Lecture 1

1. Lecture 1 CMSC 201

2. Overview • Goal: Problem solving and algorithm development. Learn to program in Python. • Algorithm - a set of unambiguous and ordered steps to accomplish a task • Algorithm Representation - Pseudocode or flowchart

3. Program Development Step 1: Understand the problem (input, process, output) Step 2: Represent algorithm in pseudocode or flowchart Step 3: Desk check the algorithm Step 4: Implement in a programming language (We will use Python, other options include C, C++, Java, etc.) Step 5: Test/Debug your program

4. Example • Develop an algorithm that will calculate an hourly employee’s weekly pay • When developing algorithms, we have 3 control structures available to us • Sequence (i.e. one line after the other) • Decision making (e.g. using if/else constructs) • Looping (e.g. using while loops) Step 1 : Understand the problem • Input : pay rate and number of hours • Process: pay = rate * hours • Output: pay

5. Example - Pseudocode • Pseudocode - looks like code, but not a real language Step 2: 1. Variables: hours, rate, pay 2. Display “Number of hours worked: ” 3. Get hours 4. Display “Amount paid per hour: ” 5. Get rate 6. pay = hours * rate 7. Display “The pay is $” , pay • Notice the importance of order and lack of ambiguity

6. Basic Flowchart Symbols Start End

7. Example - Flowchart Start Display “Amount paid per hour: ” Display “Number of hours worked: ” Get rate Get hours pay = hours * rate Display “The pay is $”, pay End

8. Flowcharts – Decision Making example If num > 0 display “Positive” Else (that means 0 or negative) display “Not Positive” num >0? True False Display “positive” Display “Not positive”

9. Flowcharts – Looping example Keep prompting the user for the number of books as long as the input is non-positive Get books books >= 0? False Display ”Error, enter number of books read: “ Get books True

10. Back to the original example Step 3:

11. Stored-Program Computer Step 4: So far, everything has been on paper. How do we get it to run on a computer? 1. Design a computer specific to solving this one problem (Not a good idea, although historically early computers were designed to just solve specific problems) 2. Use a stored-program computer (A more general approach where data/instructions are in memory and the CPU processes the code)

12. Basic Computational Model Memory Code/Data CPU Input Output

13. Computer Program • Algorithm development should be independent of the final computer language used to implement the algorithm as an executable program • Algorithm development is the hard part • Example Algorithm Python Program

14. Example: Program in C++ /************************************** C++ Implementation of the algorithm. **************************************/ #include <iostream> using namespace std; int main() { //declare the variables double hours, rate, pay; //Get user input cout<< "Number of hours worked: "; cin >> hours; cout<< "Amount paid per hour: "; cin >> rate; //Calculate the pay pay = hours * rate ; //Display the result cout << "The pay is $” << pay <<endl; return 0; } Step 4: Let’s write the program in Python (We will learn the Python programming language this semester)

15. Python Interpreter • Python Interpreter – runs your python code • Can use it in the interactive modeor script mode • Your code is compiled into byte code (.pyc file) • Python Virtual Machine (PVM) runs the byte code