Guida all'Uso delle Stringhe in Python, Schemi e mappe concettuali di Elementi di Informatica

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PROGRAMMARE CON LE STRINGHE

Capitolo 2

Introduction

• Numbers and character strings are important data types in any Python program
  • These are the fundamental building blocks we use to build more complex data structures
• In this chapter, you will learn how to work with numbers and text. We will write several simple programs that use them

Contents 2.1 Variables 2.2 Arithmetic 2.3 Problem Solving: First Do It By Hand 2.4 Strings 2.5 Input and Output

2.3 Problem Solving

DEVELOP THE ALGORITHM FIRST, THEN WRITE THE

PYTHON

Start with example values

• Givens
• Total width: 100 inches
• Tile width: 5 inches
• Test your values
  • Let’s see… 100/5 = 20, perfect! 20 tiles. No gap.
  • But wait… BW…BW “…first and last tile shall be black.”
• Look more carefully at the problem….
  • Start with one black, then some number of WB pairs
  • Observation: each pair is 2x width of 1 tile
    • In our example, 2 x 5 = 10 inches

Keep applying your solution

• Total width: 100 inches
• Tile width: 5 inches
• Calculate total width of all tiles
• One black tile: 5”
• 9 pairs of BWs: 90”
• Total tile width: 95”
• Calculate gaps (one on each end)
• 100 – 95 = 5” total gap
• 5 ” gap / 2 = 2.5” at each end

The algorithm

• Calculate the number of pairs of tiles
  • Number of pairs = integer part of (total width – tile width) / (2 * tile width)
• Calculate the number of tiles
  • Number of tiles = 1 + (2 * number of pairs)
• Calculate the gap
  • Gap at each end = (total width – number of tiles * tile width / 2
• Print the number of pairs of tiles
• Print the total number of tiles in the row
• Print the gap

2.4 Strings

String Length

• The number of characters in a string is called the length of the string. (For example, the length of "Harry" is 5).
• You can compute the length of a string using Python’s len() function: length = len("World!") # length is 6
• A string of length 0 is called the empty string. It contains no characters and is written as "" or ''.

String Concatenation (“+”)

• You can ‘add’ one String onto the end of another firstName = "Harry" lastName = "Morgan" name = firstName + lastName # HarryMorgan print(“my name is:”, name)
• You wanted a space in between the two names? name = firstName + " " + lastName # Harry Morgan Using “+” to concatenate strings is an example of a concept called operator overloading. The “+” operator performs different functions of variables of different types

Converting Numbers to Strings

• Use the str() function to convert between numbers and strings.
• Open Wing, then open a new file and type in: balance = 888. dollars = 888 balanceAsString = str(balance) dollarsAsString = str(dollars) print(balanceAsString) print(dollarsAsString)
• To turn a string containing a number into a numerical value, we use the int() and float() functions: id = int("1729") price = float("17.29") print(id) print(price)
• This conversion is important when the strings come from user input.

Strings and Characters

• strings are sequences of characters
  • Python uses Unicode characters
    • Unicode defines over 100,000 characters
    • Unicode was designed to be able to encode text in essentially all written languages
  • Characters are stored as integer values
    • See the ASCII subset on Unicode chart in Appendix A
    • For example, the letter ‘H’ has a value of 72

String Operations

Methods

• In computer programming, an object is a software entity that represents a value with certain behavior.
  • The value can be simple, such as a string, or complex, like a graphical window or data file.
• The behavior of an object is given through its methods.
  • A method is a collection of programming instructions to carry out a specific task – similar to a function
• But unlike a function , which is a standalone operation, a method can only be applied to an object of the type for which it was defined.
  • Methods are specific to a type of object
  • Functions are general and can accept arguments of different types
• You can apply the upper() method to any string, like this:
  • name = "John Smith"

  • Sets uppercaseName to "JOHN SMITH"

  • uppercaseName = name.upper()