🗊Презентация C# decision and iteration constructs

Course Object Oriented Programming Lecture 3 C# decision and iteration constructs.

Decision Statements If statement

Example int numerator, denominator; Console.WriteLine(“Enter two integer values for the 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”);

The statement body can include more than one 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; }

Nested if Statement Nested if statements occur when one 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”);

if - else - if operator If a program requires a choice from one of many cases, successive if statements can be joined together to form a if - else - if ladder.

Conditional Operator ?: There is a special shorthand syntax 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.

Example max = x >= y ? x : y; which is the equivalent of if ( x >= y) max = x; else max = y;

Switch Statement This statement is similar to the if-else-if ladder but is clearer, easier to code and less error prone.

Example Example double num1, num2, result; char op; Console.WriteLine(“Enter number operator number \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);

Iterative Statements For statement While statement Do while statement Break statement Continue statement

For Statement A statement or block of statements may 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.

Example Example int x; for (x = 1; x <= 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.

Advanced for Loops for( x = 0, sum = 0; x <= 100; x++) { Console.WriteLine(“{0}”, x); sum += x; } for( x = 0, sum = 0; x <= 100; x++) { Console.WriteLine(“{0}”, x); sum += x; } for ( ; x < 10; x++) Console.WriteLine(“{0}”, x);

Advanced for Loops int i=100,sum=0; while(i != 0) sum += i- -; Console.WriteLine(“sum is {0}”, sum);

While Statement In contrast to the for statement, the while 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.

Like for loops while loops may also be nested. Like for loops while loops may also be 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()); }

Do While Statement In both the for and while 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);

Break Statement When a break statement is encountered in 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

Continue Statement The continue statement terminates the current iteration 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.

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