Презентация на тему C# decision and iteration constructs

numerator and denominator”);
numerator = Convert.ToInt32(Console.ReadLine());
denominator = Convert.ToInt32(Console.ReadLine());
if (denominator !=

0)
Console.WriteLine(“{0}/{1} = {2}”, numerator, denominator, numerator/denominator);
else
Console.WriteLine(“Invalid operation can’t divide by 0”);

statement but make sure they are group into a

code block i.e. surrounded by curly braces.
Example

int x, y, tmp;
Console.WriteLine(“Please enter two integers”);
x = Convert.ToInt32(Console.ReadLine());
y = Convert.ToInt32(Console.ReadLine());
if ( x > y)
{
tmp = x;
x = y;
y = tmp;
}

if statement is nested within another if statement.
Example
if (x

> 0)
if ( x > 10)
Console.WriteLine(“x is greater than both 0 and 10”);
else
Console.WriteLine(“x is greater than 0 but less than or equal to 10”);
else
Console.WriteLine(“x is less than or equal to 0”);

requires a choice from one of many cases, successive

if statements can be joined together to form a if - else - if ladder.

that gives the same result as

if (expression )
true_statement;
else
false_statement;

syntax: expression

? true_statement : false_statement;

The ?; requires three arguments and is thus ternary. The main advantage of this operator is that it is succinct.

y;

which is the equivalent of

if ( x >= y)
max

= x;
else
max = y;

ladder but is clearer, easier to code and less

error prone.

\n”);
num1 = Convert.ToInt32(Console.ReadLine());
op = Convert.ToChar(Console.ReadLine());
num2 = Convert.ToInt32(Console.ReadLine());
switch(op)
{
case “+”:
result =

num1 + num2;
break;
case “-”:
result = num1 - num2;
break;
case “*”:
result = num1 * num2;
break;
case “/”:
if(num2 != 0)
{
result = num1 / num2;
break;
} //else fall through to error statement
default:
Console.WriteLine(“ERROR- invalid operation or divide by 0.0 \n”);
}
Console.WriteLine(“{0} {1},{2} = {3}\n”, num1, op, num2, result);

be repeated a known number of times using the

for statement. The programmer must know in advance how many times to iterate or loop through the statements, for this reason the for statement is referred to as a counted loop.

syntax:
for([initialisation];[condition];[action])
[statement_block];

Square braces indicate optional sections. Initialisation, condition and action can be
any valid C# expression, however, there are common expressions which are recom-
mended for each part.

initialisation: executed once only when the for loop is first entered, usually used to
initialise a counter variable.
condition: when this condition is false the loop terminates.
action: executed immediately after every run through statement_block and typically
increments the counter variable controlling the loop.

100; x++)
Console.WriteLine(“{0}”, x);

The above example prints out the numbers

from 1 to 100.

Example
int x, sum = 0;
for (x = 1; x <= 100; x++)
{
Console.WriteLine(“{0}”, x);
sum += x;
}
Console.WriteLine(“Sum is {0}”, sum);

Prints the numbers from 1 to 100 and their sum.

= 0; x

x = 0, sum = 0; x <= 100; x++)
{
Console.WriteLine(“{0}”, x);
sum += x;
}

for ( ; x < 10; x++)
Console.WriteLine(“{0}”, x);

+= i- -;
Console.WriteLine(“sum is {0}”, sum);

statement allows us to loop through a statement block

when we don’t know in advance how many iterations are required.

syntax:
while( condition )
statement_body;

Example
int sum = 0, i = 100;
while(i != 0) // this condition evaluates to true once i is not equal to 0
sum += i- -; // note postfix decrement operator, why?
Console.WriteLine(“sum is {0}”, sum);

This program calculates the sum of 1 to 100.

While Statement

nested.
Example

A program to guess a letter

char ch, letter =

“c”, finish = “y”;
while ( finish == “y” || finish == “Y”)
{
Console.WriteLine(“Guess my letter - only 1 of 26!”);
while((ch = Convert.ToChar(Console.ReadLine())) != letter)
{
Console.WriteLine(“{0} is wrong - try again\n”, ch);
}
Console.WriteLine(“OK you got it \n Lets start again.\n”);
letter += (char)3;
Console.WriteLine(“Do you wish to continue (Y/N)?”);
finish = Convert.ToChar(Console.ReadLine());
}

statements the test condition is evaluated before the statement_body

is executed. This means that the statement_body might never be executed. In the do while statement the statement_body is always executed at least once because the test condition is at the end of the body of the loop.
syntax:
do
{
statement_body;
} while ( condition );

Example
Keep reading in integers until a value between 1 and 10 is entered.

int i;
do
{
i = Convert.Toint32(Console.ReadLine());
} while( i >= 1 && i <= 10);

a for, while, do while or switch statement the

statement is immediately terminated and execution resumes at the next statement following the loop/switch statement.
Example

for (x = 1; x <= 10 ; x++)
{
if ( x > 4)
break;
Console.Write(“{0} “, x);
}
Console.WriteLine(“Next executed”);

Output is 1 2 3 4 Next executed

of a for, while or do while statement and

resumes execution back at the beginning of the statement_body of the loop with the next iteration.
Example

for (x = 1; x <= 5; x++)
{
if (x == 3)
continue;
Console.Write(“{0} “, x);
}
Console.WriteLine(“Finished loop\n”);

output is 1 2 4 5 Finished loop.