Xiaojuan Cai Computational Thinking 1 Lecture 2 Simple Programs Xiaojuan Cai （蔡小娟） Fall, 2015

Xiaojuan Cai Computational Thinking 2 Objectives To be able to understand and write Python statements ： to output information to the screen, assign values to variables, get numeric information entered, and perform a counted loop

Xiaojuan Cai Computational Thinking 3 Roadmap The software development Elements of program Names expressions Output statements Assignment statements Definite loop

Xiaojuan Cai Computational Thinking 4 Software development Analyze the problem Determine specifications Create a design (algorithm) Implement the design Test/Debug the program Maintain the program

Xiaojuan Cai Computational Thinking 5 Example Susan 刚刚来到中国，她什么都喜欢，唯独 不喜欢中国的天气预报，因为她习惯华氏度 （ Fahrenheit ），不习惯摄氏度（ Celsius ）。 她请求你的帮助，她告诉你，华氏和摄氏之 间的转换是线性的，而且 0 摄氏度 =32 华氏度， 100 摄氏度 =212 华氏度。

Xiaojuan Cai Computational Thinking 6 Roadmap The software development Elements of program Names Expressions Output statements Assignment statements Definite loop

Xiaojuan Cai Computational Thinking 7 Names File (module) has name, e.g. converter Function has name, e.g. main Variable has name, e.g. Celsius Names are called identifier.

Xiaojuan Cai Computational Thinking 8 Identifier begin with a letter or underscore _ followed by any sequence of letters, digits, or underscores. Case sensitive All these are legal identifiers: x, celsius, Spam, SPAM, x2, x_2, _x_2_, …

Xiaojuan Cai Computational Thinking 9 Reserved words

Xiaojuan Cai Computational Thinking 10 Expressions The fragments of code that produce or calculate new data values are called expressions, including literals, e.g. 3.9, 32 identifier, e.g. celsius, x combining simple ones with operators, e.g. 3.9*x*(1-x) function calls (later in this course) …

Xiaojuan Cai Computational Thinking 11 Output statement print print,, …, print,, …,, Syntax: <> is slots for other fragments, alternative … indicate any number of Semantics: evaluate, output the value

Xiaojuan Cai Computational Thinking 12 Assignment statement Syntax: = E.g., x = 3.9 * x * (1-x), x = 2 A variable can be assigned many times, with any values (of any types) Semantics: x = x x 11 x 10 “x” x = “x”

Xiaojuan Cai Computational Thinking 13 Assigning input Syntax: = input( ) : prompt the user for input E.g., x = input("Enter a number ") Semantics: evaluate, wait for an input and assign the input to

Xiaojuan Cai Computational Thinking 14 Simultaneous assignment Syntax:,…, =, …, Semantics: evaluate all the ’s, assign the values to corresponding ’s. E.g., x,y = 3,4; x,y = y,x x,y = input(“…”)

Xiaojuan Cai Computational Thinking 15 Definite loop -- syntax for in : is called loop index is any sequence of statements, indenting under the heading consists of list of values

Xiaojuan Cai Computational Thinking 16 Loops A definite loop will execute definite times. The most common definite loop is counted loop. for in range( ): is one of counted loops

Xiaojuan Cai Computational Thinking 17 Definite loop -- semantics

Xiaojuan Cai Computational Thinking 18 Example One more example – Future values Susan 在中国银行存了一笔钱，根据 中国银行的年利率，她想知道 10 年后 她能取出多少钱。

Xiaojuan Cai Computational Thinking 19 Formalize Program Future value Inputs: principal the amount of money initially apr the annual percentage rate Output: the value after 10 years

Xiaojuan Cai Computational Thinking 20 Algorithm Print an introduction Input principal and apr Repeat 10 times: principal = principal * (1 + apr) Output the principal

Xiaojuan Cai Computational Thinking 21 Conclusion def main(): print "This program illustrates a chaotic function" x = input("Enter a number between 0 and 1: ") for i in range (10): x = 3.9 * x * (1-x) print i,x main() Circle each identifier. Underline each expression. indicate the type of statement on each line