C# Programming: Expressions, conditions, and repetitions 01204111 Computer and Programming.

C# Programming:Expressions, conditions,

and repetitions01204111 Computer and

Programming

Agenda

• Expressions• Input / Output• Program flow control

– Boolean expressions– Conditions– Repetitions

Overview

BasicsVariables, expressions, input and output

BasicsVariables, expressions, input and output

Type conversion, math methodsType conversion, math methods

Flow control: Boolean expressionsFlow control: Boolean expressions

RepetitionsRepetitions ConditionsConditions

Warnings

• There are far too many topics covered in this single lecture as compared to previous ones.

• However, what we would like to reflect on is that the general ideas of programming are mostly the same no matter what language you are using.

• However, the changes are– Syntactic changes– Various language dependent features

C# Program: review

• A program starts at Main• A program is organized into a set of namespaces, classes,

and methods.• You may skip the namespace part.

using System;

namespace Sample{ class Program { static void Main() { Console.WriteLine("Hello, world"); } }}

Inside method Main

• Variable declarations• Other statements

static void Main(string[] args) { const double pi = 3.1416;

int radius; double area;

radius = int.Parse(Console.ReadLine()); area = pi*radius*radius; Console.WriteLine(area); }

Note#1: Terminating a statement

• In C#, almost every statement must end with ;

• Except compound statements, grouped by { } pairs.

int x; int y = 10;

x = int.Parse(Console.ReadLine());if(x > y) Console.WriteLine("Good");

int x; int y = 10;

x = int.Parse(Console.ReadLine());if(x > y) Console.WriteLine("Good");

if(x <= y){ Console.WriteLine("Ummm"); Console.WriteLine("Try harder.");}

if(x <= y){ Console.WriteLine("Ummm"); Console.WriteLine("Try harder.");}

Variables: review

• If we want to store any value to be used later, we shall need a variable.

• In C#, before you can use a variable, you must– Declare it– Specify its type, which specifies a set of possible

values that the variable can keep.

Standard data types in C#: Review

Type Size Description Rangebool 1 byte Store truth value true / false char 1 byte Store one character character code 0 – 255byte 1 byte Store positive integer 0 – 255short 2 byte Store integer -32,768 -- 32,767int 4 byte Store integer -2.1 x 109 -- 2.1 x 109 long 8 byte Store integer -9.2 x 1018 -- 9.2 x 1018

double 16 byte Store real number ± 5.0x10-324 -- ± 1.7x10308

string N/A Store sequence of characters

N/A

Important types

• Boolean: bool– There are two possible values, which are true and false.

• Integer: int– Stores integers with in range 2.1 x 109 -- 2.1 x 109

• Real numbers: double– Stores floating point numbers in range ± 5.0x10-324 --

± 1.7x10308

Important types

• String: string– Written only in double quotes, e.g., "Hello"– If you want double quote itself, put backslash in front of it.

• Character: char– Represents a single character, written with a single

quote.

s = "He says \"I love you.\"";Console.WriteLine(s);s = "He says \"I love you.\"";Console.WriteLine(s);

He says "I love you."He says "I love you."

Variable naming rules

• Each variable must have a name, which must satisfy the following rules:– consists of digits, English alphabets, and underlines.– first character must be English alphabets (either in upper caps or

in lower caps) or an underline.– must not be the same as reserved words

• Not that upper case and lower case alphabets are different.

Example nam

eName

_data

9point

class class_A

class_”A”

point9

Variable declaration: review

• Types must be specified.

• Can initialize the values at the same time

int radius;string firstName;double GPA;

int radius;string firstName;double GPA;

int radius = 5;string firstName = "john";double GPA = 2.4;

int radius = 5;string firstName = "john";double GPA = 2.4;

Expressions

• Computations in C# take place in expressions.

Operators

• Most arithmetic operators work as in Python, except division.– Operators: + - * /– Operators: % (modulo)

• Operators are evaluated according to their precedence, as in Python.

Divisions with/by reals, the result are realsDivisions with/by reals, the result are reals

Integer division, the results are integerInteger division, the results are integer

Operators

• What are the values– 11 + 5 – 39 / 5 – 39.0/5 – 39 % 5 – 5.0 % 2.2

167

4

0.6

7.8

Input statement

• A common method for reading a string from the user is:

Console.ReadLine()

string name = Console.ReadLine();Console.WriteLine("Your name is {0}", name);string name = Console.ReadLine();Console.WriteLine("Your name is {0}", name);

Conversion from strings

• However, if we want to perform arithmetic computation with them, we have to convert strings to appropriate types.– Mathod int.Parse turns a string to an int.– Method double.Parse turns string to a double.

string sc = Console.ReadLine();int count = int.Parse(Console.ReadLine());

int age = int.Parse(Console.ReadLine());double time = double.Parse(Console.ReadLine());

string sc = Console.ReadLine();int count = int.Parse(Console.ReadLine());

int age = int.Parse(Console.ReadLine());double time = double.Parse(Console.ReadLine());

Output statements

• Method Console.Write and Console.WriteLine are common methods for showing results to console.– Method Console.Write is similar to method Console.WriteLine, but it does not put in a new line.

• We can format the output pretty easily.

Examples usage of Console.WriteLine

• Basic usage:

• Specifying printing templates:

• Specifying extra formatting:

Console.WriteLine(”Size {0}x{1}”, width, height);Console.WriteLine(”Size {0}x{1}”, width, height);

double salary=12000;double salary=12000;Console.WriteLine("My salary is {0:f2}.", salary);Console.WriteLine("My salary is {0:f2}.", salary);

Console.WriteLine("Hello");Console.WriteLine("Hello");Console.WriteLine(area);Console.WriteLine(area);

20

Thinking Corner

• Write a program to read the height (in meters) and weight (in kg) and compute the BMI as in the following formula.

BMI =Weight in KilogramsWeight in Kilograms

(Height in Meters) X (Height in Meters)(Height in Meters) X (Height in Meters)

Enter weight (in kg): 83Enter height (in m): 1.7Your BMI is 28.72.


The solution



public static void Main(string[] args) { Console.Write("Enter weight (in kg): "); double w = double.Parse(Console.ReadLine()); Console.Write("Enter height (in m): "); double h = double.Parse(Console.ReadLine()); Console.WriteLine("Your BMI is {0}.", w / (h*h)); Console.ReadLine(); }

public static void Main(string[] args) { Console.Write("Enter weight (in kg): "); double w = double.Parse(Console.ReadLine()); Console.Write("Enter height (in m): "); double h = double.Parse(Console.ReadLine()); Console.WriteLine("Your BMI is {0}.", w / (h*h)); Console.ReadLine(); }

quite an ugly formatting

(click to show)

Basic output formatting

• When displaying floating point numbers, WriteLine may show too many digits. Console.WriteLine("Your BMI is {0:f2}.", w / (h*h)); Console.WriteLine("Your BMI is {0:f2}.", w / (h*h));

We can add formatting :f2 in to the printing template.f is for floating points, and 2 is the number of digits.We can add formatting :f2 in to the printing template.f is for floating points, and 2 is the number of digits.



Thinking Corner: Ants

• An ant starts walking on a bright whose length is m meters. The ant walks with velocity v meters/second. After arriving at one end of the bridge, it walks back with the same speed. It keeps walking for t minutes. How many rounds that the ant completely crosses the bridge and gets back to the starting point?Write a program that takes

m, v, and t , as double, and compute how many rounds that the ant walks over the

bridge.

Ideas

• From the duration and speed, we can calculate the total distance the ant walks.

• From the distance and the length of the bridge, we can calculate the number of rounds– The problem we have to solve is that how we manage

to removes all the fractions in the result by the division.

– We will start with a program that compute the number of rounds, in real, and we will fix this problem later.

Partial solution

• method Mainstatic void Main() { Console.Write("Enter m (m): "); double m = double.Parse(Console.ReadLine()); Console.Write("Enter v (m/s): "); double v = double.Parse(Console.ReadLine()); Console.Write("Enter t (min): "); double t = double.Parse(Console.ReadLine());

double dist = v * t * 60; double walk = dist / (2*m); Console.WriteLine("Number of times = {0}", walk); Console.ReadLine();}

static void Main() { Console.Write("Enter m (m): "); double m = double.Parse(Console.ReadLine()); Console.Write("Enter v (m/s): "); double v = double.Parse(Console.ReadLine()); Console.Write("Enter t (min): "); double t = double.Parse(Console.ReadLine());

double dist = v * t * 60; double walk = dist / (2*m); Console.WriteLine("Number of times = {0}", walk); Console.ReadLine();}

(click to show)

Type conversion

• Values can be converted in to different types.• C# usually converts values in smaller types to

values in bigger types automatically.

short int long

int double

Casting

• However, for other conversions, we have to explicitly specify that we really need conversion.

• The syntax for that is:

(type) value• For example, if we want to convert 2.75 to an

integer we will write (int) 2.75

• Sometimes, we will have to putparentheses on the values to be converted.

get 2, convertinga double to an int in this way always rounds everything down

get 2, convertinga double to an int in this way always rounds everything down

Solution for Thinking Corner

• Shows only method Mainstatic void Main() { Console.Write("Enter m (m): "); double m = double.Parse(Console.ReadLine()); Console.Write("Enter v (m/s): "); double v = double.Parse(Console.ReadLine()); Console.Write("Enter t (min): "); double t = double.Parse(Console.ReadLine());

double dist = v * t * 60;

int walk = (int)(dist / (2*m));

Console.WriteLine("Number of times = {0}", walk); Console.ReadLine();}

static void Main() { Console.Write("Enter m (m): "); double m = double.Parse(Console.ReadLine()); Console.Write("Enter v (m/s): "); double v = double.Parse(Console.ReadLine()); Console.Write("Enter t (min): "); double t = double.Parse(Console.ReadLine());

double dist = v * t * 60;

int walk = (int)(dist / (2*m));

Console.WriteLine("Number of times = {0}", walk); Console.ReadLine();}

(click to show)

Additional operators

• C# also provides additional useful operators related to variable updates– Operators for increasing and decreasing by one– Operators for updating values

Operators for variable updates

• As in Python, C# provides data update operators

x = x + 10 x += 10

y = y * 7 y *= 7

z = z / 7 z /= 7

Operators ++, --

• We usually increase or decrease a variable by one. In C#, it is very easy to do.

x = x + 1 x += 1 x++x++

x = x - 1 x -= 1 x--x--

Examples: printing numbers

int a = 1;while(a <= 10){ Console.WriteLine(a);

a = a + 1;}


a = a + 1;}


a += 1;}


a++;}


a++;}

Methods for computing mathematical functions

• C# provides methods for computing math functions in standard class Math.

A list of common math methodsMethod/Constant

Value returned Example Call Result

PI Value of Math.PI 3.1415927

Max(x,y) Larger of the two Math.Max(1,2) 2

Abs(x) Absolute value of x Math.Abs(-1.3) 1.3

Sqrt(x) Square-root of x Math.Sqrt(4.0) 2.0

Round(x) Nearest integer to x Math.Round(0.8) 1

Pow(x,y) xy Math.Pow(3,2) 9.0

Log(x) Natural log of x Math.Log(10) 2.302585

Ceiling(x) Smallest integer greater than or equal to x

Math.Ceiling(4.1) 5

Cos(x) Cosine of x radians Math.Cos(Math.PI) -1

Thinking corner

Write a program that reads the radius and computer the area of the circle.

Write a program that reads the radius and computer the area of the circle.Enter radius: 6The area is 113.0973Enter radius: 6The area is 113.0973

Hint: Math.PIHint: Math.PI

Program flow control

• Condition or Boolean expressions• Types of program control:

– Conditional: if, if-else– Repetition: while, do-while, for

x == 0

x > 0

x < 0

x > 0 TrueTrue

False

False

TrueTrue

False

False

x < 0

height <= 140TrueTrue

FalseFalse

Condition: Boolean expressions

• How to build an expression?– Comparison– Combining smaller expressions

Comparison operators


Boolean operatorsBoolean

operators

Data type: bool

• Type bool represents logical values, with only two possible values:– True : true– False : false As in Python

C# is case sensitive.Therefore "True" is not the same as "true".

As in PythonC# is case sensitive.Therefore "True" is not the same as "true".


Operators Meanings Examples== Equal a == 5!= Not equal done != False< Less than data < 10

<= Less than or equal

count <= 15

> Greater than money > 0>= Greater than

or equalinterest >= 0.02

Boolean operators

AND

OR

NOT

&&

||

!

Examplesx is not

equal to 0 or y is

equal to 10

(x!=0) || (y==10)

i is less than n

and x is not equal to y

(i<n) && (x!=y)

a is between b

to c

(a >= b) && (a <= c)

if statements

• if statements controls whether some statement will be executed.

if( condition ) statement

คำ��สั่��ง if-else

• if-else statements are used when you have two choices.

if( condition) statementelse statement

Examples

The sky train has 50% price reduction for children not over 10 years old. The original ticket price is 50 baht. Write a program that reads the age and shows the ticket price.

The sky train has 50% price reduction for children not over 10 years old. The original ticket price is 50 baht. Write a program that reads the age and shows the ticket price.

static void Main(){ int age = int.Parse(Console.ReadLine()); if(age <= 10) Console.WriteLine("Price = 25"); else Console.WriteLine("Price = 50");}

static void Main(){ int age = int.Parse(Console.ReadLine()); if(age <= 10) Console.WriteLine("Price = 25"); else Console.WriteLine("Price = 50");}

Be carefule

• In Python, the condition in the if-statement may not be in the parentheses.

• But in C#, the parentheses after the if-statement is part of the syntax, and cannot be removed.

if age <= 10: print("Hello!")if age <= 10: print("Hello!")

if(age <= 10) Console.WriteLine("Hello");if(age <= 10) Console.WriteLine("Hello");

Empty statements

• As in Python, C# has an empty statement that does not do anything, which is

;• Usage example:

if(x > 15) ;else Console.WriteLine("hello");

if(x > 15) ;else Console.WriteLine("hello");

Controlling many statements

• The if- and if-else- statements only control single statements following it.

• More over, indentation does not mean anything in C#.

if age <= 10: print("Hello, kid") print("Your price is 25 baht")


if(age <= 10) Console.WriteLine("Hello, kid"); Console.WriteLine("Your price is 25 baht");

if(age <= 10) Console.WriteLine("Hello, kid"); Console.WriteLine("Your price is 25 baht");

Converted to C#

Block

• We can combine many statements into a single one by embracing them with the curly brackets { }. This is call a block.

• A block behaves like a single statement.



if(age <= 10){ Console.WriteLine("Hello, kid"); Console.WriteLine("Your price is 25 baht");}

if(age <= 10){ Console.WriteLine("Hello, kid"); Console.WriteLine("Your price is 25 baht");}

converted to C#

Block

Be careful

• In C#, only curly brackets define blocks. Indentation only serves as readability enhancement.

if(x > 0) total += x; count++;

if(x > 0) total += x; count++;

if(x > 0) total += x;count++;

if(x > 0) total += x;count++;

if(x > 0){ total += x; count++;}

if(x > 0){ total += x; count++;}

is the same as

is not thesame as

Thinking corner

• From the previous example on computing the BMI, add a status message to the user according to the table below.

BMI Status

less than 18.5 Underweight

greater than or equal to 18.5 but less than 25

Normal

greater than or equal to 25.0 but less than 30

Overweight

greater than or equal to 30 Obese (Extremely

Fat)

Only write the part that prints the status. You can assume that the BMI has be computed and stored in variable bmi

Solution 1: if statements

if(bmi < 18.5) Console.WriteLine("Underweight");if((bmi >= 18.5) && (bmi < 25)) Console.WriteLine("Normal");if((bmi >= 25) && (bmi < 30)) Console.WriteLine("Overweight");if(bmi >= 30) Console.WriteLine("Extremely fat");

if(bmi < 18.5) Console.WriteLine("Underweight");if((bmi >= 18.5) && (bmi < 25)) Console.WriteLine("Normal");if((bmi >= 25) && (bmi < 30)) Console.WriteLine("Overweight");if(bmi >= 30) Console.WriteLine("Extremely fat");

(click to show)

Solution 2: nested if

if(bmi < 18.5) Console.WriteLine("Underweight");else if(bmi < 25) Console.WriteLine("Normal"); else if(bmi < 30) Console.WriteLine("Overweight"); else Console.WriteLine("Extremely fat");

if(bmi < 18.5) Console.WriteLine("Underweight");else if(bmi < 25) Console.WriteLine("Normal"); else if(bmi < 30) Console.WriteLine("Overweight"); else Console.WriteLine("Extremely fat");

(click to show)

Be careful when using nested if

if(a > 5) if(b < 10) Console.WriteLine("Hello"); else Console.WriteLine("Good-bye");


if(a > 5) if(b < 10) Console.WriteLine("Hello");else Console.WriteLine("Good-bye");


Both programs are the same, because in C#

indentation does not change the meaning of the

program.

Which "if" that the only "else" in the program is referring to?

Which "if" that the only "else" in the program is referring to?

Matching else and if





The "else" is always paired up with the closed first if-statementThe "else" is always paired up

with the closed first if-statement

Using blocks

• If we want "else" to pair up with other "if", we have to explicitly put them into a single block.

if a > 5: if b < 10: print("Hello")else: print("Good-bye")

if a > 5: if b < 10: print("Hello")else: print("Good-bye")

if(a > 5){ if(b < 10) Console.WriteLine("Hello");}else Console.WriteLine("Good-bye");

if(a > 5){ if(b < 10) Console.WriteLine("Hello");}else Console.WriteLine("Good-bye");

Example: Tax deduction

• When calculate tax deduction, if you have children, you can deduct 15,000 baht per children. You can use that deduction for at most 3 children.

• We would like to write a program as below.

Do you have any children (Y/N)? NYour deduction is 0 baht.Do you have any children (Y/N)? NYour deduction is 0 baht.

Do you have any children (Y/N)? YHow many children do you have? 2Your deduction is 30000 baht




Deduction: plans

• Keep the deduction in variable deduction.• Write the first part where the program asks if the

user has any children.• Then, add more details later.

Deduction: first steppublic static void Main(string[] args){ Console.Write("Do you have any children (Y/N)? "); string ans = Console.ReadLine(); int deduction = 0; if(ans == "Y") { // dealing with the case where the user has children } else deduction = 0; Console.WriteLine("Your deduction is {0} baht.", deduction); Console.ReadLine();}

public static void Main(string[] args){ Console.Write("Do you have any children (Y/N)? "); string ans = Console.ReadLine(); int deduction = 0; if(ans == "Y") { // dealing with the case where the user has children } else deduction = 0; Console.WriteLine("Your deduction is {0} baht.", deduction); Console.ReadLine();}

Y or y

• If we would like to allow the user to answer with either Y or y, how should we change the program?

if((ans == "Y") || (ans == "y")) { // ……… }

if((ans == "Y") || (ans == "y")) { // ……… }

Use "or" ( || )

if(ans == "Y") if(ans == "Y")

Deduction: final steppublic static void Main(string[] args){ Console.Write("Do you have any children (Y/N)? "); string ans = Console.ReadLine(); int deduction = 0; if((ans == "Y") || (ans == "y")) { Console.Write("How many children do you have? "); int ccount = int.Parse(Console.ReadLine()); if(ccount > 3) ccount = 3; deduction = ccount * 15000; } else deduction = 0; Console.WriteLine("Your deduction is {0} baht.", deduction); Console.ReadLine();}

public static void Main(string[] args){ Console.Write("Do you have any children (Y/N)? "); string ans = Console.ReadLine(); int deduction = 0; if((ans == "Y") || (ans == "y")) { Console.Write("How many children do you have? "); int ccount = int.Parse(Console.ReadLine()); if(ccount > 3) ccount = 3; deduction = ccount * 15000; } else deduction = 0; Console.WriteLine("Your deduction is {0} baht.", deduction); Console.ReadLine();}

Iteration

• We will look at two closely-related statements for iteration

while-statementchecks the condition before start or continue

do-while-statementdoes the loop first, then checks the condition

do-while-statementdoes the loop first, then checks the condition

while-statement

• Works as in Python.

while( condition) statement

63

condition

statement 1

true

statement 2:

statement n

false

do-while-statement

• Python does not have the do-while statement

do statementwhile( condition );

64

condition

true

statement 1statement 2:

statement n

false

Examples

• Reads the password, until the user enters "hellocsharp"

string pwd;

pwd = Console.ReadLine();while(pwd != "hellocsharp") pwd = Console.ReadLine();

string pwd;

pwd = Console.ReadLine();while(pwd != "hellocsharp") pwd = Console.ReadLine();

written with while

string pwd;

do pwd = Console.ReadLine();while(pwd != "hellocsharp");

string pwd;

do pwd = Console.ReadLine();while(pwd != "hellocsharp");

written with

do-while

Which one looks easier?

Examples

• Reads an integer n, print numbers from 1 to n, one per line.

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.WriteLine(i); i++;}

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.WriteLine(i); i++;}

written with while

int n = int.Parse( Console.ReadLine());int i = 1;do{ Console.WriteLine(i); i++;}while(i <= n);


written withdo-while

Be careful

• The while-statement and the do-while-statement are very similar. The only differences is that the do-while-statement does not check the condition on the first run of the loop.



In previous example if n is 0, the program still prints 1.

In previous example if n is 0, the program still prints 1.

Thinking corner

• Reads an integer n, the prints numbers from 1 to n. Prints at most 8 numbers per line; separate each pair of numbers with a space.

Printing numbers: plans

• Write a program that prints all number in a single line.

• Add some code that prints new lines at appropriate place.

Printing numbers: 1

• Start with a program that prints every in a single line.– Next step: add more spaces.

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.Write(i); i++;}

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.Write(i); i++;}

201234567891011121314151617181920

201234567891011121314151617181920

Printing numbers: 2

• Change the template to "{0} "– Next step: add new lines

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.Write("{0} ", i); i++;}


201 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

201 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Printing numbers: 3

• We shall add statements that print new lines.• Note that we print new lines after printing 8 numbers,

i.e., when i is 8, 16, 24, …



201 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

201 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

if(___________________) Console.WriteLine();

Printing numbers: Final

• Question: if we add the code after statement i++ what would the output be like?

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.Write("{0} ", i); if(i % 8 == 0) Console.WriteLine(); i++;}

int n = int.Parse( Console.ReadLine());int i = 1;while(i <= n){ Console.Write("{0} ", i); if(i % 8 == 0) Console.WriteLine(); i++;}

201 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

201 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Conclusion

• This lecture presents the syntax of C# related to basic control flows.

• Note that even though the programming language changes, many of the concepts carry through the new language.

Date post:	12-Jan-2016
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C# Programming: Expressions, conditions, and repetitions 01204111 Computer and Programming.

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