Lecture 2 - stat.purdue.edu · Lecture 2 Xiaoguang Wang STAT 598W January 16th, 2014 (STAT 598W)...

Lecture 2

Xiaoguang Wang

STAT 598W

January 16th, 2014

(STAT 598W) Lecture 2 1 / 41

Outline

1 GNU compiler and debugger

2 Pointers and Arrays

3 Structures

4 Compilation Process

5 Exercises


Outline



3 Structures


5 Exercises


GNU compiler

GNU Compilers: gcc (for C compiling), g++ (for C++ compiling)

-c : compile the code file into a machine code file

-o : produce the final executable file/output

-g : enable the executable file to be debugged with GDB debugger

-Wall : have the compiler generate many warnings about syntacticallycorrect but questionable looking code

-lm : Compile a C program that uses math functions such as ”sqrt”.

-lefence : Compile a C program with the ”electric fence” library.

For more information, please read the tutorial from the link below:

http://www3.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html


GDB

GDB: GNU Debugger.

Debugger for many programming languages: C, C++, FreePascal,Fortran, etc.

Like GCC, it is part of the GNU project. (http://www.gnu.org/)

Command line interfacing only. But emacs provides a nice front-end.

It debugs executable files only.


Some GDB commands

gdb name_program: start gdb.

b [file:]line: set breakpoint at line number in file.

run [arglist]: start your program with an arglist.

p expr: display the value of an expression.

c: continue running a program.

n: next line (stepping over function calls)

s: next line (stepping into function calls)


More information

http://www.gnu.org/software/gdb/documentation/

http://sourceware.org/gdb/current/onlinedocs/gdb/

http://refcards.com/docs/peschr/gdb/gdb-refcard-a4.pdf


Outline



3 Structures


5 Exercises


Pointers

Pointer: variable that contains the address of a variable.

Pointers and arrays are closely related.

The operator & gives the address of an object. For example:

p = &c

assigns the address of c to the variable p. (p is pointing to c).

The operator * is the “indirection” operator. When applied to apointer it accesses the object the pointer points to.

Pointer declaration:

type *variable_name


Example

int x=1,y=1;

int *ip;

ip=&x;

y=*ip;

*ip=0;

Can you explain what this code is doing?


Pointers and Functions

By default C passes arguments to functions by value.

The only way to change a variable in the calling function is by usingpointers.

What is the problem with the following code?

void swap(int x, int y){

int temp;

temp=x;

x=y;

y=temp;

}


Exercise

Fix the swap function using pointers.


Arrays

Syntax: type name[size]. Example:

int a[10];

This notation defines an array with elements a[0],a[1],...,a[9].

Suppose that pa is a pointer to a[0]. Then we have to state:

int *pa;

pa= &a[0];

Clearly, if i=1,...9, then:

pa+i is the address of the component a[i] and therefore pa+i=&a[i].a[i] has the same value than *(pa+i).

Then there is a 1-1 relationship between pointers and arrays.


Arrays

Then the relationship pa=&a[0] can be written as pa=a.

Then when array is passed to a function, what is passed is thelocation of the first element.

Example: (include this function in your current directory)

int strlen(char *s){

int n;

for(n=0; *s != ’\0’; s++)

n++;

return n;

}


Pointer operations

As long as two pointers point to the same array:

When you substract them the result is the number of elementsbetween those pointers.

One pointer is greater than the other if it points beyond where theother one points.

You can compare them by equality and inequality.

Example:

int array1[10], array2[10];

int *ip1, *ip2 = &array2[0];

int *ep = &array1[10];

for(ip1 = &array1[0]; ip1 < ep; ip1++)

*ip2++ = *ip1;


Increment and Decrement Operators

The ”for” loop of the code from last page works as follows:

*ip2 = *ip1;

ip2 = ip2 +1;

Note that even though the increment operator ”++” has the highestprecedence, there is still a difference between ”prefix” and ”postfix”. Theprefix usage ”++i” first increases i by 1 and then the expression takes asits value the new stored value of i. However, the postfix usage ”i++”causes the value of i incremented by 1 while the value of the expression isstill the value of i before its increase.


Memory Allocation

It is problematic and inefficient to work with fixed-size arrays.

malloc: returns a pointer to n bytes of memory. Example:

#include <stdlib.h>

char *line;

int linelen = 100;

line = malloc(linelen);

Note that 1 char uses 1 byte of memory.

If we need to allocate space for a different class of variables, usesizeof() operator:

int *ip = malloc(100 * sizeof(int));


Memory allocation

If malloc fails in the memory allocation, it returns a NULL pointer.Then it is important to include this warning in your code:

int *ip = malloc(100 * sizeof(int));

if(ip == NULL)

{

printf("out of memory\n");

return 0;

}

After the memory is used, we can deallocate it by using:

free(ip);

Obviously, it is not mandatory if you use your memory only onceduring execution.


Memory allocation

Sometimes you need to re-define your allocated memory. Use:

ip = realloc(ip, 200 * sizeof(int));


Always Allocate memory for pointers

Once you define a pointer, always ask yourself what the pointer points to,i.e., the location the pointer pointing to. What’s the problem of the codebelow?

int *fun()

{

int *point;

*point=12;

return point;

}


Always Allocate memory for pointers

Don’t forget to allocate memory for your pointer!Correction:

int *fun()

{

int *point = malloc(sizeof *point); /* Mandatory. */

*point=12;

return point;

}


Pointers to functions

Syntax:

type (*name)(arguments)

Examples:

double (*func)(double x);

double (*myFunc)(double x);

double (*AnyFuncName)(double x);



Recall:

Any pointer to function has the syntax:

type (*name)(arguments);

You can simplify the pointer definitions by using typedef, forexample:

typedef int (*cmpFcn)(const string &, const string &);

and afterwards, you can initialize a function pointer:cmpFcn pf2=lengthcompare;

A pointer to function argument can be declared as:

int (*)(const string &, const string &);

in your header files.



If you use a pointer to function inside any function, you don’t need touse the * operator to call the pointed function, just use the pointer’sname instead.


Pointers to functions: Example

How to interprete the following lines?

int (*pfi)();

int *pfi();

typedef int (*funcptr)();

funcptr pfi;

\\\\\\\\\\\

extern int f1();

pfi = &f1;

pfi = f1;

\\\\\\\\\\\

int (*pfr)(arg1, arg2);

int *pfr(arg1, arg2);

(*pfr)(arg1, arg2);


Outline



3 Structures


5 Exercises


Basics

Collection of one or more variables. They are grouped under a singlename.

Example:

struct point {

int x;

int y;

}

You can declare your structure as usual:

struct point pt={2,1};

And you can call its members:

printf("%d,%d", pt.x, pt.y);


Operations on structures

We can do several operations on structures:

Copy.

Assign.

Take its address.

Access its members.


Pointers to structures

We can declare pointers to structures, in the usual way:

struct point *pp;

Note that *pp is the structure and (*pp).x, (*pp).y are themembers.

Pointers to structures admit the following notation:

(*pp).x = pp->x ;

(*pp).y = pp->y ;

Using structures we can introduce nice data schemes, like trees andlinked lists. If you are interested there are a lot of web resources, forexample:http://publications.gbdirect.co.uk/c_book/chapter6/structures.html


Important commands

enum: we can construct a list of named constants which areassociated with integer values. Example:

enum Colours {Red, Green, Blue, Indigo, Violet};

Colours myCol;

Colours myCol2 = Green;

typedef: add a new name for an existing type of variable. Example:

typedef int Int32;

Int32 myVar = 8;


Outline



3 Structures


5 Exercises


Compilation Process


C Preprocessor

It is a macro processor (cpp) that is used by the C/C++ compiler inorder to transform your program before compilation.

The most usual macros are:

#include: File inclusion.#define: macro substitution.


File Inclusion

The macro #include<filename> or #include"filename" takessource from “filename” to be used by the compiler.

If the filename is quoted, the searching process starts where thesource program is found.

If the filename is enclosed by < >, searching begins at /usr/includefolder.


Macro substitution

A definition has the form:

#define name replacement_text

Then subsequent ocurrences of the token “name” will be replaced by“replacement text”.

Example:

#define MAXIMUM 300

#define forever for(;;)

#define printmax if(a<b) printf("%i",b);

#define sqr(x) ((x)*(x));


Example

Suppose we have the following C code:

#include<stdio.h>

#define MAXI 20

int main(){

int i=4;

printf("%i\n", i+MAXI);

return 0;

}

If you use gcc compilex.c -o compilex you can do the entirecompiling process.


Assembling

Once the C preprocessor has stripped the source code of thecomments and expanded the preprocessor commands, the compilertranslates the C code to assembly language (machine level code).

Use the command gcc -S compilex.c to create compilex.s(assembly file).

This code contains instructions that manipulate memory andprocessor directly.


Compiling

The compiler takes the assembly code and converts the machine-levelinstructions into binary code.

The binary file will be called compilex.o. You can see it with thecommand: gcc -c compilex.c


Linking

In order to create the executable file we need to use the linker toprocess the main function, its arguments and link it with other binaryfiles that our program needs.

The linker will call the printf binary file on the standard library. (if wehave header files or libraries called with #include, the process is thesame).

The linking process can be done withgcc compilex.o -o compilex. The final output is the executablefile.


Outline



3 Structures


5 Exercises


Midpoint Rule

Recall that the definite integral of a real-valued function f (x) can beapproximated by: ∫ b

af (x)dx ≈

n∑i=1

f (xi )∆xi

where ∆xi = b−an and xi = xi−1 + ∆xi , x1 = a + ∆xi

2 .

Implement the Midpoint rule using as inputs: pointer to a functionf (x), a, b and number of intervals n.


Midpoint Rule

Test your program with the integrals (Duffy, 2004):∫ 1

0x3dx

∫ 1

0

log(x)

1 − x2dx


Lecture 2 - stat.purdue.edu · Lecture 2 Xiaoguang Wang STAT 598W January 16th, 2014 (STAT 598W)...

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