CS102B - Lesson 7

Pointer Examples

#include <stdio.h>

int main()
{
    int x=1, y=2, z[10];
    int *ip;            // ip is a pointer to an int

    ip = &x;            // ip now points to x
    printf("ip=%x\n", ip);   // prints address of x

    y = *ip;            // y is now 1
    printf("y=%d\n", y);

    *ip = 0;            // x is now 0
    printf("x=%d\n", x);

    ip = &z[0];         // ip now points to z[0]
    printf("ip=%x\n", ip);

    return 0;
}

• Every pointer points to a specific data type...

with One Exception - pointer to void

#include <stdio.h>

int main()
{
    char *ptr = NULL; // Null pointer
    printf("%c", *ptr);

    return 0;
}

• Segmentation Fault when you try to deference it!!

Pointer Unary Operators * and & with Arithmetic Operators

#include <stdio.h>

void printArray(int arr[])
{
    for (int i=0; i<10; i++)
        printf("%d ", arr[i]);
    printf("\n");
}

void printArrayAddress(int arr[])
{
    for (int i=0; i<10; i++)
        printf("%x ", &arr[i]);
    printf("\n");
}

int main()
{
    int intArr[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    printArray(intArr);
    printArrayAddress(intArr);

    int *intPtr = intArr;           // defaults to &intArr[0]
    printf("intPtr=%x\n", intPtr);
    printf("*intPtr=%d\n", *intPtr);

    int *intPtr2 = &intArr[2];      // address of intArr[2]
    printf("intPtr2=%x\n", intPtr2);
    printf("*intPtr2=%d\n", *intPtr2);

    printf("\n");

    *intPtr2 = *intPtr2 + 10;       // adds 10 to arr[2]
    printf("intPtr2=%x\n", intPtr2);
    printf("*intPtr=%d\n", *intPtr2);

    int y;
    y = *intPtr2 + 1;               // gets arr[2] and adds 1
    printf("y=%d\n", y);

    *intPtr2 += 10;                 // adds 10 to arr[2]
    y = *intPtr2;
    printf("y=%d\n", y);

    ++*intPtr2;                     // adds 1 to arr[2]
    y = *intPtr2;
    printf("y=%d\n", y);

    (*intPtr2)++;                   // adds 1 to arr[2]
    y = *intPtr2;
    printf("y=%d\n", y);

    *intPtr2++;                     // increments pointer to arr[3]! That's it!
    y = *intPtr2;
    printf("y=%d\n", y);
    return 0;
}

• Be careful with order of operations!!

Wrong Swap

#include <stdio.h>

// K&R Pg. 95
void swap(int x, int y)
{
    int temp;

    temp = x;
    x = y;
    y = temp;
}

int main()
{
    int x = 1;
    int y = 2;
    printf("before: x=%d, y=%d\n", x, y);

    swap(x, y);
    printf("after:  x=%d, y=%d\n", x, y);
}

Correct Swap

#include <stdio.h>

// K&R Pg. 96
void swap(int *px, int *py)
{
    int temp;

    temp = *px;
    *px = *py;
    *py = temp;
}

int main()
{
    int x = 1;
    int y = 2;
    printf("before: x=%d, y=%d\n", x, y);

    swap(&x, &y);
    printf("after:  x=%d, y=%d\n", x, y);
}

getint

#include <stdio.h>
#include <ctype.h>

// K&R Pg. 97
int getch();
void ungetch(int);

#define BUFSIZE 100

char buf[BUFSIZE];  // buffer for ungetch
int bufp = 0;       // next free position in buf

int getch(void) // get a (possibly pushed back) charater
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c) // push character back on input
{
    if (bufp >= BUFSIZE)
        printf("ungetch:too many characters\n");
    else
        buf[bufp++] = c;
}

// getint: get next integer from input into *pn
int getint(int *pn)
{
    int c, sign;

    while (isspace(c = getch())) // skip white space
        ;

    if (!isdigit(c) && c != EOF && c != '+' && c != '-')
    {
        ungetch(c); // it's not a number
        return 0;
    }
    sign = (c == '-') ? -1 : 1;

    if (c == '+' || c == '-')
        c = getch();

    for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');

    *pn *= sign;

    if (c != EOF)
        ungetch(c);

    return c;
}

int main()
{
    int c, pn;
    c = getint(&pn);
    printf("c=%d pn=%d", c, pn);
    return 0;
}

strlen

#include <stdio.h>

// K&R Pg. 99
// strlen:return length of string s

int strlen(char *s)
{
    int n;

    for (n=0; *s != '\0'; s++)
        n++;
    return n;
}

int main()
{
    char str[] = "ABCDEFG";
    printf("strlen=%d\n", strlen(str));
    return 0;
}

Another strlen

#include <stdio.h>

// K&R Pg. 103
// strlen:return length of string s

int strlen(char *s)
{
    char *p = s;

    while (*p != '\0')
        p++;
    return p - s;
}

int main()
{
    char str[] = "ABCDEFG";
    printf("strlen=%d\n", strlen(str));
    return 0;
}

alloc

#include <stdio.h>

// K&R Pg. 101

#define ALLOCSIZE 10000 // size of available space

static char allocbuf[ALLOCSIZE]; // storage for alloc
static char *allocp = allocbuf;  // next free position

char *alloc(int n) // return pointer to n characters
{
    if (allocbuf + ALLOCSIZE - allocp >= n) // it fits
    {
        allocp += n;
        return allocp - n; // old p
    }
    else // not enough room
        return 0;
}

void afree(char *p) // free storage pointed to by p
{
    if (p >= allocbuf && p < allocbuf + ALLOCSIZE)
        allocp = p;
}

int main()
{
    char *spacePtr;
    int n = 10;
    spacePtr = alloc(n);
    printf("spacePtr=%x, allocp=%x\n", spacePtr, allocp);

    char *spacePtr2;
    spacePtr2 = alloc(n);
    printf("spacePtr2=%x, allocp=%x\n", spacePtr2, allocp);

    afree(spacePtr2);
    printf("spacePtr2=%x, allocp=%x\n", spacePtr2, allocp);

    afree(spacePtr);
    printf("spacePtr=%x, allocp=%x\n", spacePtr, allocp);
    return 0;
}

Strings

#include <stdio.h>
#include <string.h>

int main()
{
    char amessage[] = "now is the time"; // an array
    char *pmessage = "now is the time"; // a pointer

    printf("amessage=%s\n",amessage);
    printf("pmessage=%x\n",pmessage);
    printf("pmessage=");
    for(; *pmessage!='\0'; ++pmessage)
        printf("%c",*pmessage);
}

strcpy with Arrays

#include <stdio.h>

// K&R pg. 105
// strcpy: copy t to s; array subscript version
void strcpy(char *s, char *t)
{

    int i;
    i = 0;
    while ((s[i] = t[i]) != '\0')
        i++;
}

int main()
{
    char str[10] = "My string";
    char newStr[10];
    strcpy(newStr, str);

    printf("str=%s\n", str);
    printf("newStr=%s\n", newStr);
    return 0;
}

strcpy with Pointers

#include <stdio.h

// K&R pg. 105
// strcpy: copy t to s; poitner version 1

void strcpy(char *s, char *t)
{
    while ((*s = *t) != '\0') {
        s++;
        t++;
    }
}

int main()
{
    char str[10] = "My string";
    char newStr[10];
    strcpy(newStr, str);

    printf("str=%s\n", str);
    printf("newStr=%s\n", newStr);
    return 0;
}

strcmp with Arrays

#include <stdio.h>

// K&R pg. 106
// strcmp: return <0 if s<t, 0 if s==t, >0 if s>t
int strcmp(char *s, char *t)
{
    int i;

    for (i = 0; s[i] == t[i]; i++)
        if (s[i] == '\0')
            return 0;
    return s[i] - t[i];
}

int main()
{
    char s[] = "ABC";
    char t[] = "ABC";
    printf("%d", strcmp(s,t));
    return 0;
}

strcmp with Pointers

#include <stdio.h>

// K&R pg. 106
// strcmp: return <0 if s<t, 0 if s==t, >0 if s>t
int strcmp(char *s, char *t)
{
    for ( ; *s == *t; s++, t++)
        if (*s == '\0')
            return 0;
    return *s - *t;
}

int main()
{
    char s[] = "ABC";
    char t[] = "ABC";
    printf("%d", strcmp(s,t));
    return 0;
}

Example with class

Simple Date Example

#include <stdio.h>

// K&R pg. 111

static char daytab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};

// day of year: set day of year from month & day
int day_of_year(int year, int month, int day)
{
    int i, leap;

    leap = year%4 == year%100 != 0 || year%400 == 0;
    for (i = 1; i < month; i++)
        day += daytab[leap][i];

    return day;
}

// month_day: set month, day from day of year
void month_day(int year, int yearday, int *pmonth, int *pday)
{
    int i, leap;

    leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
    for (i = 1; yearday > daytab[leap][i]; i++)
        yearday -= daytab[leap][i];
    *pmonth = i;
    *pday = yearday;
}

int main()
{
    int day;
    day = day_of_year(2018, 2, 15);
    printf("day=%d\n", day);

    int pmonth, pday;
    month_day(2018, 46, &pmonth, &pday);
    printf("pmonth=%d, pday=%d", pmonth, pday);

    return 0;
}

Initialization of Pointer Arrays

#include <stdio.h>

// K&R pg. 113

// month_name: return name of n-th month
char *month_name(int n)
{
    static char *name[] = {
        "Illegal month",
        "January", "February", "March",
        "April", "May", "June",
        "July", "August", "September",
        "October", "November", "December"
    };

    return (n < 1 || n > 12) ? name[0] : name[n];
}

int main()
{
    char *name = month_name(2);
    printf("name=%s", name);

    return 0;
}