c intro2

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C Introduction / Basics 1/35

Introduction to C

Part 2: Basics

C Language TutorialSystem Programming 251-0053

Fall Semester 08

Mathias Payer, RZ H11
mailto:[email protected]:[email protected]

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Outline

Tutorial teaches basics of C

4 Lectures 09/25: Overview

10/02: Basic programming

10/10: Advanced programming

10/17: Environment

Hands-on work handled in exercise classes

Debugger examples

Inlining assembler instructions

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Overview (this lecture)

Variables

Declaration and handling Conversion and Casting

Automatic conversion, upcasts and downcasts

Assignments and Operators

Input / Output

The Unix I/O paradigm

Control Flow

if and switch statements and different loop types

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Variable declaration

Variables must be declared before they can be used

Variables are declared at the beginning of the codeblock

All variable declarations first, code later

C99 allows declaration in code blocks too

Variable names must be valid identifiers Keywords (void, int, struct, ...) are not allowed

A-Z, a-z and _ are allowed as first character

After the first character 0-9 are allowed as well

Watch out: C is case-sensitive -> fooBar != FoObAr

Type Identifier int foo;double bar;char ch;

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Initial value & literals

Variables can have an initial value (assigned at decl.)

int foo = 4;double xx = 3.1415;

Other literals and types:

Literals are used in the code to specify a numeric value

int: 23

long: 2342l (l as in Lancelot)

double: 23.4223.42d

2.3e42 (=2.3 * 1042)

float: 42.23f

4.2e-23 (=4.2 * 10-23)

char: 'c'

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Constants

A constant is a variable that cannot be changed afterits initialisation

C has the keyword const to tell the Compiler that avalue is constant

const int MAX_CHAR = 255;

const double PI = 3.1415;

The keyword is a prefix to the variable type

Only a (compile) warning message is printed if aconstant is overwritten in the program

Aliasing can overwrite constants Leads to strange effects (register vs. fetch from memory)

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Variable scope

int x; // in global scope

int main(){

while (...) {

...

}

}

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Variable scope

int x;

int main(){

int x ,y; // in local scope, this x hides top x

while (...) {

...

}

}

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Variable scope

int main(){

while (...) {

int x; // in (other) local scope

...

}

}

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Type Conversion

long long int

int

short

double

float

Automatic (implicit) conversion to 'bigger'/larger types Automatic (implicit) conversion from short or int to

float and double (possible loss of precision int->float)

Otherwise: Loss of precision and explicit cast needed

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Strange conversion

#include

int main() {int a, c;float b;a = 2147483647; // max int valueb = a; // int -> floatc = b; // float -> intprintf("Values: %d %f %d\n", a, b, c);

return 0;}

Values: 2147483647 2147483648.000000 -2147483648

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Explicit casting

The cast operator permits explicit casting:

valueInTypeA = (TypeA) valueInTypeB;

Cast from floating point to integer truncates value

int myInt;double myDouble = 23.42;

myInt = myDouble; // --> trunc to 23myDouble = (int)myDouble; // --> trunc to 23myDouble = myInt/2 ; // int.div. 23/2 = 11myDouble = (double)myInt/2; // flt.div. 23/2=11.5myDouble = myInt/2.0; // flt.div. 23/2=11.5

Floating point division,if one or both

operands are of typefloat

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Assignments

An assignment statement in C is specified by theoperator =

foo = bar + 2; foo

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Arithmetic Operations

C knows the following arithmetic operations:

+ Addition x = 2 + 3; - Subtraction x = 2 3;

* Multiplication x = 2 * 3;

/ Division x = 2 / 3;

% Modulo x = 2 % 3; Division is integer iff both operand types are int

Modulo means reminder of the division (5%3 = 2)

A ith ti h th d

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Arithmetic shorthands

Assignments which assign one operand can becompressed

x = x OP y x OP= y

+= x += y; x = x + y;

-= x -= y; x = x y;*= x *= y; x = x * y;/= x /= y; x = x / y;%= x %= y; x = x % y;

Short Form Translation

I t & D t

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Increment & Decrement

Another short hand for +1 and -1

x = x + 1; x++;

x = x 1; x--;

Prefix and postfix possible:

x++ vs. ++x

y = x++; y = x; x = x + 1;

y = ++x; x = x + 1; y = x;

x++ returns the current value of x and then increments x

++x increments the value of x and then returns x

Be aware of the difference!

B l L i

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Boolean Logic

C uses integers to represent boolean results

false (int)0 true (int)(any other value)

boolean type was introduced in C99

Operator==

!=

> 23 > 42

< 23 < 42

>= 10 >= 9

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Logic Operators

Boolean expressions can be linked and evaluatedtogether

AND Both operands need to be true ( a && b )

OR One of the operands needs to be true ( a || b )

NOT The operand is negated ( !a )

Precedence: ! > && > ||

Same as in algebraic calculations

AND OR NOTa b a && b a || b !aF F F F T

F T F T T

T F F T F

T T T T F

Operands

Lazy Evaluation

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Lazy Evaluation

C is lazy at the evaluation of logic comparisons

Careful with side effects!

(( bool1 ) && ( bool2 ))

bool2 is only evaluated if bool1 is true

(( bool1 ) || ( bool2 ))

bool2 is only evaluated if bool1 is false int bool = ( a == 0 ) && ( ++a );

a is only incremented if it was 0 beforehand

if a = 0 -> bool = 1, a = 1

if a = 1 -> bool = 0, a = 1

if a = 2 -> bool = 0, a = 2

bool1 && xx++

Bitwise logical operators

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Bitwise logical operators

Every bit in a variable is examined and changed

different from logical operators that use all bits to representtrue or false

Precedence: ~, , &, ^, |

Operator Name~ ~0011 => 1100

1011>>2 => 0010& AND 10 & 11 => 10^ XOR 10 11 => 01| OR 10 | 11 => 11

Examplenot

shift left

shift right

I/O Input and Output

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I/O Input and Output

The printf(...) function writes to the console

use #include to include the header file

printf("Hello World!\n");

takes at least one argument (more later)

Strings must be contained in double quotes ( "" )

Strings can contain ANSI-C escape sequences

\n (newline)

\t (tab stop)

\" (double quote)

\\ (backslash)

Formatted Output

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Formatted Output

Not only strings can be printed to the console!

The first argument to printf is a String that maycontain special format characters (with leading %)

printf(char*, arg1, arg2, ..., argn);

Format characters:

Type%d, %i

%o%x, %X

%u%c

%s%f%e, %E

%p%%

Token Outputint Decimalint Octalint Hexadecimalint Unsigned

char Character

char* String (until \0)double Decimaldouble Scientific (2.3e10)pointer Memory Addressnone Character '%'

Output example

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Output example

#include int main() {

int myInt = 23;float myFloat = 42.23;

printf("myInt = %d\n", myInt);printf("myFloat = %f\n", myFloat);printf("&myInt = %p\n", &myInt);return 0;

}

myInt = 23myFloat = 42.230000&myInt = 0xbffeefdc

Reading from std input

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Reading from std. input

scanf is very similar to printf

First argument is a format string

Other arguments are pointers to variables

scanf reads a string, parses it according to the formatstring and writes the interpreted data to the arguments

scanf(char*, &arg1, &arg2, ..., &argn);

&arg1 is the pointer to the memory location where arg1 is

Remember: C uses call by value (so passing arg1 wouldnot work)!

The format string can contain the same characters as theprintf format string

scanf("%d", &anInt);

reads an integer from the command line and saves it tothe address of anInt

if Statement

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if Statement

if (boolean_expression) statement;

statement is executed if boolean_expression is true

optional else block if statement is not true

if there are multiple statements, then a statement block canbe used

statement = '{' stmt1; stmt2; ... stmtn; '}'

if (foo==bar) {foo = 15;printf(equals);

} else

printf(does not equal);

true?

stmt stmt

noyes

...

Dangling-Else Problem

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Dangling Else Problem

if (a == 1)if (b == 1)a = 42;

elseb = 42;

C does not care aboutindentation

To which if does the else partbelong?

Compiler maps else to theclosest if without else

Use statement blocks to make itexplicit

One of the most commonmistakes!

if (a == 1) {if (b == 1)a = 42;

} else {b = 42;

}

Shorthand if-assignment

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Shorthand if assignment

Often an assignment is directly determined by the ifcondition

(condition) ? expression1 : expression2;

Is itself evaluated to an expression

Can be used inline

Handy for e.g. return values

Can be very cryptic if used inappropriately

if (foo>bar)max = foo;

elsemax = bar;

max = (foo>bar) ? foo : bar;

while loop

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e oop

Executed if and as long as the condition is true

Loop is executed 0 or more times

Condition is an expression that evaluates to a boolean

while (condition) {stmt1;stmt2;...

}...

true?

stmts

...

no

yes

for loop

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p

For loops have at least 4 statements

Initial statement (executed before loop is entered)

Loop condition (evaluated at loop head) Loop statement (executed at end of loop)

Loop body (executed if loop condition is true)

for (init_stmt; loop_condition; loop_stmt) loop_body;

loop_body can be replaced by a statement block#include int main(){

int i, sum=0;for (i=0; i

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Every while loop can be rewritten as a for loop andvice versa

init;while (cond) {stmt1;

stmt2;...end;

}...

for (init; cond; end) {stmt1;stmt2;

...}...

do .. while loop

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p

Executed once and as long as the condition is true

Loop is executed 1 or more times

Condition is an expression that evaluates to a boolean

do {stmt1;stmt2;...

} while(condition);... true?

stmts

...

no

yes

break & continue

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break can be used to 'break out' of a loop to thenext higher scope

continue immediately jumps to the beginning of theloop (without evaluating the rest)

#include int main(){

int x;for ( x=0; x

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if is good for 2 alternatives, but does not scale

C has the switch statement to select betweenmultiple statements

After a test the program continues at the correspondingcase

The default is executed if no match is found (and nothing is

executed if there is no default)

switch (expression) {case constant1: stmt1; stmt2; ... break;case constant2: stmt1; stmt2; ... break;case constant3: stmt1; stmt2; ... break;default: stmt1; stmt2; ...

}

Duff's Device

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for (count=NRITR; count>0; count--) mul *= count;

count = NRITR;switch (count % 8) /* count > 0 assumed */ {

case 0: do { mul *= count--;

case 7: mul *= count--;case 6: mul *= count--;case 5: mul *= count--;case 4: mul *= count--;case 3: mul *= count--;case 2: mul *= count--;case 1: mul *= count--;

} while ((count -= 8) > 0);}

Method for (handmade, compiler independent) loopunrolling

http://en.wikipedia.org/wiki/Duff's_device

Questions
http://en.wikipedia.org/wiki/Duff's_devicehttp://en.wikipedia.org/wiki/Duff's_device

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