University:
Boston University
Course:
MET CS 201 | Introduction to Programming
Academic year:

2021
Views:

238

Pages:

20
Author:

sapela015a

Introduction to C Part 3 Pointers A pointer in C a variable whose value is the memory address of another variable. i ... 51 0 4 ip 8 int i = 51; int *ip = &i; 12 4 1024 1028 1032 1036 // regular integer variable // pointer to an integer variable 2 Pointers int i; int *ip; // regular integer variable // pointer to an integer variable i = 51; ip = &i; // get address of i and store it in ip printf( "address of i is %p\n", &i); printf( "value stored at the address %p is %d\n", ip, *ip); 3 Pointers Copy pointer.c file from RCS examples directory $ cp /project/scv/examples/c/tutorials/ex010/*.c . $ gcc –o pointer pointer.c $ ./pointer 4 Pointers Notice: Here we use * to declare that ip is a pointer to an integer value int *ip; printf( "value stored at the address %p is %d\n", ip, *ip); Here we use * to get the value stored at the address of ip Pointers Hands-on exercises: Declare 3 variables: char, int and double and assign values to them. For each variable assign a variable that points to it. Using sizeof() function find the memory size that is required to store each variable 6 Pointers Hands-on exercises: Declare int variable i and a pointer ip to it. Use ip variable to modify the value stored in i – increase the value by 1. 7 Pointers – common mistkes float x = 3; float *p; // p is a pointer (address) but x is not p = x; // Error // &x is address but *p is not *p = &x; // Error // both &c and pc are addresses p = &x; // Correct // both c and *pc values *p = x; // Correct 8 Pointers and arrays The name of an array can be treated as a pointer and vice versa float x[5] = { 1.0, 2.1, 3.2, 4.3, 5.4}; float *p = x; // p will point to x[0] p = p + 2; printf ("p points to %d\n", *p); // prints 3.2 So x[2] and *(p+2) refer to the same element 9 Pointers and arrays Copy parrays.c file from RCS examples directory $ cp /project/scv/examples/c/tutorials/ex11/*.c . Compile and run: $ gcc –o parrays parrays.c $ ./parrays 10 Pointers and Strings A C string is an array of characters So the name of the string variable is also a pointer: char my_string[] = "Hello"; or char my_string[] = {'H', 'e', 'l', 'l', 'o' , '\0’}; printf("third letter in the string is %c\n", my_string + 2); 11 Memory allocation Suppose we do not know how many values we need to store in an array. We can then first declare it as a pointer and allocate memory later. malloc ( size_in_bytes ) returns a pointer to a memory region that you can then assign to a variable of whatever type you like To free the memory use free() function 12 Memory allocation Suppose we do not know how many values we need to store in an array. We can then first declare it as a pointer and allocate memory later: #include float *x; //allocate memory to store 10 elements x = (float*) malloc ( 10 * sizeof(float) ); // free memory free(x); 13 Memory allocation Hands-on exercises: Copy mem.c file from RCS examples directory $ cp /project/scv/examples/c/tutorials/ex12/*.c . Declare int array with unknown size (pointer). Using malloc() function allocate 100 elements to be stored in this array. Using for loop fill array with integer values from 1 to 100. 14 Command line arguments Input parameters to the program can be passed using command line: $ my_program 100 The number of arguments that were passed to the program and their values can be accessed using argc and argv variables: int main (int argc, char **argv){ } 15 Command line arguments argc – is equal to the number of arguments passed to the program. The first argument is always the program name. So argc is always greater or equal to 1. argv is an array of strings (hence **). Each string contains the value of an argument. argv[0] contains the name of the program int main (int argc, char **argv){ } 16 Command line arguments To convert values stored in argv to integers or floating values, use atoi() and atof() functions respectfully: i = atoi( argv[1] ); or x = atof ( argv[2] ); 17 Command line argument Hands-on exercises: Copy mem.c file from RCS examples directory $ cp /project/scv/examples/c/tutorials/ex13/*.c . Compile the program and run passing various integer values to the program: $ ./mem 20 18 Putting it all together • Let’s define a function that takes a value N, allocates an integer array, fills it with values from 1 to N and returns a pointer to this array. • Define another function mean() that takes array as an argument and calculates mean (average) of this array. • Save both functions in a file with a name mean_fun.c • In the main.c file define main() function. This function will read the command line and assign the input to N and then it will call first function array_init() and then mean(). Print the result at the end. 19 Putting it all together Hands-on exercises: Copy files from RCS examples directory $ cp /project/scv/examples/c/tutorials/ex14/*.c . Compile the program and run passing various integer values to the program: $ gcc -c mean_fun.c $ gcc -c main.c $ gcc -o mean main.o mean_fun.c 20