C Programming - Structures

Structures containing arrays A structure member that is an array does not ‘behave’ like an ordinary array When copying a structure that contains a member which is an array, the array is copied element by element  Not just the address gets copied  For example - array_member.c Reminder – ordinary arrays can’t be copied simply by using the ‘=‘ operator  They must be copied using a loop

Structures containing arrays When passing the structure to a function by value  Changing the array field inside the function won’t change it in the calling function Reminder – when passing an ordinary array to a function, all that gets passed is the address of its first element  Hence every change to the array within the function, changes the array in the calling function

C Programming Pointers

A new type of variable Its value is the address of another variable We declare a pointer variable by adding a ‘*’ before the variable name: type *variable_name;

Understanding Pointers int i = 5; TypeNameaddressvalueinti20005 int j = 10; intj200410int*p int* p = &i; int*q int* q = p; j = *p; address of i The value of q equals the value of p Evaluate the right hand side of the assignment: Go to the variable pointed by p and take its value

Referencing The unary operator & gives the address of a variable The statement: ptr = &c; assigns the address of c to the pointer variable ptr, and now ptr points to c

Referencing - Example int n; int *iptr; /* Declare iptr as a pointer to int */ n = 7; iptr = &n; n 34…… …… iptr

Dereferencing The unary operator * is the dereferencing operator Applied ONLY on pointers Access the object the pointer points to The statement: *iptr = 5; puts 5 in the variable pointed by iptr

Dereferencing printf(“%d”, *iptr); /* Prints out ‘7’ */ *iptr = 181; printf(“%d”, n); /* Prints out ‘181’ */ iptr = 181; /* This is unadvisable!! */ n 34…… …… iptr

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2] …

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2]

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2]

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2]

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2]

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2]

Pointers Example int x=1, y=2, z[3]={5,6,7}; int *ip; ip = &x; y = *ip; *ip = 0; ip = z; ip = &z[2]; *ip = 1; xy zip Z[0]Z[1]Z[2]

Common errors It is an error to define pointers to constants or expressions.  i = &3  j = &(k + 5) It is an error to change a variable’s address (because it is not for us to determine!).  &a = 150  &a = &b Dereferencing un-initialized pointers int* a;... *a = 5;

What will be printed? int main() { int a=3,b=6,c; int *x=&a; printf(“c=%d\n”,c); } c=b*(*x);c=b**x;

Function Arguments Functions receive their arguments “by value” Cannot change the value in the caller void increment_counter(int counter) { counter++; }

Pointers as Function Arguments If we want to change a variable in the function we can pass its address – a pointer! Call “by reference” void increment_counter(int* counter) { (*counter)++; }

CallerCallee main incerement_counter(X) X by value increment_counter(int counter) by reference increment_counter(int* counter) copy of X counter main incerement_counter(&X)

Wrong Swap A swap that gets integers as variables does not change the value in the original variables. void swap(int x, int y) { int tmp = x; x = y; y = tmp; }

How can we fix it? We can define swap so it gets pointers to integers instead of integers void swap(int *x, int *y) { …swap *x and *y… } We then call swap by swap(&x, &y); This is passing values by address

Right Swap – add_swap.c void swap(int *x, int *y) { int tmp = *x; *x = *y; *y = tmp; }

Back to scanf We can now understand the & in scanf("%d", &a); The argument list in scanf is simply passed by address, so scanf can change its content

Exercise Implement the function: void split(double d, int* int_part, double* frac_part) The function 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.

Solution void split(double num, int *int_part, double *frac_part) { *int_part = (int)num; *frac_part = num - *int_part; } int main(void) { double num, fraction; int integer; printf("Please enter a real number: "); scanf("%lf", &num); split(num, &integer, &fraction); printf("The integer part is %d\n", integer); printf("The remaining fraction is %g\n", fraction); return 0; }

Pointers and Arrays An array is consecutive bytes in memory The name of the array is the address of the first element. arr is the address of arr[0] Unlike pointers this address cannot be changed

Pointers Vs. Arrays Characteristics: Assignment: change content can changeaddress xxx pointer xx array xx const pointer arraypointer xx array xxx const pointer

Pointers Vs. Arrays - Example int arr[3] = {1, 2, 3}; int *ptr; const int* cptr; arr = ptr; ptr = arr; *ptr = 3; cptr = arr; *cptr = 5; ptr = cptr; *arr = 6; same as arr[0] = 6

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 p++, p+i, and p += i also make sense

Pointer arithmetic If p and q point to elements in an array, q-p yields the number of elements between p and q. You can’t add two pointers There is a difference between pointer arithmetic and “regular” arithmetic.

Pointer arithmetic - example int main(void) { int a[3] = {17,289,4913}, *p, *q; p = a; /* p points to the beginning of a, that is &a[0] */ q = p+2; /* q points to a[2]. Equivalent to q = &a[2] */ printf(“a is %p\n", a); printf("p is %p, q is %p\n", p, q); printf("p points to %d and q points to %d\n", *p, *q); printf("The pointer distance between p and q is %d\n", q-p); printf("The integer distance between p and q is %d\n", (int)q-(int)p); return 0; } a is 0012FECC p is 0012FECC, q is 0012FED4 p points to 17 and q points to 4913 The pointer distance between p and q is 2 The integer distance between p and q is 8

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; } Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘!’

void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; } Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘!’

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; } Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘!’

void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; } Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘!’

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘s’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; } Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘s’‘!’

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘s’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘s’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

Strcpy – step by step ‘y’‘e’‘s’‘\0’‘%’ src dest ‘y’‘e’‘s’‘\0’‘!’ void my_strcpy(char *dest, char *src) { while (*src != '\0') { *dest = *src; dest++; src++; } *dest = '\0'; }

Pointers, Arrays and Function Arguments Consider the following function prototypes These are all identical void foo(char str[10]); void foo(char str[]); void foo(char* str);

Exercise Write a function with the prototype: void replace_char(char *str, char c1, char c2); It replaces each appearance of c1 by c2 in the string str. Do not use the [] operator! Demonstrate your function with a program that uses it

Solution void replace_char(char *str, char c1, char c2) { while (*str != '\0') { if (*str == c1) *str = c2; ++str; }

Exercise Write a function with the prototype: int *find_max (int *array, size); that returns a pointer to the largest element of the array Do not use the [] operator! Use pointer arithmetics

Solution int* find_max(int *array, int size) { int *ptr = array, *max = array; for (ptr=array;ptr<array+size; ptr++) { if (*ptr > *max) max = ptr; } return max; }