1 more on pointers. 2 reminder 3 pointers pointer is a variable that contains the address of a...

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More on More on PointerPointers s

2

Reminder

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Pointers

Pointer is a variable that contains the address of a variable

Here P is said to point to the variable C

C

7 3 4… …

173172 174 175 176 177 178 179 180 181

174 3 4… …

P

833832 834 835 836 837 838 839 840 841

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Brief Summary

&x – address (pointer) of variable x

*y – content in address y (common) usage: int z = *y; (common) usage:

printf(“%d”,*y);

int */double */char * - define pointer to the corresponding primitive

(common) usage: int * p = &x;

int x = 8;

int *y = &x;

*y = 10;

printf(“%d”,*(&x));

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Common errors It is impossible to define pointers to

constants or expressions. i = &3; j = &(k+5);

It is also impossible to change a variable’s address (because it is not for us to determine!). &a = &b; &a = 150;

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Exercise (from last week)

Write a function that accepts a double parameter and returns its integer and fraction parts

Write a program that accepts a number from the user and prints out its integer and fraction parts, using this function

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dbl_split.c

void split(double num, int *int_part, double *frac_part) {

*int_part = (int)num;*frac_part = num - *int_part;

}

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dbl_split.c

int main(void) {double num, fraction;int integer;printf("Please enter a double number: ");scanf("%lf", &num);split(num, &integer, &fraction);printf("The integer part is %d\n", integer);printf("The fraction part is %g\n", fraction);return 0;

}

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Pointers and Arrays

Recall that an array S holds the address of its first element S[0]

S is actually a pointer to S[0] int S[10]; int *P; P=S; /* From now P is equivalent to S */

Both P and S are now pointing to S[0]

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Pointer versus Array

Array is a pointer Array declaration and memory

allocation A pointer is uninitialized when

declared (like a primitive variable) The value of an array variable (the

address it is pointing) cannot be changed – unlike pointers!

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Pointer arithmetic

Pointers can be incremented and decremented

If p is a pointer to a particular type, p+1 yields the correct address of the next variable of the same type

How? p++, p+i, and p += i also make

sense

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Pointer arithmetic - example

If p and q point to elements in an array, q-p yields the number of elements between p and q.

int a[] = {0,1,2,3};int * b = a;b += 4;printf("last element: %d, diff: %d\n",*(b-1),b-a);

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array_strcpy

void array_strcpy(char dest[], char src[]){

int i = 0;

while (src[i] != '\0'){

dest[i] = src[i];i++;

} dest[i] = '\0';

}

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array_strcpy – step by step


int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

0

i

‘!’ ‘!’ ‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

0

i

‘!’ ‘!’ ‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){

dest[i] =src[i];i++;

} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

1

i

‘y’

‘!’ ‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

1

i

‘y’

‘!’ ‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

1

i

‘y’

‘e’

‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

2

i

‘y’

‘e’

‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

2

i

‘y’

‘e’

‘!’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

2

i

‘y’

‘e’

‘s’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

3

i

‘y’

‘e’

‘s’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

3

i

‘y’

‘e’

‘s’ ‘!’ ‘!’

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int i = 0;

while (src[i] != '\0'){


} dest[i] = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

3

i

‘y’

‘e’

‘s’ ‘\0’

‘!’

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Pointer arithmetic strcpyvoid my_strcpy(char *dest, char *src){

while (*src != '\0'){

*dest = *src;dest++;src++;

} *dest = '\0';

}

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Strcpy – step by stepvoid my_strcpy(char *dest, char *src){

while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘!’ ‘!’ ‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘!’ ‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘!’ ‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘!’ ‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘!’ ‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘!’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘s’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘s’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘s’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘s’ ‘!’ ‘!’

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while (*src != '\0'){


} *dest = '\0';

}

‘y’

‘e’

‘s’ ‘\0’

‘%’

src

dest

‘y’

‘e’

‘s’ ‘\0’

‘!’

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An additional use

A function that accepts an array and searches for something within it (a char, a number, etc.)

What should the function return? The index where the ‘something’ was

found A pointer to the place where it was

found

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Functions that return pointers Like any other data type, functions

can return pointers For example, a prototype for a

function returning a pointer to char will be –char *func(…);

But how would we indicate failure Suppose we searched the array and

didn’t find anything – what should we return?

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The NULL pointer

The NULL pointer is an ‘empty’ pointer that points to nothing.

It is the address number 0. If a pointer p is set to NULL, trying

to access *p results in a run-time (not compilation) error.

Often used to indicate failure

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strchr – pointer arithmeticchar *my_strchr(char str[], char c){

char *search = str;

while (*search != '\0'){

if (*search == c)return search;

search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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char *search = str;



search++; }

return NULL;}

‘h’

‘e’

‘l’ ‘l’ ‘o’

str

‘\0’

c‘l’

search

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Example

strchr.c

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Mystery – what does this do?int main(void){

char s[LENGTH+1];char *p;

scanf("%100s", s);

p = strchr(s, ',');while(p!=NULL){

*p = ' '; p = strchr(p+1, ',');

}

printf("The result is - %s\n", s);return 0;

}

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Exercise

Implement the string.h function strrchr that returns a pointer to the last occurrence of a character inside a string.

Write a program that accepts a string and char from the user, displays what remains of the string after the last occurrence of that char.

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strrchr.cchar* my_strrchr(char str[], char c) {

char *search = str + strlen(str) - 1; /* search now points to the beginning of str */while(search >= str) {

if(*search == c)return search;

search--; /* advance pointer by one */}/* if we're here, we didn't find */return NULL;

}

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strncmp

Declared in string.h, with prototypeint strncmp(char *s1, char *s2, int n);

Returns 0 if the first n letters of s1 are equal to those of s2

Returns non-zero if first n letters are different

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Exercise Using strncmp, implement the following

function – Input – two strings str1, str2 Output – a pointer to the first instance of

str2 in str1, or NULL

Write a program that accepts two strings from the user and reports whether the first contains the second

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strstr.cchar* my_strstr(char str1[], char str2[]) {

char *search = haystack;/*search now points to the beginning of str*/int len1 = strlen(str1), len2 = strlen(str2);int i;for(i = 0; i <= len1-len2; i++) {

/* strncmp(s1, s2, n) compares the first n letters of s1 and s2 */

if(strncmp(search, str2, len2) == 0)return search;

search++;}/* if we're here, we didn't find */return NULL;

}

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Exercise @ home Implement the following function –

Input – two strings str1, str2 Output – pointer to the first instance in str1

of any of the characters contained in str2

Write a program that accepts a string from the user and replaces all punctuation signs (,.;:!?) with spaces

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solution

strcspn.c

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Some functions of string.h Implementations of functions from

string.h : strcpy.c – copies one string into the other strchr.c – returns a pointer to the first

occurrence of a character within a string strstr.c – returns a pointer to the first

occurrence of one string within another strcspn.c – returns a pointer to the first

occurrence of a single char from one string in the second

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