Introduction to C

Introduction to C

Topics Covered • Creating a program in C • editing, compiling, linking, running • C basics • program structure, variables, statements • Conversion from pseudocode • if, case, for, while, etc.. • Functions

Creating a C Program Externallibraries Texteditor Compiler Linker C source file Object file Executablefile Plain text (ASCII) file, human readable Machine code, readable by computer Final executable, contains code to load and run program

Creating Source Files • C source code is a simple text file • Can be created in any text editor • notepad • vi (simple Unix text editor) • gedit, kate, etc, etc.. • Has the extension .c • .C, .cc, .cpp are for C++ • .java for Java, etc.. • C source code cannot be run directly, it must be compiled and linked first – converted into machine code, then combined into executable form

Compiling the C Source Code • Compiling is the process of turning source code (human readable) into machine code (machine readable) • This is done by a compiler • the standard compiler is cc or gcc • syntax: gcc myprogram.c • The output of the compiler is an object file • is made up of machine code and symbol tables • cannot be run directly, as it contains no loader, and libraries are not yet linked • If source code is incorrect in any way, the compiler will return an error and terminate, or give a warning and continue

Linking to Create an Executable • Object files must be linked before they can be run • The linker performs the following tasks: • resolves external references to other libraries • printing to screen, graphics, etc.. • add operating specific code to load the program into memory • Unix linker is ld • this is also a part of gcc, which by default links immediately after compiling! • however they are still two separate processes • use the –c flag to prevent linking • Linking can also produce errors • external references not found in supplied libraries • no ‘main’ program to load into memory • Default output filename is a.out (in Unix, in Windows a.exe) • can be changed using the –o option of gcc

Running the Program • Once successfully linked, the program is just like any other executable file • to run, simply enter the filename and path at the command line ./a.out ./my_program • Unlike DOS and Windows, executable files in Unix do not end in .exe or .com • can have any name, any extension • executable status is indicated by permission bit ‘x’ • coloured green where available in ls listing • ls –l will show all permission bits in first column

Anatomy of a C Program • A C source file consists of many parts • pre-processor directives • comments • any section enclosed by /* */ • any part of line following the // symbol • function declarations and bodies • Every program must have one function called main • this is the PROGRAM module • other functions can be declared to represent other modules in the pseudocode • C is case-sensitive • While != while, my_Program != my_program, etc.. • C does not care about whitespace • tabs, spacing, newlines, etc, are meaningless, but still good programming practice!

Simple C Program #include <stdio.h> /* HelloWorld program Author: Andrew Busch Date: 14/08/06 */ int main ( void ){ printf ( “Hello World!\n" ) ; // another comment here }

Another C Program #include <stdio.h> /* Addnumbers program A simple program to add two numbers Author: Andrew Busch Date: 14/08/06 */ int main ( void ){ // variables are declared up here int num1, num2 ; int total ; // all the printf function from the stdio library printf ( "Please enter the first number\n" ) ; // now call scanf to read from the keyboard scanf ( "%d", &num1 ) ; printf ( "Now enter the second number\n" ) ; scanf ( "%d", &num2 ) ; // now calculate the total total = num1 + num2 ; // and display it printf ( "The total of %d and %d is %d\n", num1, num2, total ) ; }

C Syntax • C programs are comprised of functions • these are the equivalent of modules • one function must be called ‘main’ • A C function is defined by: • its name • the number and type of its inputs • its return type • The body of the function is contained within braces { } • this is the same as the pseudocode statements which make up a module

C Function Example function name inputs (parameters, arguments) int sum ( int num1, int num2 ){ // statements go here! } return type

C Functions • Functions other than main should be declared before they are used • this means telling the compiler that they exist, even if you have not written them yet, or they appear later in the file • it is not strictly necessary, but if it is NOT done, the compiler will make assumptions about these functions which may cause errors • Declaring a function is done by replacing the body in { } with a semicolon int sum ( int num1, int num2 ) ;

C Statements • A function is a collection of statements • A statement can be: • a variable declaration • an expression (including assignment) • a conditional statement (if, case) • a loop (for, while, do..while) • a function call • other special statements ( break, return, continue, etc..) • Combinations of the above are also possible • eg, a function call is used within an expression • Each statement is terminated with a semicolon ; • statements which have bodies (loops, conditionals) terminate when the loop body finishes – after the closing { }

Variable Declarations • Variables in C must be declared before than can be used • Syntax: typevar_name [ = initial_value ] ; • type can be any valid C data type • var_name can contain letters, digits, and _ • but cannot start with a number!! • initial value is optional • Multiple variables of the same type can be declared in a single statement int a ; int b = 10 ; double c, d = 15.23 ;

Variables Declarations • Variables should be declared at the start of a function • this was historically mandatory • modern compilers allow some flexibility • Variables inside the same function cannot have the same name • but variables in different functions can • these are still different variables! • Variables are only accessible within the function they are declared in • this is known as ‘scope’ • the variable only exists while the function runs, then it is destroyed • Variables can be declared outside of any function • ‘global’ variables – accessible to any function • global variables live for the duration of the program – they are not destroyed • should be used sparingly, or not at all

Expressions • Expressions range from very simple to extremely complex • An expression (formally) is either: • a literal value • a variable • a function call (with any necessary paramters) • an operator (with arguments) • arguments are also expressions! • Clearly, expressions can be quite long! • Examples: 3 ; a = b + 4 ; a = ( 4 < sqrt (7) ) ; printf ( “%f”, ( sin ( 17 – ( 5 / a % 13 )) + b / 4 ) / ( z + 3 – a == 13 )) ;

Operators in C • Classified by the number of operands • unary – 1 operand • binary – 2 operand • trinary – 3 operand • Some operators have restrictions on what type of expressions each operand may be • eg, LHS of = operator must be a variable • Classes of operators: • arithmetic: +, -, /, *, % • assignment: = • relational: <, <=, ==, >=, >, != • logical: &&, ||, ! • bit operators: ~, &, |, ^, <<, >> • parentheses: (), [] • special: *, ., &, ->, ?:

Operators • All operators have a ‘result’ associated with them! • even those that don’t ‘make sense’ • assignment (=), etc.. • Some operators have side-effects as well, eg ‘=‘, ‘++’, ‘+=‘, etc.. • Operators follow rules of precedence • (), [], ., -> L-R • all unary operators R-L • multiplicative L-R • additive L-R • bitshift L-R • inequality L-R • equality L-R • bitwise logical • AND L-R • XOR L-R • OR L-R • logical (same order as above • conditional L-R • assignment R-L

Shortcut Arithmetic Operators • Increment operators: • ++ either before or after a value • known as prefix or postfix increment operators • both increase the variable by 1 • postfix does this after returning the value • prefix does this before returning the value • Examples: • b = 10 ; • a = b++ ; // a is assigned 10 • a = ++b ; // a is assigned 11 • Decrement operators (--) are the same for -1

Shortcut Operators • Assignment can be integrated with most arithmetic and logical operations • operators are: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>= • variable is modified and overwritten with the result • eg: • a += 15 ; // 15 is added to a • b %= 4 ; // b is divided by 4, rem stored // in b • z <<= 2 ; // z is bit shifted two places left

Operator Examples • What is performed by the following C statements: int a, b, c, d = 4 ; a = b = c = d + 10 ; a += --b ; c %= d ; d = a + ( b = c <<= 1 ) ;

Logical and Relational Operators • In C, FALSE is represented by 0 • TRUE is equal to anything else • The output of logical operators is 0 (FALSE) or 1 (TRUE) • 7 || 0 equals 1 • 5 && 0 equals 0 • Care must be taken not to mix = and ==, as they are both VALID, but have completely different meanings! if ( a = 10 ){ // this statement is ALWAYS TRUE!! }

Input and Output in C • C has no ‘built-in’ IO functions • they must be imported from a library • the standard C IO library is <stdio.h> • to use this library, put this line in the preprocessor block: #include <stdio.h> • The functions of most interest are: • printf writes formatted output to the screen • scanf reads formatted input from the keyboard • ‘Formatted’ means that you can specify how the output will be written or the input will be read • as an integer, character, floating point number, string, etc..

printf Statement • Syntax: printf ( format_string, var1, var2, … ) ; • printf is somewhat unique in that it takes a varying number of parameters • depending on the format string, zero or more variables are also required • The format string can be any string of text • it will be directly copied onto the screen • the ‘%’ character however has a special meaning, depending on the next character to follow it

printf Statement • printf formatting characters: • %d, %i integer value • %f, %g floating point value • %c character • %x, %X hexidecimal number • %o octal number • %s string • The behaviour of some formatting chars can be modified, eg: • %lf, %lg is a long float, ie a double • %.2g specifies 2 digits of precision after decimal point • The return value of printf is the number of characters written to the screen – this value is rarely used • Each time a formatting (%) character appears, it is replaced by the corresponding variable in the parameter list, by position • ie, the first % symbol is matched with the variable immediately following the format string • printf will NOT perform checking to ensure the variables match, so be careful!

printf Statement • Other characters in printf statements are ‘special’ • escaped characters are preceded by a backspace • some common examples are: \n newline \r linefeed \t tab • others can be found with ‘man printf’ command in linux • Examples of printf: printf ( “Hello world\n” ) ; printf ( “The value of x is %i\n”, x ) ; printf ( “The average mark for CSE is %1.2g\n”, avg ) ; printf ( “%s is %d years old\n”, name, age ) ;

scanf Statement • scanf is the input equivalent of printf • The syntax is identical, ie scanf ( format_string, var1, var2, … ) ; • However, the address of the variable to store the input in must be passed, not the variable itself • this means, put a & in front of the variable name! • Eg, to read a single integer from the keyboard: int i ; scanf ( “%d”, &i ) ; • The return value of scanf is the number of items successfully read • this should always be checked to ensure that the input was in the correct format

Conditionals in C • C has conditional statements equivalent to the IF and CASE in pseudocode • IF..THEN..ELSE = if..else • CASE..IS = switch..case • The syntax of these statements is quite similar to their pseudocode equivalents

if Statements • Syntax: if ( expression ) statement ; else statement ; • The else part of this statement is optional • By default, only the single statement forms part of the if statement • to overcome this, multiple statements are surrounded in braces { } if ( expression ){ statement1; statement2; } else { statements… } • These if statements can be nested as in pseudocode

Common Errors with ‘if’ • Forgetting the braces can have unexpected results: if ( x < 10 ) x++ ; printf ( “x is now %d\n”, x ) ; • The printf statement is executed regardless of the value of x! • it is NOT part of the if statement, only the first statement is • Using a ‘=‘ instead of ‘==‘ can also have unexpected consequences if ( x = 0 ){ // this is ALWAYS false and x is now 0!! … }

switch Statements • switch is the C equivalent to CASE • Syntax: switch ( expression ){ case value1: statements ; … break ; case value2: statements ; … break ; default: statements ; … break ; } • The ‘break’ statements are essential – without them execution will continue into the next case block! • As with pseudocode, the case values must be literals, they cannot be variables!

Loops in C • C has loops equivalent to each of those used in pseudocode • WHILE..DO = while • REPEAT..UNTIL = do..while • FOR..DO = for • The syntax for each is a little different • Like conditionals, loops only process the next single instruction by default • this can be overcome by surrounding multiple statements inside { }

while Loop • Syntax: while ( expression ){ statements… } • expression is evaluated at the start of each iteration • the loop continues while it evaluates as TRUE (!=0) • If the braces are omitted, only the very next statement is looped

do..while Loop • Syntax: do{ statements ; } while ( expression ) ; • Identical to while loop, however expression is evaluated at the end of the loop • guaranteed to loop at least once • Unlike REPEAT statement, continues while the expression evaluates to TRUE, not FALSE

for Loop • Most complex C loop • very different syntax to pseudocode version • Syntax: for ( init_statement ; cont_expr ; incr_expr ){ statements ; } • It works as follows: • init_statement is executed ONCE only, before the loop starts the first time • cont_expr is executed before each loop, loop only continues if it evaluates to TRUE (same as while loop) • incr_expr is executed at the END of every loop • Typically: • init_statement is used to initialise the loop variable • cont_expr is used to specify the finishing value for the loop • incr_expr is used to update the loop variables (increment or decrement) • However they can be used for anything, and can be empty!

for Loop Example • This is typical example of a for loop int i ; for ( i = 0 ; i < 10 ; i++ ){ printf ( “%d\n”, i ) ; } • This would print out the values 0..9 to the screen

Functions • All program code in C must be contained within a function body: • the main function is the program block • other functions are modules • Functions in C can have only one direct output • this is the first type in the declaration, and does not have a name as in pseudocode • The output of a function is achieved using the return statement: return ( value ) ;OR return value ; • This indicates that the given value is the output of the function, and immediately terminates the function!

Function Example float par_resist ( float r1, float r2 ){ float total ; total = r1*r2 / ( r1 + r2 ) ; return ( total ) ; } return type arguments return statement

Calling Functions • Functions are called with the following syntax: function_name ( arg1, arg2, arg3 ) ; • Note that the types of the arguments are omitted • If a function has NO arguments, you still need the ()! • If the function returns a value, it can be assigned to a variable or used in another expression. Consider the previous example.. int main ( void ){ float r1 = 123.25 ; float r2 = 2300 ; float total ; total = par_resist ( r1, r2 ) ; }

Introduction to C