The control structures are used to give to give conditional
decisional statements to the computer. That is, it tells the computer what to
do at a particular time.
TYPES OF CONTROL STATEMENTS
1.
If
statement
2.
If else statement
3.
Nested
If statement
4.
Switch/case
statement
5.
For loop
statement
6.
While
loop statement
7.
Do while
statement
8.
Go to
statement
9.
Break
statement
10.
Continue
statement
11.
Return
statement
1. If
statement
The If statement is a conditional statement in
which a condition have to be meant. If the condition is not meant, there is no
result.
The syntax of the If statement is:
If (condition)
{
Statement;
}
Let us consider the program below;
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace
myfirstprogram
{
class
Program
{
static void Main(string[] args)
{
double y, result;
Console.WriteLine("enter
value of y");
y = Convert.ToDouble(Console.ReadLine());
if (y != 5)
{
result = 10 *
y;
Console.WriteLine("result="
+ result);
}
}
}
}
From the program we can see that whenever a user
enters a number that is not 5, result will be display but if a user enters 5
nothing will happen.
2. If
… else statement
The If … else statement is a conditional
statement in which a condition is meant and if not another alternative is
provided.
The syntax of the If … else statement is:
If (condition)
{
Statement;
}
Else
{
Statement;
}
In a case where you have more than one condition
you can also apply logical AND (&&) and OR (||).
Consider the program below on the use of If …
else statement;
using System;
using System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace
myfirstprogram
{
class
Program
{
static void Main(string[] args)
{
double y, result;
Console.WriteLine("enter
value of y");
y = Convert.ToDouble(Console.ReadLine());
if (y != 5)
{
result = 10 *
y;
Console.WriteLine("result="
+ result);
}
else
{
result = 0 * y;
Console.WriteLine("result="
+ result);
}
}
}
}
From the program we can see that whenever a user
enters a number that is not 5, the statement after the if statement will react
but the user enters 5 the output result will be 0.
3. Nested
If statement
A situation where we have more than one If …
else statement in a program in a program is called the nested if statement.
The syntax of the nested if statement is:
If (condition1)
{
Statement1;
}
Else if (condition2)
{
Statement2;
}
.
.
.
Else
{
Statement N;
}
Let’s take a quadractic equation solver (program
to solve the root of a quadractic equation) to demonstrate this. Many students
in programming class always find this difficult to solve the root of a
quadractic equation. I will take my time to explain how to go about it before
writing the code.
Recall that quadractic equation is of the form:
ax2+bx+c=0
and the formular is: x1x2 = (-b
Let d=b2-4ac
Remember that quadractic equation has 3
conditions
1.
When d>0 (we have two different roots) x1 = (-b + sqrt(b2-4ac))/
When d>0 (we have two different roots) x1 = (-b + sqrt(b2-4ac))/
X2= (-b
2.
When d=0
(we have two equal roots) x1=x2=-b/2a
3.
When
d<0 (complex root). For now we won’t solve this, but it will be later done.
Now let us write the program;
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace QE
{
class
Program
{
static void Main(string[] args)
{
double a, b, c, d, x1, x2;
Console.WriteLine("Enter
the value of a");
a = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("Enter
the value of b");
b = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("Enter
the value of c");
c = Convert.ToDouble(Console.ReadLine());
d = b * b - 4 * a *
c;
if (d == 0)
{
x1 = -b / (2 *
a);
x2 = x1;
Console.WriteLine("X1="
+ x1);
Console.WriteLine("X2="
+ x2);
}
else if (d > 0)
{
x1 = (-b + Math.Sqrt(d)) / (2 * a);
x2 = (-b - Math.Sqrt(d)) / (2 * a);
Console.WriteLine("X1="
+ x1);
Console.WriteLine("X2="
+ x2);
}
else
{
Console.WriteLine("complex
root");
}
}
}
}
4. Switch/case
statement
The switch statement is more readable because it
eliminates the problem of writing plenty code compare to that of the If … else
statement. The switch and the case statement go hand-in-hand. Just like the If
… else statement. The advantage the switch/case statement have over the if …
else statement is that it reduces data redundancy.
The syntax of the switch/case is:
Switch(condition)
{
Case1:
Statement;
Break;
Case2:
Statement;
.
.
.
Case N:
Statement;
Break;
}
Let us find the GPA of two courses in a
department. You can modify this code to find the CGPA of students in the whole
school.
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace
myfirstprogram
{
class
Program
{
static void Main(string[] args)
{
//gpa computation
string csc101,csc102;
double csc101g,csc101u,csc102g,csc102u,gpa;
csc101=Convert.ToString(Console.ReadLine());
csc102=Convert.ToString(Console.ReadLine());
csc101g = 5;
csc101g = 4;
csc101u = 4;
csc102g = 5;
csc102u = 3;
switch (csc101)
{
case "A":
csc101g =
5;
csc101u =
4;
break;
case "B":
csc101g =
4;
csc101u =
4;
break;
}
switch (csc102)
{
case "A":
csc102g =
5;
csc102u =
3;
break;
case "B":
csc102g =
4;
csc102u =
3;
break;
}
gpa = (csc102g *
csc102u + csc101g * csc101u) / (csc101u + csc102u);
Console.WriteLine(gpa);
}
}
}
5. For
loop statement
The for loop statement is used for looping a
particular set of code either forward (++) or backward (--). It is one of the
most widely used control statements for performing iterations in a loop. The
For loop statement is a PRETEST loop which means if first tests if a condition
is true and only executes if it is.
Syntax:
For (int expression; condition; increment
expression)
{
Statement;
}
Program to solve Jacobi iteration of 3 x 3
matrix is shown below. The matrix is of the form;
ax+by+cz=d …………(1)
ex+fy+gz=h ………….(2)
ix+jy+kz=l ……………(3)
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace QE
{
class
Program
{
static void Main(string[] args)
{
double a, b, c, d, e, f, g, h, i, j, k, l, x, y, z;
double tempx, tempy, tempz, iteration;
tempx = tempy =
tempz = 0;
Console.WriteLine("enter
value of a");
a = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of b");
b = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of c");
c = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of d");
d = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of e");
e = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of f");
f = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of g");
g = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of h");
h = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of i");
i = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of i");
j = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of k");
k = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of l");
l = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("enter
value of iteration");
iteration = Convert.ToDouble(Console.ReadLine());
for (int m = 0; m
< iteration; m++)
{
x = (d - b *
tempy - c * tempz) / a;
y = (h - e *
tempx - g * tempz) / f;
z = (l - i *
tempx - j * tempy) / k;
tempx = x;
tempy = y;
tempz = z;
Console.WriteLine("x="
+ x);
Console.WriteLine("y="
+ y);
Console.WriteLine("z="
+ z);
}
}
}
}
Exercise:
Try the above matrix form using Gaus Scidel iteration method.
6. While
loop statement
The while loop statement is a conditional loop
statement that behaves like the For loop statement. The while loop statement
executes until the whole condition is true.
Syntax:
While (condition)
{
Statement
}
Consider the program below as an example.
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace
myfirstprogram
{
class
Program
{
static void Main(string[] args)
{
int man = 0;
while (man < 10)
{
Console.WriteLine(man.ToString());
man++;
}
}
}
}
7. Do
… while statement
The do … while loop statement is a conditional
loop statement that executes a statement first and then checks if the condition
is true. If the condition is true, the loop continues until it is false.
Because of this condition, the do ….while loop is regarded as a post-test loop.
Syntax:
Do
{
Statement;
}
While (condition);
Consider the program below;
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace
myfirstprogram
{
class
Program
{
static void Main(string[] args)
{
int man
= 0;
do
{
Console.WriteLine(man.ToString());
man++;
}
while (man < 10);
}
}
}
8. Goto
statement
The goto statement is an unconditional jump
statement. When the goto statement is encountered program flow jumps to the
location specified by the goto.
9. Break
statement
In a simple term, the break statement leaves the
current loop. It exists from a loop or a switch immediately after a condition
is meant. The break statement can be used in a switch statement (check the
program under the switch statement, in a For statement, etc.
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace QE
{
class
Program
{
static void Main(string[] args)
{
for (int i = 1; i
<= 50; i++)
{
if (i == 20)
{
break;
}
Console.WriteLine(i);
}
Console.WriteLine("exists");
Console.ReadKey();
}
}
}
Console.ReadLine() and Console.ReadKey() behaves
the same way.
10.
Continue statement
The continue statement behaves exactly like the
break statement. In a simple word, the continue statement goes to the next
iteration of the current loop.
Program: write the same program under break but replace
break with continue.
The difference between the break statement and
the continue statement is that the break statement makes control exists the
entire loop while the continue statement only skips the current iteration.
11.
Return statement
The return statement returns from a method
before the end of that method is reached. It can be a value or not, depending
on the method that calls it.
using System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
namespace QE
{
class
Program
{
static void Main(string[] args)
{
int result = 101 + 20;
if (result
> 90)
{
Console.WriteLine("the
result is large");
return;
}
}
}
}
From the example above the return statement
returns nothing
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