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Introduction to Programming

(in C++)

Introduction

Jordi Cortadella, Ricard Gavaldà, Fernando Orejas Dept. of Computer Science, UPC

Outline

• Programming examples
• Algorithms, programming languages and

computer programs

• Steps in the design of a program

cin

cout

sum 8 13 21 sum

• 15 9
• 6

Calculate x

y

• Algorithm: repeated multiplication

x  x  x    x

y times

y x i p=x

i

Prime factors

• Decompose a number in prime factors

• Example: input 350 output 2 5 5 7

• Intuitive algorithm:

• Try all potential divisors d, starting from 2
  • If divisible by d, divide and try again the same divisor
  • If not divisible, go to the next divisor
• Keep dividing until the number becomes 1

Prime factors

n d divisible write 350 2 yes 2 175 2 no 175 3 no 175 4 no 175 5 yes 5 35 5 yes 5 7 5 no 7 6 no 7 7 yes 7 1 finish The algorithm will never write a non-prime factor. Why?

ALGORITHMS, PROGRAMMING LANGUAGES AND COMPUTER PROGRAMS

An algorithm

• An algorithm is a method for solving a

problem. It is usually described as a sequence

of steps.

• Example: How can we find out whether a

number is prime?

• Read the number (N).
• Divide N by all numbers between 2 and N-1 and

calculate the remainder of each division.

• If all remainders are different from zero, the

number is prime. Otherwise, the number is not

prime.

A computer program

• A computer program is an algorithm written in a in a

programming language that executes a certain task.

• Examples of tasks a computer program can execute:
  • Calculate the square root of a number
  • Find the number of times the word “equation” appears in a math book
  • Play a music file
  • Find the shortest path between two cities

A computer system

CPU Instruction Memory Data Memory Input devices (keyboard, mouse, microphone, etc.) Output devices (display, printer, speakers, etc.) Program (machine language) Program (high-level language) Compiler Loader This course:

• Design of programs
• Language: C++

Assembly and machine language

(From http://en.wikipedia.org/wiki/Assembly_language)

STEPS IN THE DESIGN OF A

PROGRAM

Example

• Design a program that
  • given a natural number representing a certain

amount of time in seconds (N),

• calculates three numbers (h, m, s) that represent

the same time decomposed into hours (h),

minutes (m) and seconds (s)

• Example
  • Given N=3815,
  • Calculate h=1, m=3, s=

Specification

• Precondition:
  • Specification of the data before the program is

executed

• Postcondition:
  • Specification of the data after the program is

executed

• Example
  • Precondition: N ≥ 0
  • Postcondition: 3600 h + 60m + s = N