Programming in C
Variables, Controls, Functions
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
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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, ciel)
– 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 functions
or 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”
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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;
}
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Compiling and Running
a Java Program
unix> javac -d . *.java
Java
Code
Java
Bytecode
Java interpreter
translates bytecode to
machine code in JRE
javac Hello.java
Java compiler
Hello.java
JRE for
Linux
Hello.class
JRE for
Windows
unix> java hello.Hello
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Compiling and Running
a C Program
unix> gcc -o hello hello.c
printf.o
hello.c
Source
program
(text)
Prehello.i
processor
(cpp)
Modified
source
program
(text)
unix> hello
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Compiler hello.s Assembler hello.o
(cc1)
(as)
Assembly
program
(text)
Relocatable
object
programs
(binary)
Linker
(ld)
hello
Executable
object
program
(binary)
Language Commonality
• C and Java syntax have much in common
–
–
–
–
–
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Some Data Types
Arithmetic operators
Logical Operators
Control structures
Other Operators
Data Types
•
•
Both languages support the integral types int, short, long.
Both languages support the floating point types double and
•
•
•
•
Both languages support the character data type char
Both languages support the Boolean data type bool*
Both languages support enums
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).
•
•
float
* bool is a built-in type in Java. bool is available with the current C99 standard, but requires <stdbool.h>
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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
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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
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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 ( {, } ) begin and end of blocks
 Loop Practice
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Other Operators
These other operators are the same in C and Java
 ?: (tri-nary “hook colon”)
printf(“x is %s greater than y”, (x <= y ? “ not “ : “”));




<<, >>, &, |, ^ (bit operators*)
<<=, >>=, &=, |=,^=
[ ] (brackets for arrays)
( ) parenthesis for functions and type casting
*much more on these later
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Functions and 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
– Have (optional) 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
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C Function Examples
/* returns the average of two integer parameters */
int average( 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”);
}
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Function Prototypes
• A function prototype defines the function’s name,
(note the semi-colon).
return type, and parameters.
• For example from the previous slide
int average( int a, int b );
void bark( int n );
• Before a function may be called, you must provide
its prototype (or the function itself) so that the C
compiler can verify the function is being called
correctly.
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A Simple C Program
/* convertTemps.c */
#include <stdio.h>
/* prototype for CtoF */
double CtoF( double 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 */
double CtoF(double celsius)
{
double fahrenheit;
fahrenheit = 9.0 / 5.0 * celsius + 32;
return fahrenheit;
}
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Typical C Program
includes
#include <stdio.h>
double CtoF( double celcius);
defines, data type
definitions, global
variable declarations
function prototypes
main()
function definitions
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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 */
double CtoF(double celsius)
{
double fahrenheit;
fahrenheit = 9.0 / 5.0 * celsius + 32;
return fahrenheit;
}
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”.
gcc –o convert convertTemps.c
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tells the compiler to
create executable file
with the name convert
tells the compiler
the name of the input
file.
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
#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
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CtoF.h
/* CtoF.h */
/* #includes needed by prototypes */
/* #defines needed for prototypes */
/* prototype for functions defined in CtoF.c */
double CtoF( double celsius );
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CtoF.c
/* CtoF.c */
/* necessary #includes, #defines */
/* converts temp in Celsius to Fahrenheit */
double CtoF(double celsius)
{
double fahrenheit;
fahrenheit = 9.0 / 5.0 * celsius + 32;
return fahrenheit;
}
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convertTemps.c revisited
#include <stdio.h>
#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;
}
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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 myprogram.c
gcc -c CtoF.c
gcc -o myprogram myprogram.o CtoF.o
•
Or it can be done all in one step
– gcc -o myprogram myprogram.c CtoF.c
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Recursion
• C functions may be called recursively.
– 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
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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 )
printd( n / 10 );
/* more than 1 digit */
/* recursive call- n has 1 less digit */
printf( “%c”, n % 10 + ‘0’); /* base case --- 1 digit */
}
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Miscellaneous
• Comments
– Both languages support block comments surrounded by
/* and */,
– C99 standard also supports // end-of-line style
• Declaring variables
– Starting with the C99 standard, C variables may be
declared anywhere in the code, just like Java
• #defines
– C supports the definition of macros using #define
• #define MAX 42
// a simple name of an int
• More complex macros later
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Java/C Project example
• Number Theory - a problem about integers
• http://www.cs.umbc.edu/courses/undergradaute/3
13/fall09/code/numbertheory.txt
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