Lecture 02: Operators, Expressions & Statements
Objectives:
Learn about the C# operators and how they are evaluated in
expressions
Learn the Loop Constructs
Learn the Jump and Selection Constructs
1. C# Operators & Expressions:
C# has almost identical set of operators as Java as the table
below shows.
Complex expressions can be built by combining these operators
with constants, variables and method calls.
When an expression contains multiple operators, the precedence
of the operators control how the operators are evaluated.
When the operators have the same precedence, their
associativity determines the order of evaluation.
Except for assignment operator, all other binary operators are
left-associative and are evaluated from left to right.
The assignment operator, the unary operator and the conditional
operator are evaluated from right to left.
For example, the following expressions are equivalent:
1+2+3*4 ((1 + 2) + (3 * 4))
The following table list the operators according to their
precedence:
Category Operators
Primary x.y f(x) a[x] x++ x-- new typeof
checked unchecked
Unary + - ! ~ ++x --x (T)x
Multiplicative * / %
Additive + -
Shift << >>
Relational and type testing < > <= >= is as
Equality == !=
Logical AND &
Logical XOR ^
Logical OR |
Conditional AND &&
Conditional OR ||
Conditional ?:
Assignment = *= /= %= += -= <<= >>= &= ^= |=
The typeof operator is used to obtain the System.Type object for a
type.
Example:
using System; output:
class Test
{ System.Int32
static void Main() { System.Int32
Type[] t = { System.String
typeof(int), System.Double[]
typeof(System.Int32),
typeof(string),
typeof(double[]),
};
for (int i = 0; i < t.Length; i++) {
Console.WriteLine(t[i].FullName);
}
}
}
The as operator is used for type-conversion (down-casting). For
example, consider the following statements:
Student s = new GraduateStudent(…);
GraduateStudent gs;
Then the following statements are equivalent – well almost.
gs = (GraduateStudent) s; gs = s as GraduateStudent;
The only difference is when the object in s is not compatible with
GraduateStudent. In that case, the first statement throws
InvalidCastException, while the second assign null to gs.
The is operator is like the instanceof operator in Java. It checks
if an object is compatible with a type. Example:
if (s is GraduateStudent)
gs = s as GraduateStudent;
2. Loop Constructs:
The usual for, while, and do-while loops exists in exactly the
same syntax as in Java. Example:
while loops
syntax: while (expression) statement[s]
A 'while' loop executes a statement, or a block of statements, repeatedly until the condition specified
by the boolean expression returns false.
int a = 0; Output:
while (a < 3){
System.Console.WriteLine(a); 0
a++; 1
} 2
do-while loops
syntax: do statement[s] while (expression)
In 'do-while' the condition is evaluated after the block of code
specified in the 'do' clause has been run. So even where the
condition is initially false, the block runs once. For instance, the
following code outputs '4':
int a = 4; Output:
do{
System.Console.WriteLine(a); 4
a++;
} while (a < 3);
for loops
syntax: for (statement1; expression; statement2)
statement[s]3
Statement1 is executed before the loop is entered. The loop which
is then executed corresponds to the following 'while' loop:
while (expression) {statement[s]3; statement2}
for (int a = 0; a<3; a++) { Output:
System.Console.WriteLine(a); 0
} 1
2
In addition, C# has the foreach loop which has the following
syntax:
foreach (itemType variable1 in variable2)
Statement[s];
Where, variable2 is a collection whose items are of type
itemType and which implements the IEnumerable interface.
All C# arrays implement the IEnumerable interface, so we can
have a statement such as the following:
int[] a = {1, 3, 5, 7, 9};
foreach (int i in a)
Console.WriteLine(i);
3. Jump and Selection Constructs:
C# provide the following Jump constructs for jumping out of
loops, switch statements or methods.
break
The 'break' statement breaks out of the 'while' and 'for' loops and
out of a case in a 'switch' statement – discussed below.
continue
The 'continue' statement can be placed in any loop structure. When
it executes, it moves the program counter immediately to the next
iteration of the loop. The following code example uses the 'continue'
statement to count the number of values between 1 and 100
inclusive that are not multiples of seven. At the end of the loop the
variable y holds the required value.
int y = 0;
for (int x=1; x<=100; x++){
if ((x % 7) == 0)
continue;
y++;
}
goto
The 'goto' statement is used to make a jump to a particular labelled
part of the program code. It is also used in the 'switch' statement
described below. We can use a 'goto' statement to construct a loop,
as in the following example (but again, this usage is not
recommended):
int a = 0;
start:
System.Console.WriteLine(a);
a++;
if (a < 5)
goto start;
Other jump statements are:
return -- exit out of a method
throw -- throws an exception and exit out of a block
Selection Statements
C# offers the same basic types of selection statements as Java, as
described below:
if - else
'If-else' statement is exactly as in java as shown by the following example:
if (a == 5)
System.Console.WriteLine("A is 5");
else
System.Console.WriteLine("A is not 5");
If statements can also be emulated by using the conditional
operator. The conditional operator returns one of two values,
depending upon the value of a boolean expression.
int i = (myBoolean) ? 1 : 0 ;
sets i to 1 if myBoolean is true, and sets i to 0 if myBoolean is
false.
switch
The switch statement also exists in the same syntax as Java.
However, C# does not allow automatic fall through between
cases, which is the default in Java if a break statement is not
used.
In C# you must explicitly use a break or goto statement to
indicate where control should jump to. The following shows an
example:
int a = 2;
switch(a) {
case 1:
Console.WriteLine("a>0");
goto case 2;
case 2:
Console.WriteLine(" and a>1");
break;
default:
Console.WriteLine("a is not set");
break;
}
An exception to this rule is when a case does not specify an
action as in the following example:
switch(a) {
case 1:
case 2:
Console.WriteLine(" and a>0");
break;
default:
Console.WriteLine("a is not set");
break;
}
Example:
The following example shows how to use some of the control
structures. It also shows how to tokenize a string in C#.
using System;
public class ControlStructures
{
public static void Main()
{
String input;
do
{
Console.Write("Type int values to add or stop to
exit: ");
input = Console.ReadLine();
if (input.ToLower() != "stop")
{
char[] delimiters = {' ', '\t', ','};
String[] tokens = input.Split(delimiters);
int sum = 0;
foreach (String token in tokens)
sum += int.Parse(token);
Console.WriteLine("The sum is: "+sum);
}
} while (input.ToLower() != "stop");
}
}
There are a number of points to observe in this example:
String tokenization is achieved by using the Split method of the
string object. This takes an array of characters as argument and
returns an array of string tokens in the string, using the
characters in the character array as delimiters.
Note also that we can compare string objects using the != and
== operators.
There are two types of arrays in C#. One is exactly like the one
in Java. It is one dimensional, but it can be used to simulate
multi-dimensional arrays, as arrays of arrays. The resulting
multi-dimensional can be ragged. The declaration is exactly as in
Java. It also has a Length (with capital L) property that returns
the length of the first dimension. This type of array is sufficient
for us in this course.
The other type of array is an actual multi-dimensional array
which is not ragged and is declared in a slightly different way.
We may not need to use this type of array in this course.
