COMPSCI 101 S1 2014 Principles of Programming

COMPSCI 101 S1 2014Principles of Programming 13 Revision

Agenda • The Three Rules • Name Conversion • Types • Variables • Statements • Operators • selections • Loops • Lists COMPSCI101

The Three Rules • Rule 1: • Think before you program • Rule 2: • A program is a human-readable essay on problem solving that also happens to execute on a computer • Rule 3: • The best way to improve your programming and problem solving skills is to practice. COMPSCI101

Python Name Conventions • must begin with a letter or underscore _ • Ab_123 is OK, but 123_ABC is not. • may contain letters, digits, and underscores • this_is_an_identifier_123 • may be of any length • upper and lower case letters are different • Length_Of_Rope is not length_of_rope • the standard way for most things named in python is lower case with separate words joined by an underline: • my_list NameError: name 'length_of_rope' is not defined COMPSCI101

Python “Types” • integers: 5 • floats: 1.2 • booleans: True/False • strings: "anything" or 'something' • lists: [] ['a',1,1.3] • … • What is a type? • a type in Python essentially defines two things: • the internal structure of the type (what is contains) • the kinds of operations you can perform • 'abc'.isalpha()is a method you can call on strings, but not integers COMPSCI101

Type Conversion • int(some_var)returns an integer • float(some_var)returns a float • str(some_var) returns a string ERROR int(2.1) int('2') int('2.1') float(2) float('2.0') float('2') float(2.0) str(2) str(2.0) str('a') COMPSCI101

Operators • Integer • addition and subtraction: +, - • multiplication: * • division • quotient: / • integer quotient: // • remainder: % • exponentiation: ** • Floating point • add, subtract, multiply, divide: +, -, *, / COMPSCI101

Two Types of Division • The standard division operator (/) yields a floating point result no matter the type of its operands: • Integer division (//) yields only the integer part of the divide (its type depends on its operands): 2 / 3 4.4 / 2 5 / 2 0.6666666666666666 2.2 2.5 0 2.0 2 2.0 2 // 3 4.4 // 2 5 // 2 5 // 2.0 COMPSCI101

Modulus & Exponentiation • The modulus operator (%) give the integer remainder of division: • Again, the type of the result depends on the type of the operands. • The exponentiation operator (**) 2 1.0 5 % 3 7.0 % 3 2 ** 3 4.0 ** 3 4.0 * 3 8 64.0 12.0 COMPSCI101

Order of operations and parentheses • Remember, parentheses always takes precedence COMPSCI101

Exercises • What is the result of the following expression? 25 / 4 + 4 * 10 % 3) 25 - 7 * 3 + 12 / 3 17 % 3 * 2 - 12 + 15 COMPSCI101

Rounding • The round(x) function returns the value x rounded to an integer • Note: The round(x) function rounds 'halfway' numbers (e.g. 2.5, 4.5, -0.5) to the nearest eveninteger print (round(2.3), round(3.2), round(3.9)) print (round(2.5), round(3.5), round(4.5)) COMPSCI101

Variable & Statements • Variable • A variable is a name we designate to represent an object (number, data structure, function, etc.) in our program • We use names to make our program more readable, so that the object is easily understood in the program • Statements • Statements are commands in Python. • They perform some action, often called a side effect, but they do not return any values COMPSCI101

Printing output • The print() function takes a list of elements in parentheses separated by commas • if the element is a string, prints it as is • if the element is a variable, prints the value associated with the variable • The print statement inserts a blank between objects when objects are separated by “,” • after printing, moves on to a new line of output my_var = 12 print('My var has a value of: ', my_var) a = 1 b = 2 c = 3 print(a, b, c) COMPSCI101

Assignment Statement "=" • In many computer languages, = means assignment. • my_int = my_int + 7 • lhs = rhs • What assignment means is: • evaluate the rhs of the = • take the resulting value and associate it with the name on the lhs • It does not stand for equality! • What does the following code print to standard output? x = 9 y = 4 s = " + " print(x - y , s , y + y) a = 5 b = 10 c = 0 c = a a = b b = c print (a, b, c)) COMPSCI101

Exercise • What does the following code print to standard output? a = 5 b = 10 a = a + b b = a - b a = a - b print (a, b) a = 5 b = a + 2 c = b + 2 d = c + 2 c = c // 2 * 2 b = b // 2 * 2 print (a, b, c, d) a = 3 b = 12 c = 6 d = 1 d = d * a c = c + 2 * a d = d - b // c c = c * b % c b = b // 2 print (a, b, c, d) a = 9 b = a % 11 c = a / 2 d = a % 2 print (a, b, c, d) COMPSCI101

Modules • Modules are files that can be imported into your Python program. • Example is the math module • we import a module to use its contents • we use the name of the module as part of the content we imported • Example: import math print(math.pi) COMPSCI101

Selection • Selection is how programs make choices, and it is the process of making choices that provides a lot of the power of computing if condition : indented_Code_Block if condition : indented_Code_Block_For_True_Condition else: indented_Code_Block_For_False_Condition COMPSCI101

if, elif, else, the process • evaluate Boolean expressions until: • the Boolean expression returns True • none of the Boolean expressions return True • if a boolean returns True, run the corresponding block. Skip the rest of the if • if no boolean returns True, run the else block if condition1 : indented_Code_Block_For_True_Condition_1 elif condition2 : indented_Code_Block_For_True_Condition_2 elif condition3 : indented_Code_Block_For_True_Condition_3 else: indented_Code_Block_ForEachCondition_False COMPSCI101

Exercises • What does the following code print to standard output? x = 10 y = 5 if x < 10: if y != 5: print ("a") else: print ("b") elif y > 10: print ("c") else: if y != 5: print ("d") else: print ("e")) a = 4 b = 12 c = 37 d = 51 if a < b : print( "a < b" ) if a > b : print( "a > b" ) if d <= c : print( "d <= c" ) if c != d : print( "c != d" ) COMPSCI101

Repeating statements • Besides selecting which statements to execute, a fundamental need in a program is repetition • repeat a set of statements under some conditions • The for statement is useful for iteration, moving through all the elements of data structure, one at a time. for item in sequence: indented_Code_Block COMPSCI101

Exercises • What does the following code print to standard output? my_list = [1,2,3] b = 0 for y in my_list: b = y print (b) my_list = [1,2,3] a = 0 for y in my_list: a = a + y print (a) COMPSCI101

Terms Test • Section A (75 marks) • 20 MCQ (2.25 minutes per question) • What is the output of the following code… • The following code should…. Which expression should be used in …? • Given the following code… which one of the following function calls will… when executed? • Section B (25 marks) • 3 questions (15 minutes for 3 questions) • Tracing Code • Write a Function • Understanding Python code COMPSCI101

Tuesday 1st April 7:30pm - 8:30pm. • The test is worth 15% of your final mark. • Please bring your Student Id card, a pencil and an eraser • Material to be examined: Lectures 1 - 9 and Labs 1 - 4 • The test is a closed book test, so you cannot refer to any material during the test. • Calculators are also not permitted • Please notify Angela Chang if you have a test clash • Please arrive by 7:15pm as you will be given 5 minutes' reading time. • Please read the instructions on filling out a Teleform sheet before you go to the test COMPSCI101

Teleforms • Fill in your Student ID Number in the STUDENT ID# section. • Also fill in one column for each digit of your Student ID Number. • The example below shows a Student ID Number of 8677777 (with 7 digits) COMPSCI101

Room Allocations • Based on the first letter of your surname: • A - C: Room LgeChem/301-G050 • D - J: Room LibB28/109-B28 • K - L: Room MLT1/303-G23 • M - P Room PLT1/303-G20 • Q - T Room HSB1/201N-346 • U - Z: Room LibB15/109-B15 COMPSCI101

COMPSCI 101 S1 2014 Principles of Programming