConsole
{
private static string spaces(double val)
{
string SpaceString = new String(' ', ((int)(val * 10.0)) + 10);
return SpaceString;
}
o This produces a string (not covered
[STAThread]
yet.) It produces a string of concatenated
static void Main(string[] args)
spaces eg 5 spaces -> “ “
{
for (float i = 0; i < Math.PI * 2.0F; i += 0.3F)
{
Console.WriteLine("The sine of {0,10:F} = {1,-10:F6}" +
spaces(Math.Sin(i)) + "*", i, Math.Sin(i));
}
Console.ReadLine();
}
The 10 says
The - says “right
}
a “field width
justified”. The 6
o This “calls” the spaces function
above to produce a series of spaces
proportional to the value of
Math.Sin(i).
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of 10.
The F says
decimal
says 6 decimal
places.
15
A More Difficult Compound Interest Problem
(Optional for FM students)
£100 is invested at 10% interest for 1 year. How much will we have at the end of the year?
Of course £110. ie 100 * (1 + .10)
Lets now say that we get 5% interest every half year. How much will we have at the end of the
year?
Ans: 100 * (1 + .05) after 6 months and then THIS amount will accrue interest, so in total:
100 * (1 + .05) * (1 + 05) which gives 110.25 which of course is more than the £110.
This can be generalized to:
100 * (1 + .10/2) * (1 + .10/2) = 100 * (1 + .10/2) 2.
Lets now say that we get 2.5% (1%/4) interest every quarter. How much will we have at the end
of the year?
Ans: 100 * (1 + .025) after 3 months and then this amount will accrue interest for the next 3
months and then THIS amount will ….. etc.
So at the end of the year we will have:
100 * (1 + .025) * (1 +.025) * (1 +.025) * (1 +.025) which gives 110.38 which of course is even
more than the £110.
This can be generalized to:
100 * (1 + .10/4) * (1 + .10/4) * (1 + .10/4) * (1 + .10/4) = 100 * (1 + .10/4) 4
To further generalize: If we are paid the interest n times per year, the total amount that we will
have at the end of the year will be:
A = 100 * (1 + .10/n)
n
Even if you don’t quite understand
how we got this formula just use it!
Write a program that will give you the total amount at the end of the year for a variable number of
intervals.
eg for 1 to 10 intervals , the output should be:
Here’s the $64,000 question:
If we kept dividing up the interval
FOREVER ie an infinite number of payoffs,
how much will we have Is this possible?
Continually compounding interest!
For 10 intervals, the end of the year we will
have £105.114.
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