[Steve Oualline] 13. Simple Pointers

Steve Oualline
Practical C Programming

13. Simple Pointers


183p
Pointers are also called address variables because they contain the addresses of other variables.

184p
Pointers can be used as a quick and simple way to access arrays. (...) Pointers can be used to create new variables and complex data structures such as linked lists and trees.

185p
The operator ampersand (&) returns the address of a thing which is a pointer.
The operator asterisk (*) returns the object to which a pointer points.

Operator - Meaning
* - Dereference (given a pointer, get the thing referenced)
& - Address of (given a thing, point to it)

The operator ampersand (&) returns the address of a thing which is a pointer.
The operator asterisk (*) returns the object to which a pointer points.

int *thing_ptr; // declare a pointer to a thing
thing_ptr = &thing; // point to the thing
*thing_ptr = 5; // set "thing" to 5
// The expression &thing is a pointer to a thing. The variable thing is an object
// thing_ptr points to any integer. It may or may not point to the specific variable thing.

The & (address of operator) gets the address of an object (a pointer).
The * (dereference operator) tells C to look at the data pointed to, not hte pointer itself.

http://www.cplusplus.com/doc/tutorial/pointers.html

http://www.cprogramming.com/tutorial/lesson6.html


187p
Several pointers can point to the same thing.

188p
Pointers as Function Arguments

The only result of a function is a single return value.

http://en.wikipedia.org/wiki/Call_by_value#Call_by_value

http://www.cplusplus.com/doc/tutorial/functions2.html

NULL pointer
locale.h
http://www.cplusplus.com/reference/clibrary/clocale/

189p
const Pointers

const char *answer_ptr = "Forty-Two";

char *const name_ptr = "Test";

const char *const title_ptr = "Title";

191p
Pointers and Arrays

(reminding...) 서 교수님:
number[10] 이라고 배열 선언을 하면 컴파일러는 10개의 연속된 데이터를 위한 공간을 확보하고 그 첫번째 공간의 주소를 number 에 넣습니다. 그래서 number 는 number[0] 의 주소를 가지는 것입니다.

number[1] 은 배열에서  number[0] 다음의 값을 가지는데 그 공간의 주소값을 알고 싶으면 &number[1] 이라고 하든지 number+1  이라고 하면 됩니다.

거꾸로, number 에서 시작하여 두번째 즉 number[1] 의 값을 주소로부터 얻고 싶으면 number[1] 이라고  하든지 *(number+1) 로 하여 얻을 수 있습니다.

192p
A pointer can be used to find each element of the array.

197p
Using Pointers to Split a String

strchr()
http://www.cplusplus.com/reference/clibrary/cstring/strchr.html


201p
Pointers and Structures

Instead of having to move a lot of data around, we can declare an array of pointers and then sort the pointers.


Command-Line Arguments

main (int argc, char *argv[])
{

The parameter argc is the number of arguments on the command line (including the program name).
The array argv contains the actual arguments.

(reminding...)
터미널에서, 말씀하신대로 "./V2008122-01 input.txt"라고 치면 다음과 같이 나옵니다.

argc=2
argv[0]=./V2008122-01
argv[1]=input.txt

서 교수님:
argv[0] = 실행프로그램 이름; 1번째는 항상 실행프로그램의 패스/이름 이 들어갑니다.
이번에는 argv[1] 에 들어갈 두 번째 값을 주었기 때문에 그 값이 프린트 된 것입니다.
command-line arguments는 shell 프로그램이 fileio 함수를 호출할 때 매개변수로 주는 것이고, 좀 더 엄밀하게는 OS 가 main 함수를 호출할 때 주는 것입니다.

http://www.cplusplus.com/reference/clibrary/cstdio/fprintf.html

Example 13-12: print.c

// formats files for printing
// usage: print [options] file(s)

#include <stdio.h>
#include <stdlib.h>

int verbose = 0; // verbose mode (default = false)
char *out_file = "print.out"; //output filename
char *program_name; // name of the program for erros
int line_max = 66; // number of lines per page

void do_file(char *name)
{
    printf("Verbose %d Lines %d Input %s Output %s\n",
           verbose, line_max, name, out_file);
}

void usage (void)
{
    fprintf(stderr, "Usage is %s [options] [file-list]\n",
            program_name);
    fprintf(stderr, "Options\n");
    fprintf(stderr, "    -v    verbose\n");
    fprintf(stderr, "    -l<number> Number of line\n");
    fprintf(stderr, "    -o<name>    Set output filename\n");
    exit(8);
}

int main (int argc, char *argv[])
{
    program_name =argv[0];
   
    while ( (argc>1) && (argv[1][0] == '-') )
        // argv[1][1] is the actual option character
    {
        switch (argv[1][1]) {
            case 'v':
                verbose = 1;
                break;
               
            case 'o':
                out_file = &argv[1][2];
                break;
           
            case 'l':
                line_max = atoi(&argv[1][2]);
                break;
           
            default:
                fprintf(stderr, "Bad option %s\n", argv[1]);
                usage();
        }
       
        ++argv;
        --argc;
    }
   
    if (argc == 1) {
        do_file("printf.in");
    }
    else {
        while (argc>1)
        {
            do_file(argv[1]);
            ++argv;
            --argc;
        }
    }
   
    return(0);
}

Xcode 실행창:

Verbose 0 Lines 66 Input printf.in Output print.out

terminal:

999:~/cintro/ch13/eg12 lym$ ./ch13eg12
Verbose 0 Lines 66 Input printf.in Output print.out
999:~/cintro/ch13/eg12 lym$ ./ch13eg12 i am tired
Verbose 0 Lines 66 Input i Output print.out
Verbose 0 Lines 66 Input am Output print.out
Verbose 0 Lines 66 Input tired Output print.out
999:~/cintro/ch13/eg12 lym$ ./ch13eg12 -v -l128 -0title xfile yfile zfile
Bad option -0title
Usage is ./ch13eg12 [options] [file-list]
Options
        -v      verbose
        -l<number> Number of line
        -o<name>        Set output filename
999:~/cintro/ch13/eg12 lym$ ./ch13eg12 -v -l128 -otitle xfile yfile zfile
Verbose 1 Lines 128 Input xfile Output title
Verbose 1 Lines 128 Input yfile Output title
Verbose 1 Lines 128 Input zfile Output title

208p
A pointer does not create any new space for data, but just refers to data that is created elsewhere.

http://www.cplusplus.com/doc/tutorial/pointers.html
The identifier of an array is equivalent to the address of its first element, as a pointer is equivalent to the address of the first element that it points to, so in fact they are the same concept.

An array can be considered a constant pointer.

http://www.cprogramming.com/tutorial/lesson6.html
Arrays can act just like pointers.