CMPSC 16
Problem Solving with Computers I
Spring 2014
Instructor: Tevfik Bultan
Lecture 12: Pointers continued, C strings
Strings in C
• A char array, terminated by '\0'
• String literal – anything enclosed in double quotes
– e.g., "cat" – automatically includes terminator
c
a
t
• Must allocate memory to store the characters
char sentence[50];
/* up to 49 chars plus '\0' */
char word[] = "cat";
/* length is 3, but size is 4 */
• Point to such memory with char* (pointer to char)
char *s = sentence;
s = word;
\0
Strings in C
• Equivalent declarations of a string
char filename[11] = “sensor.txt”;
char filename[] = “sensor.txt”;
char filename[] = {‘s’, ‘e’, ‘n’, ‘s’, ‘o’,
‘r’, ‘.’, ‘t’, ‘x’, ‘t’, ‘\0’};
Using pointer arithmetic to traverse a string
• Example: compute the length of a string by traversing it by
incrementing a pointer until reaching the end of the string
• The difference between the final value of the pointer and
the beginning of the string array will give you the length
char *p = word;
while (*p++ != '\0')
; /* empty loop moves p to end */
printf("length: %d", p – word - 1);
• This is just an example to demonstrate pointer arithmetic,
use strlen function in string.h if you want to get the
length of a string
Copying string t to string s
void stringcopy(char *s, char *t)
•One way to implement – use subscript notation:
int i = 0;
while ((s[i] = t[i]) != ‘\0’) i++;
•Another way – use the pointer parameters:
while ((*s = *t) != ‘\0’)
{ s++; t++; }
•Can just increment in the while header:
while ((*s++ = *t++) != ‘\0’);
•And it’s possible to be even more cryptic:
while (*s++ = *t++);
/* works because value of ‘\0’ is 0 */
•These are just examples demonstrating pointer arithmetic,
use strcpy function in string.h to copy strings.
String library functions
• The standard C library contains a number of functions for
strings which are defined in string.h:
• strlen(t): Returns the length of the string s.
• strcopy(s, t): Copies string t to string s and returns
a pointer to s.
• strcat(s, t): Concatenates string t to the end of
string s.
• strcmp(s, t): Compares string s to string t in an
element-by-element comparison. A negative values is
returned if s < t, zero is returned is s is equal to t and a
positive value is returned if s > t;
Why do we have to deal with pointers?
• Given that programming using pointers seems somewhat
challenging, why do we need them?
• One reason is, as we have seen earlier, pointers allow us
to write functions with side-effects, i.e., the changes made
in the function body are reflected to the caller
– You have to use this carefully since side-effects are
hard to understand when someone else looks at your
code.
• A more significant reason for using pointers is that,
without pointers, we would be forced to know the size of
each memory we plan to allocate before runtime
– We would need to know the sizes of the arrays we plan
to allocate for example
– But sometimes, the size of an array we plan to allocate
may depend on the size of the input. What can we do?
Two ways to allocate memory
• Static memory allocation – done at compile-time
int x; double a[5];
/* allocates space for 1 int, 5 doubles */
– Both size and type are clearly specified ahead of time
– x can only hold int values, a only doubles
• Dynamic memory allocation – done during execution
– Must use library methods like malloc
• malloc allocates specific amount of memory,
returns void *
– Cast to appropriate pointer type – then use as always
int *ip = (int *)malloc(sizeof(int));
• Notes: returns NULL if memory not available so
must check that; cast is optional
– Must free the memory when done with it: free(ip);
What is sizeof ?
• A unary operator – computes the size, in bytes, of any
object or type
– Usage: sizeof object or sizeof(type)
– If x is an int, sizeof x == sizeof(int) is true
• Works for arrays too – total bytes in whole array
• Actual type of result is size_t
– An unsigned integer defined in <stddef.h>
– Similarly, ptrdiff_t is result type of pointer
subtraction
• Especially useful to find the sizes of data structures
The malloc function
• malloc is used for dynamic memory allocation
• The argument of the malloc function is the number of
bytes of memory required (where each byte is 8 bits).
• For example, if we want to allocate a memory for 10
integers, we need to figure out how many bytes we need
per integer and then multiple that by 10.
– We can use sizeof(int)to get the number of bytes
used per integer value
• Note that this value is system dependent
• If there is not enough memory for allocation, then malloc
returns NULL
• calloc : like malloc but initializes the allocated
memory to 0
• realloc : can be used to change the size of the
Pointers to functions
• Functions are also stored in memory too
– A function’s name “points” to its address
• Awkward declarations of function pointers
– e.g., int (*func)(int, int) declares func as a
pointer to a function that takes two ints and returns an
int
– Like all pointers, especially useful as function
parameters (in other functions)
• See …/demos/funcptr.c
Pointer tutorial
Pointer fun
http://www.cs.ucsb.edu/~mikec/cs16/slides/binky.html
Topics we have discussed after the first midterm
• Generating “random” values
• File IO
• Functions
– Parameter passing
– Scope, global vs. local variables
– Recursive functions
• Arrays
– Sorting an array
– Multi-dimensional arrays
– Arrays as function parameters
• Software life-cycle, testing, assertions
• Pointers
– Address and dereference operators
– Pointers and arrays
– C strings
– dynamic memory allocation
Practice problems
Match the following
•black-box testing
•assertion
•driver
to the following phrases
•A Boolean expression that is expected to hold at a
particular program point
•A piece of code that provides the necessary inputs to the
module under test
•A testing method that is synonymous to functional testing
Practice Problems
• Given the following piece of code
int *a, *b;
a = (int *)malloc(sizeof(int));
b = (int *)malloc(sizeof(int));
*a = 5; *b = 17;
printf("Values: %d, %d\n", *a, *b);
printf("Values: %u, %u\n", a, b);
• What will be printed ?
• What will be printed if we put the assignment a = b;
before the print statements?
• How can you swap the values stored at a and b ?
• What would happen if we tried this: b = 17; ?
Practice problems
• What does the following function do?
void switch(int a, int b) {
int hold;
hold = a;
a = b;
b = hold;
return;
}
Practice problems
• Write a function that computes the sum of elements in an
array
• Write a recursive function that computes the sum of
elements in an array
Practice problems
• The practice problems and problems in your text book!