Programming in C

1. Programming in C Variables, Controls, Functions

2. Different Kinds of Languages • Java is an object-oriented programming (OOP) language • Problem solving centers on defining classes that model “things” like Trucks, Persons, Marbles, Strings, and CandyMachine • Classes encapsulate data (instance variables) and code (methods) C is a procedural language • Problem solving centers on defining functions that perform a single service like getValidInt( ), search( ) and inputPersonData( ). • Data is global or passed to functions as parameters • No classes

3. Libraries • Because Java is an OOP language, its libraries consist of predefined classes that you can use in your applications • ArrayList, Scanner, Color, Integer • Because C is a procedural language, its library consists of predefined functions. • Char/string functions (strcpy, strcmp) • Math functions (floor, ceil, sin) • Input/Output functions (printf, scanf) • On-line C/Unix manual -- the “man” command • Description of many C library functions and Unix commands • Usage: man <function name> for C library functionsor man <command name> for Unix commands • man printf • man dir • Search for applicable man pages using the “apropos” command or “man -k” • Learn to use the man command using “man man”

4. Hello World In JAVA -- Hello.java is in a class package hello; public class Hello { public static void main( String[ ] args ) { System.out.println( “Hello World” ); } } In C -- Hello.c stands alone #include <stdio.h> int main( ) { printf( “Hello World\n” ); return 0; }

5. JRE for Linux Java Code Java Bytecode java Hello javac Hello.java Java interpreter translates bytecode to machine code in JRE Java compiler Hello.java Hello.class java Hello JRE for Windows Compiling and Runninga Java Program unix> javac -d . *.java unix> java hello.Hello

6. Compiling and Running a C Program unix> gcc -o hello hello.c printf.o Pre- processor (cpp) Compiler (cc1) Assembler (as) Linker (ld) hello.c hello.i hello.s hello.o hello Source program (text) Modified source program (text) Assembly program (text) Relocatable object programs (binary) Executable object program (binary) unix> hello

7. Language Commonality • C and Java syntax have much in common • Some Data Types • Arithmetic operators • Logical Operators • Control structures • Other Operators

8. Data Types • Both languages support the integral types int, short, long. • Both languages support the floating point typesdoubleandfloat • Both languages support the character data typechar • Both languages support enumerations - enum • C allows the signed (default) and unsigned qualifiers for integral types (char, int, short, long) • C does NOT support byte (use char instead) • Both languages support arrays using [ ] and indexing starting with zero (0).

9. True and False • Java supports the bool data type and the keywords true and false bool isRed = true; bool isLarge = false; • C does not support the bool data type. Integer variables and expressions are used as instead • Any non-zero value is considered “true” • A zero integer value is considered “false” int isRed = 55; /* “true” */ int isLarge = 0; /* “false” */

10. Arithmetic Operators Arithmetic operators are the same = is used for assignment +, -, (plus, minus) *, /, % (times, divide, mod) ++, -- (increment, decrement (pre and post)) Combinations are the same +=, -=, (plus equal, minus equal) *=, /=, %= (times equal, divide equal, mod equal) • Arithmetic Practice • Assignment Practice

11. Logical Operators Logical operators are the same in C and Java and result in a Boolean value. • && (and) • || (or) • ==, != (equal and not equal) • <, <= (less than, less than or equal) • >, >= (greater than, greater than or equal) • Boolean Logic Practice

12. Control Structures Both languages support these control structures which function the same way in C and Java • for loops • But NOT -- for (int i = 0; i < size; i++) • while loops • do-while loops • switch statements • if and if-else statements • braces ( {, } ) are used to begin and end blocks • Loop Practice

13. Other Operators These other operators are the same in C and Java • ?: (tri-nary “hook colon”) int larger = (x : y ? x : y); • <<, >>, &, |, ^ (bit operators*) • <<=, >>=, &=, |=,^= • [ ] (brackets for arrays) • ( ) parenthesis for functions and type casting *much more on these later

14. #defines • The #define directive can be used to give names to important constants in your code. This makes your code more readable and more easily changeable. • The compiler replaces every instance of the #define name with the text that it represents. • Note that there is no terminating semi-colon #define MIN_AGE 21 ... if (myAge > MIN_AGE) ... #define HELP “There is no hope” ... printf( “%s\n”, HELP); ...

15. typedefs • C allows you to define new names for existing data types (NOT new data types) • This feature can be used to give application-specific names to simple types typedef int Temperature; typedef long Width; • Or to give simple names to complex types - more on this later • Using typedefs makes future changes easier and makes the code more relevant to the application

16. Enumeration Constants • C provides the enum as a list of named constant integer values (starting a 0 by default) • Names in enums must be distinct • Often a better alternative to #defines • Example enum boolean { FALSE, TRUE }; enum months { JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC };

17. Miscellaneous • Comments • Both languages support block comments surrounded by /* and */ • C DOES NOT support // comment style* • Declaring variables • All variables must be declared at the beginning of the “block” in which they are used • Typecasting • Both languages allow typecasting - temporarily treating a variable as a different type • temp = (double) degrees / 4; • Ask me about C99

18. Functions vs. Methods • Java classes include methods which can be called from any code with appropriate access (recall public methods) • C functions are like Java methods, but they don’t belong to any class. Functions are defined in a file and may be either global to your program or local to the file in which they are defined*. • Like Java methods, C functions • Have a name • Have a return type • May have parameters • Pass arguments “by value” • Unlike Java methods, a function in C is uniquely identified by its name. Therefore, there is no concept of method overloading in C as there is in Java. There can be only one main( ) function in a C application. *more on this later

19. C Function Examples /* returns the average of two integer parameters */ int intAverage( int a, int b ) { return ( (a + b) / 2 ); } /* Simulates dog barking */ void bark (int nrTimes) { int n; for (n = 0; n < nrTimes; n++) printf(“Arf!\n”); }

20. Function Prototypes (note the semi-colon). • A function prototype defines the function’s name, return type, and parameters. • For example from the previous slide void bark( int n ); int average( int a, int b ); • Before a function may be called, you must provide its prototype (or the function definition itself) so that the C compiler can verify the function is being called correctly.

21. Recursion • C functions may be called recursively. • Typically a function calls itself • A properly written recursive function has the following properties • A “base case” - a condition which does NOT make a recursive call because a simple solution exists • A recursive call with a condition (usually a parameter value) that is closer to the base case than the condition (parameter value) of the current function call • Each invocation of the function gets its own set of arguments and local variables

22. Recursion Example /* print an integer in decimal ** K & R page 87 (may fail on largest negative int) */ #include <stdio.h> void printd( int n ) { if ( n < 0 ) { printf( “-” ); n = -n; } if ( n / 10 ) /* (n / 10 != 0) -- more than 1 digit */ printd( n / 10 ); /* recursive call: n has 1 less digit */ printf( “%c”, n % 10 + ‘0’); /* base case --- 1 digit */ }

23. A Simple C Program /* convertTemps.c */ #include <stdio.h> /* includes prototype for printf, etc */ typedef double Temperature; /* prototype for CtoF */ Temperature CtoF( Temperature degreesCelsius); int main( ) { int temp; for (temp = 0; temp < 100; temp += 20) printf(“%2d degrees F = %5.2f degrees C\n”, temp, CtoF( temp )); return 0; } /* converts temperature in Celsius to Fahrenheit */ Temperature CtoF(Temperature celsius) { Temperature fahrenheit; fahrenheit = 9.0 / 5.0 * celsius + 32; return fahrenheit; }

24. Typical C Program includes #include <stdio.h> typedef double Temperature; Temperature CtoF( Temperature celcius); int main( ) { int temp; for (temp = 0; temp < 100; temp += 20) printf(“%2d degrees F = %5.2f degrees C\n”, temp, CtoF( temp )); return 0; } /* converts temperature in Celsius to Fahrenheit */ Temperature CtoF(Temperature celsius) { Temperature fahrenheit; fahrenheit = 9.0 / 5.0 * celsius + 32; return fahrenheit; } defines, typedefs, data type definitions, global variable declarationsfunction prototypes main() function definitions

25. Compiling on Unix Traditionally the name of the C compiler that comes with Unix is “cc”. On the UMBC GL systems, use the GNU compiler named “gcc”. The default name of the executable program that is created is a.out unix> gcc convertTemps.c

26. Compiler Options • -c • Compile only (create a .o file), don’t link (create an executable) gcc -c convertTemps.c • -o filename • Name the executable “filename” instead of a.outgcc -o project1 convertTemps.c • -Wall • Report all warningsgcc -Wall convertTemps.c • -ansi • Force adherence to the C ANSI standardsgcc -ansi convertTemps • Use them together gcc -ansi -Wall -o project1 convertTemps.c

27. Header Files • When a file contains functions to be reused in several programs, their prototypes and important #defines are usually placed into a header ( .h ) that is then #included where needed. • Each .h file should be “stand alone”. That is, it should and #defines needed by the prototypes and #include any .h files it needs to avoid compiler errors. • In this example, the prototype for CtoF() would be placed into the file CtoF.h which would then be #included in convertTemps.c or any other .c file that used CtoF( ) • The code for CtoF( ) would be placed int CtoF.c

28. CtoF.h /* CtoF.h */ /* #includes needed by prototypes */ /* relevant #defines */ /* relevant typedefs */ typedef double Temperature; /* prototype for functions defined in CtoF.c */ Temperature CtoF( Temperature celsius );

29. CtoF.c /* CtoF.c */ /* necessary #includes, #defines */ #include “CtoF.h” /* converts temp in Celsius to Fahrenheit */ Temperature CtoF(Temperature celsius) { Temperature fahrenheit; fahrenheit = 9.0 / 5.0 * celsius + 32; return fahrenheit; }

30. convertTemps.c revisited #include <stdio.h> /* note angle brackets */ #include “CtoF.h” /* note quotes */ int main( ) { int temp; for (temp = 0; temp < 100; temp += 20) printf(“%2d degrees F = %5.2f degrees C\n”, temp, CtoF( temp )); return 0; }

31. Compiling and linking • When a program’s code is separated into multiple .c files, we must compile each .c file and then combine the resulting .o files to create an executable program. • The files may be compiled separately and then linked together. The -c flag in the first two command tells gcc to “compile only” which results in the creation of .o (object) files. In the 3rd command, the presence of .o extension tells gcc to link the files into an executable gcc -c -Wall -ansi convertTemps.c gcc -c -Wall -ansi CtoF.c gcc -Wall -ansi -o myProgram convertTemps.o CtoF.o • Or it can be done all in one step • gcc -Wall -ansi -o myProgram convertTemps.c CtoF.c

32. Java/C Project example • Number Theory - a problem about integers • http://www.cs.umbc.edu/courses/undergradaute/313/fall09/code/numbertheory.txt