Introduction%20to%20Programming

1
Introduction to Programming

• What is an algorithm?
• Input and output
• Sequential execution
• Conditional execution
• Reading
• Chapter 1 in either text
• Lewis Loftus, or Savitch

2
Problems and Algorithms

• problem A task to be performed
• algorithm A procedure for solving a problem
• Algorithms are
• precise and unambiguous
• methodical and effective
• abstract

3
Sample Problems

• Is a given integer even or is it odd?
• For each integer in a list, is it even or odd?
• Play tic-tac-toe so you never lose
• Play tic-tac-toe so you always win
• Play chess so you always win

4
Programs and Computers

• program An algorithm written down in some
  language, e.g., English, Russian, Java, MatLab.
• computer A device that can execute programs
  written in programming languages, e.g., Java,
  MatLab, C, C, Fortran, Basic.

5
Algorithms and Programs
the thing with the property threeness
6
Problem 1

• Is a given integer even or odd?
• more precisely
• An integer is written on a card. If the integer
  is even, say even, otherwise say odd.

7
Algorithm / Program A

• 1. Get the integer from the card.
• 2. Divide the integer by 2.
• 3. If the remainder is 0, say even, otherwise
  say odd.
• Test
• 123456789

8
Algorithm / Program B

• 1. Get the integer from the card.
• 2. If the rightmost digit is 0, 2, 4, 6, or 8 say
  even, otherwise say odd.
• Test
• MCMXCIX

9
Input and Output

• input Bring data into the program, e.g., the
  integer from the card.
• output Send values out from the program, e.g.,
  the text even or odd.

10
Sequential and Conditional Execution

• sequential execution Perform sequence of
  computation steps in order, e.g., step 1, step 2,
  step 3.
• conditional execution Perform one of several
  steps based on a test, e.g., either print even
  or print odd based on whether the remainder is
  0 or not.

11
Algorithm / Program A

• Problem statement
• An integer is given as input data. If it is even,
  output even, otherwise output odd.
• Program in English
• 1. Get the integer and call it num
• 2. Divide num by 2 and call the remainder rem
• 3. If rem is 0, output even, otherwise output
  odd
• Program Segment in Java
• num in.readInt()
• rem num 2
• if ( rem 0 )
• System.out.println("even")
• else
• System.out.println("odd")

12
Initial Observations

• A segment is a part of a program
• Java is like stylized English
• Sequence of imperatives, known as statements
• Statements are not numbered
• Layout is essential for readability, but
  irrelevant for meaning, e.g., this code segment
  has the same effect
• numin.readInt()remnum2if(rem0)
  System.out.println("even")else
  System.out.println("odd")
• English version
• names num and rem
• Java version
• memory locations num and rem
• Memory locations are known as variables

13
Complete Program

• / Input an integer and output "even" if it
• is even, otherwise output "odd. /
• import java.io.
• public class OddEven
• public static void main(String args)
• int num // Input integer.
• int rem // Remainder of num / 2.
• // Initialize Text object in to read
• // from standard input.
• TokenReader in new TokenReader(System.in)
• // Set num to be the input integer.
• num in.readInt()
• // Set rem to be the remainder of num/2.
• rem num 2

14
Things to Do Now

• Web Browse CS100J web pages
• http//courses.cs.cornell.edu/cs100j/2002sp/
• Program 1 get description from web (Homework),
  and start
• Read relevant textbook chapter
• Lecture (Thursday) attend
• Sections pick one and attend. (Start next
  Tuesday or Wednesday)