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Algorithms

Algorithms. CSC1310 Fall 2009. What Is Programming?. Programming means writing down a series of instructions that tell a computer what to do. Properties of these instructions: Computation proceeds in discrete steps. Each step is precisely defined.

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Algorithms

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  1. Algorithms CSC1310 Fall 2009

  2. What Is Programming? • Programming means writing down a series of instructions that tell a computer what to do. • Properties of these instructions: • Computation proceeds in discrete steps. • Each step is precisely defined. • The order in which steps are performed may be important.

  3. Algorithms • A set of instructions with these properties is said to be an algorithm. • The steps in an algorithm are not always short and simple. • Some steps may involve a series of smaller steps. • Some steps may involve making decisions. • Some steps may repeat. • Algorithms are common in the real world.

  4. The Recipe 1. Melt slowly and keep warm: 1/2 cup butter. 2. Barely heat: 1 1/2 tablespoons lemon juice, dry sherry, or tarragon vinegar. 3. Place in the top of a double boiler over—not in—hot water: 3 egg yolks. 4. Beat the yolks with a wire whisk until they begin to thicken. Add: 1 tablespoon boiling water. 5. Beat again until the eggs begin to thicken. Repeat this process until you have added: 3 more tablespoons water. 6. Then beat in the warm lemon juice. 7. Remove double boiler from heat. 8. Beat the sauce well with a wire whisk. Continue to beat while slowly adding the melted butter and: 1/4 teaspoon salt; a few grains cayenne. 9. Beat until the sauce is thick. 10.Serve at once.

  5. The Recipe as an Algorithm • This recipe satisfies most of the requirements for an algorithm: • It involves discrete steps. • Each step is more or less precisely defined. • The order of the steps matters, at least to some extent. • Some steps require making decisions and repeating actions. • This recipe is detailed enough for an experienced cook, but it might pose problems for a novice. • Computer algorithms need to be much more precise.

  6. Computer Algorithms • Computer algorithms often involve obtaining input, performing a calculation, and producing output. • An algorithm for converting from Fahrenheit to Celsius: 1. Ask the user to enter a Fahrenheit temperature. 2. Obtain the input entered by the user. 3. Convert the user’s input into numerical form. 4. Calculate the equivalent Celsius temperature, using the formula C = (F – 32)  (5 / 9) 5. Convert the Celsius temperature into character form and display the result.

  7. Boil Water for a Tea Algorithm • Input: empty kettle, stove, faucet • Output: boiled water in the kettle. • Algorithm? • Reuse of algorithm.

  8. Ways to Express Algorithms • Natural languages. Allows anyone who understands that language to read the algorithm, but lacks precision. • Programming languages. Precise, yet simple enough for computers to understand. • Pseudocode. A mixture of natural language and a programming language. More precise than natural language but less precise than a programming language. Often easier to read (and to write) than a programming language.

  9. Fahrenheit to Celsius Conversion in Python 1. Ask the user to enter a Fahrenheit temperature. 2. Obtain the input entered by the user. 3. Convert the user’s input into numerical form. >>>x=float(raw_input(“Please, enter Fahrenheit temperature:\n”)) 4. Calculate the equivalent Celsius temperature, using the formula C = (F – 32)  (5 / 9) 5. Convert the Celsius temperature into character form and display the result. >>>print “The Celsius temperature is ” +str((x-32)*(5.0/9))

  10. Chapter 9 IF Statement CSC1310 Fall 2009

  11. If Statement • The main statement used for selecting from alternative actions based on test results • It’s the primary selection tool in Python and represents the Logicprocess ( “==”, “!=”, “>”, “>=”, “<”, “<=”)

  12. Simple If statement • It takes the form of an if test • if <test>: <statement>

  13. Simple If statement >>>if 4>3: print “it is true that 4>3” >>>if 1: print “true” >>>a=10 >>>if a==10: print “a=10”

  14. If... else statement • It takes the form of an if test, and ends with an optional else block • if <test>: <statement1> else: <statement2>

  15. If... else statement >>>if 3>4: print “3 is larger than 4” else: print “3 is NOT larger than 4” >>>if {}: print “true” else: print “false”

  16. If… else if … else statement • It takes the form of an if test, followed by one or more optional elif tests, and ends with an optional else block if <test1>: <statement1> elif <test2>: <statement2> else: <statement3>

  17. If… else if … else statement >>>a=10 >>>if a>10: print “a > 10” elif a==10: print “a = 10” else: print “a < 10”

  18. If… else if … else statement example >>>number = 23 >>>guess = int(raw_input('Enter an integer : ')) >>>if guess == number: print 'Congratulations, you guessed it.' # New block starts here print "(but you do not win any prizes!)" # New block ends here elif guess < number: print 'No, it is a little higher than that' # Another block else: print 'No, it is a little lower than that'

  19. Syntax Rules • Statements execute one after another, unless you say otherwise. • Block and statement boundaries are detected automatically (no “end/begin” delimiters). • Header “:” indented statements (pattern of all compound statements). • Statement delimiters: • Statement may span lines if you’re continuing an open syntactic pair((),{},[]); • Statement may span lines if they end with backslash. • Block literals. • Several non-compound statements could be written at the same line being separated by semicolons. • You could move body of compound statement up to header line.

  20. Truth Test • X and Y is true if both X and Y are true. >>>a=5;b=3;c=2 >>> a>b and a>c >>> b>c and b>a >>>2 and 3, 3 and 2, {} and [], 3 and [] • X or Y is true if either X or Y are true. >>> b>c or b>a >>> b>a or b>c >>>2 or 3, 3 or 2, {} or [], 3 or [] • not X is true if X is false. >>> not 2, not {}, not [1,2,3,4,5]

  21. Non empty set • Assume user may input some data to A, B and C. • Program does not know if data was provided. >>>X = A or B or C or None

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