Lecture 1.pptx
- 1. 1
- 2. Some influential ones: FORTRAN science / engineering COBOL business data LISP logic and AI BASIC a simple language 2 Languages
- 3. Programming basics code or source code: The sequence of instructions in a program. syntax: The set of legal structures and commands that can be used in a particular programming language. output: The messages printed to the user by a program. console: The text box onto which output is printed. 3
- 4. Compiling and interpreting Many languages require you to compile (translate) your program into a form that the machine understands. Python is instead directly interpreted into machine instructions. 4 compile execute output source code Hello.java byte code Hello.class interpret output source code Hello.py
- 5. Dimensions of aPL • Functional • Treatscomputation as the evaluation ofmathematical functions (e.g. Lisp, Scheme, Haskell, etc.) • Imperative • describes computation in terms of statements that changea program state (e.g. FORTRAN, BASIC, Pascal, C, etc. ) • Logical (declarative) • expresses the logic of a computation withoutdescribingits control flow (e.g. Prolog) • Object oriented • uses "objects"– data structures consistingof data fields and methods together with their interactions– to design applications and computer programs (e.g. C++, Java, C#, Python, etc.)
- 6. Slide credit: Thomas J. Cortina C (1973) • Developed by Ken Thompson and Dennis Ritchie at AT&T Bell Labs for use on the UNIXoperating system. – now used on practicallyevery operatingsystem – popular languagefor writing system software • Features: – An extremely simple core language, with non-essential functionality provided by a standardizedsetof library routines. – Low-level access to computer memory via the use of pointers. • C ancestors: C++, C#, Java 9
- 7. Slide credit: Thomas J. Cortina Python • Created by Guido van Rossum in the late 1980s • Allows programming in multiple paradigms: object- oriented, structured,functional • Uses dynamic typing and garbage collection
- 8. 8 Building a simple program in Python (as compared to C) • Compilers versusinterpreters • Variable declarations • Whitespace • The printf() function • Functions
- 9. 9 Compilers versus interpreters • One major difference between C and Python is how the programs written in these two languages are executed. • With C programs, you usually use a compiler when you are ready to see a C program execute. • By contrast, with Python, you typically use an interpreter.
- 10. 10 Compilers versus interpreters • An interpreter reads the user-written programand performs it directly. • A compiler generates a file containing the translation of the program into the machine's native code. – The compiler does not actually execute the program! – Instead, you first execute the compiler to create a native executable, and then you execute the generatedexecutable.
- 11. 11 Variable declarations • C requires variable declarations, informing the compiler about the variable before the variable is actuallyused. • In C, the variable declaration defines the variable'stype. • No such thing in Python!
- 12. 20 Declaring a Variable • Declaring a variable is simpleenough. • You enter the variable's type, somewhitespace, the variable's name, and a semicolon: double x; • Value assignment is similar toPython: x=3; • x will actually hold the floating-pointvalue 3.0 rather than the integer3. • However, once you declare a variable to be of a particular type, you cannot change itstype!
- 13. 13 Declaring a Variable • In C, variable declarations belong at the top of the function in which they areused. • If you forget to declare a variable, the compiler will refuse to compile theprogram: – A variable is used but is notdeclared. • To a Python programmer, it seems a pain to have to include these variable declarations in a program, though this gets easier with morepractice.
- 14. 14 Whitespace • In Python, whitespace characters like tabs andnewlines are important: – You separate your statements by placingthem on separate lines, and you indicate the extent of a block using indentation. – like the body of a while or if statement • C does not use whitespace except for separatingwords. • Most statements are terminated with a semicolon ';', and blocks of statementsare indicated using a set of braces, '{' and '}'.
- 15. Whitespace 15 C fragment disc = b * b - 4 * a * c; if (disc < 0) { num_sol = 0; } else { t0 = -b / a; if (disc == 0) { num_sol = 1; sol0 = t0 / 2; } else { num_sol = 2; t1 = sqrt(disc) / a; sol0 = (t0 + t1) / 2; sol1 = (t0 - t1) / 2; } } Python equivalent disc = b * b - 4 * a * c if disc < 0: num_sol = 0 else: t0 = -b if disc / a == 0: num_sol = 1 sol0 = t0 / 2 else: num_sol = 2 ** t1 = disc sol0 sol1 = (t0 + = (t0 - 0.5 / a t1) / 2 t1) / 2
- 16. 16 Whitespace • As said, whitespace is insignificant in C. • The computer would be just as happy if the previous code fragment is written as follows: disc=b*b-4*a*c;if(disc<0){ num_sol=0;}else{t0=-b/a;if( disc==0){num_sol=1;sol0=t0/2 ;}else{num_sol=2;t1=sqrt(disc/a; sol0=(t0+t1)/2;sol1=(t0-t1)/2;}} • However, do not write your programs like this!
- 17. 30 Functions • Unlike Python, all C code must be nested within functions, and functions cannot be nested withineach other. • A C program's overall structure is typicallyvery straightforward. • It is a list of function definitions, one after another, each containing a list of statements to be executed when the function iscalled.
- 18. 18 Functions • A C function is defined bynamingthe return type, followed bythe function name, followed bya set of parentheses listing the parameters. • Each parameter is described by includingthe type of the parameter and the parametername. • Here's a simple example of a function definition: float expon(float b, int e) { if (e == 0) { return 1.0; } else { return b * expon(b, e - 1); } } This is a function named expon, which takes two arguments, first a floating point number and next an integer, and returns a floating point number.
- 19. 19 Functions • If you have a function that does not have any useful return value, then you'd use void as the return type. • Programs have one special function named main, whose return type is aninteger. • This function is the “starting point” for the program: – The computer essentially calls the program's mainfunction when it wants to execute theprogram. – The integer return value is largely meaningless; we'll always return 0 rather than worrying about howthe return value might beused.
- 20. Functions 20 C program int gcd(int a, int b) { if (b == 0) { return a; } else { return gcd(b, a % b); } } int main() { printf("GCD: %dn“, gcd(24,40)); return 0; } Python program def gcd(a, b): if b == 0: return a else: return gcd(b, a % b) print("GCD: " + str(gcd(24, 40)))