Programming for MSc Part I

Transcription

1 Herbert Martin Dietze University of Buckingham July 24, 2001 Abstract The course introduces the C programming language and fundamental software development techniques. These lectue notes by no means cover the course contents as they are merely intended to support the lecture. Thanks to Prof. Dr. Uwe Schmidt for teaching me C and giving me a lot of inspiration

2 July 24, 2001 Contents 1. Fundamentals (a) Programming Language Elements (b) Compiling, Linking and Running Programs (c) C Language Elements (d) Branching 2. Scalar Data Types (a) What Scalar Types are (b) Operations on Scalar Types (c) Introduction to Boolean Algebra (d) Enumerations (e) Defining own Types (f) Pointers (g) Increment and Decrement 3. Aggregate Data Types (a) Arrays (b) Structs 4. Control Flow (a) Loops (b) Advanced Branching 5. Dynamic Data structures (a) Allocating and Releasing Memory (b) List Structures 6. The Preprocessor and Macros Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 1

3 July 24, 2001 (a) The Preprocessors (b) Macros Herbert Martin Dietze 2

4 Part 1: Fundamentals (a) Programming Language Elements Computers and Humans Humans can use intuition to solve problems The computer only executes a sequences of instructions Simple operations (addition, comparison), but high speed Questions: Which instructions and in what order? Describe rules to solve a problem Algorithm Simple examples: A cooking recipe, or x 2 = x x A computer can t understand an algorithm without our help Not every algorithm can be made a computer program Computer Programs and Programming Languages A Computer Program can be executed on a computer A computer program implements an algorithm The computer program is written in a Programming Language The programmer creates a Source File Then a special computer program, the Compiler is invoked The Compiler generates machine code from the source file Every processor type uses its own machine language Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 3

5 Part 1: Fundamentals Programming Language Elements Low and High Level Programming Languages We call low level programming languages Assembly Languages Assembly language is a processor s language s readable form Primitive instructions, very close to the processor High level programming languages are processor independent More complex instructions, high abstraction level Some high level programming languages: Pascal, C, C++,... The C Programming Language Developed in the early seventies for the Unix operating system Later: ANSI standard Operating systems written in C: Windows, Unix, OS/2,... Popular general purpose programming language Small set of instructions, relatively little abstraction Example C Source Source file fiveptwo.c: int main (void) { 5*5; return 0; } Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 4

6 Part 1: Fundamentals Programming Language Elements Human Languages Analogy A programming language is just a language It consists of Characters, Words and Sentences We call them Tokens, Expressions and Statements Tokens A token is the smallest unit C can handle Atom Expressions consist of one or more tokens Examples: Text C Tokens hugo hugo *9 6, *, 9 (4+2)*9 (, 4, +, 2, ), *, 9 The compiler will always form the longest-possible token To avoid ambiguity rules for creating tokens are needed Expressions Expressions are what sentences are made of They can be just constants: 42 Operators can make expressions more complex: 6*9 Every expression has a value Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 5

7 Part 1: Fundamentals Programming Language Elements Statements A statement ( sentence ) is a command we want to execute It ends with a semicolon ; or a closing brace } An Empty statement contains nothing: ; An Expression statement consist of one expression: 5*5; A Compound statement bundles many to one: {5*5; 6*6;} There are some more categories of statements Example Let s take another look at fiveptwo.c: int main (void) { 5*5; return 0; } A list of two statements between { and } That consist of expressions: 5*5 and return 0 That consist of tokens: {, 5, *, 5, ;, return, 0, ;, } And some stuff we don t know yet Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 6

8 Part 1: Fundamentals (b) Compiling, Linking and Running Programs From the Source to an Executable Program The compiler compiles every file to machine language The result is one Object File for every source file The Linker bundles object files to an executable file The executable file can only be run on the target platform.c File.c File.c File.c File cc cc cc cc.o File.o File.o File.o File ld Library Executable Program Herbert Martin Dietze 7

9 Part 1: Fundamentals Compiling, Linking and Running Programs What a Program can do Technically a program only pushes 1s and 0s around From a our point of view it talks with the operating system Several categories of operations Input read something from a file or the keyboard Output write something to a file or the screen Arithmetics calculate something Data manipulation e.g. text, images etc. Operating systems and toolkits provide help for complex tasks: GUI windows Database access etc. These operations come in Libraries We don t want to reinvent the wheel! The C standard library The C language itself is rather primitive The C standard library is the standard toolkit for C programs It provides things like high mathematics (e.g. trigonometric functions), input and output etc. The linker will search the standard library automatically No useful C program without using the standard library Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 8

10 Part 1: Fundamentals Compiling, Linking and Running Programs A closer Look at the Compiler Compiling a C source involves two steps: 1. The Preprocessor expands Preprocessing directives that can be used to create macros or include other source files 2. The Compiler analyzes the result and generates machine code from it if there were no errors Source files that are not syntactically correct will be rejected Object files are no complete executable files! The Linker The linker bundles code from object files and libraries It also adds things needed by the operating system Complex programs consist of many modules The linker resolves dependencies Running an Executable File The operating system loads the program code into memory In every program there must be a entry point for execution The operating system now jumps to that entry point and starts executing instruction by instruction In a C program execution starts at main () Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 9

11 Part 1: Fundamentals Compiling, Linking and Running Programs C in the Unix Enviroment On Unix systems the compiler for C sources is cc (or gcc) The linker is ld The Linker normally gets invoked by the compiler The following commands compile and link the source fiveptwo.c to creae an executable file fiveptwo: herbert@paola:~/src $ gcc -c fiveptwo.c herbert@paola:~/src $ gcc -o fiveptwo fiveptwo.o herbert@paola:~/src $ _ Or all in one command: herbert@paola:~/src $ gcc -o fiveptwo fiveptwo.c herbert@paola:~/src $ _ We can now run the program: herbert@paola:~/src $./fiveptwo herbert@paola:~/src $ _ Does it do anything at all??? Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 10

12 Part 1: Fundamentals Compiling, Linking and Running Programs So what s 5*5 now? fiveptwo calculates fine, but how do we know the result? Solution: We need to output the result We therefore have to extend our source: #include <stdio.h> int main (void) { printf ("%d\n", 5*5); return 0; } printf () is a utility from the Standard C Library It outputs text to the screen The #include statement is an instruction to the preprocessor Now we can see the result: herbert@paola:~/src $ gcc -o fiveptwo fiveptwo.c herbert@paola:~/src $./fiveptwo 25 herbert@paola:~/src $ _ Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 11

13 Part 1: Fundamentals (c) C Language Elements Syntax Like a human language there are a set of rules The compiler can only work on syntactically correct sources Some rules: Statements end with semicolons or a closing braces There is a set of operators There is a set of reserved keywords On syntax errors the compiler will issue an error message Example for a syntax error in source file foo.c (does not end with semicolon or closing brace): #include <stdio.h> int main (void) { printf ("Hello World.\n") return 0; } herbert@paola:~/src $ gcc -c foo.c foo.c: In function main : foo.c:6: parse error before return herbert@paola:~/src $ _ Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 12

14 Part 1: Fundamentals C Language Elements Types Any kind of data in C has a type A type is a name for a category of data C has a list of builtin types (e.g. int for integer numbers or char for characters) You can t compare apples and oranges! Functions A function calculates something and returns the result We already know functions: Entering a number and then pressing the sin key on a pocket calculator In C a Function Definition looks like this: Return-Type Name (Arguments) Compound-Statement Between all those elements we can have any number of whitespaces (tabulators, newlines etc.) Example: int answer (void) { return 6*9-12; } Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 13

15 Part 1: Fundamentals C Language Elements Definitions vs. Declarations A complete function with body is called Function Definition There can be only one definition of the same function A function declaration looks like this: Return-Type Name (Arguments); The same funtion can be declared more than once Important: The compiler uses them to check the way we call the functions If missing: a function returning int is assumed Function declarations are also called Function Prototypes Definition and declaration must be consistent Header Files Header files are usually #include d by the preprocessor They contain declarations of all kinds: function prototypes, types, macros System headers are included by #include <headername.h> For our own headers we use #include "headername.h" Example: #include <stdio.h> for printf() and friends Header Files are the glue between modules in C Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 14

16 Part 1: Fundamentals C Language Elements Variables A variable is an object in memory that can store data Values are assigned using the assignment operator = Example: Output the results of 6 9 and Inefficient solution: printf ("%d\n", 6*9); printf ("%d\n", 6*9-12); Better: Store the result of 6 9 in a variable, output it, then calculate on and output the second result: int result; result = 6*9; printf ("%d\n", result); result = result - 12; printf ("%d\n", result); Creating Variables There are different kinds of variable declarations The simplest form is: Type Name; We can also create more than one variable: int a, b; This can only be done at the start of a Compound Statement or outside functions Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 15

17 Part 1: Fundamentals C Language Elements Variables Properties A variable is identified by its name A variable must be given a type Name and type chosen for a variable must be legal A variable must have been declared before use Values assigned to variables must be of compatible types After creation a variable has a random value Global and Local Variables Variables defined in compound statements are local They are invisible outside their block Variables defined outside compound statements are global They are visible through the whole source file Global variables should not be used side effects Example: int global; int foo (void) { int local = global; return local; } Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 16

18 Part 1: Fundamentals C Language Elements Function Arguments Function arguments are a special kind of variables They get initialized when the function is called They are ordinary local variables to their function Example: #include <stdio.h> int plustwo (int value); int main (void) { int fourty = 40; printf ("%d + 2 is %d\n", fourty, plustwo (fourty)); return 0; } int plustwo (int value) { return value + 2; } Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 17

19 Part 1: Fundamentals C Language Elements Comments Comments are used to document source code They get ignored by the compiler Comments are all between /* and */ It is good style to comment function heads or blocks Nesting comments (/* /*... */ */) does not work! There are a lot of different ways to format comments Matter of taste! Example: /* * This function adds 2 to value and * returns the result */ int another_plustwo (int value) { /* using the previous one-liner is more elegant, still this works well, too! */ value = value + 2; return value; } Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 18

20 Part 1: Fundamentals C Language Elements Names Names are used e.g. for functions or variables They can be almost freely chosen, but: C s builtin keywords are reserved They re not allowed to start with numbers They re not allowed to contain any special characters but the underscore Names starting with are reserved for system use There can be only one sybol of name name in one block Names Examples: legal illegal j j5 system name CaPiTaLS or NoT 5J $name int bad%$&name Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 19

21 Part 1: Fundamentals C Language Elements Reserved Words These words are reserved by the C language: auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while Basic Input and Output The printf() utility performs formatted output Example: printf ("A number: %d\n", 42) The scanf() utility performs formatted input Input must be written to a variable s address We get a variable s address using the & operator The Format string is mostly like in printf() Normally only the placeholders are needed Example: scanf ("%d", &var) But what if the user does not enter an integer? scanf() does not help us here! More sophisticated methods will come later! Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 20

22 Part 1: Fundamentals C Language Elements Example using printf and scanf #include <stdio.h> int getnum (void); /* main program: get input and display * the result. */ int main (void) { int value = getnum (); printf ("That was %d\n", value); return 0; } /* Prompt the user for an integer number * and return the result. No checks for * valid input format are performed. */ int getnum (void) { int num; printf ("Enter integer number!\n"); scanf ("%d", &num); return num; } Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 21

23 Part 1: Fundamentals C Language Elements Simple Arithmetics and Logical Expressions For integer numbers there are several arithmetic operators: a + b a - b a * b a / b a % b adds a to b subtracts b from a multiplies a by b divides a by b discarding the rest returns the rest of the division a by b Comparing expressions: a == b a!= b a > b a < b a >= b a <= b a equals b (not the same as a = b!) a does not equal b a is greater than b a is less than b a is greater or equal to b a is less or equal to b Combining expressions: expr1 && expr2 expr1 expr2!expr expr1 and expr2 are both true At least expr1 or expr2 is true The opposite of expr In C logical expressions are of the type int A false condition resolves to 0 A true condition resolves to something not equal to 0 Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 22