1. Programming Languages and Paradigms
The C Programming Language
(pointers, arrays, and strings)

2. Pointer types
In C, any existing type may have an associated pointer type
Example (for the int type)
int i; /* defines an int variable */
int *p; /* defines a pointer-to-int variable */ p i 5 5
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

3. Pointer values: addresses
Pointer variables hold addresses of memory locations
e.g.,
Here, address 9006 is a memory location that contains the integer value 5 p
9006
9006: 5
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

4. Pointer operations Address operator & : from variables to addresses
int num = 10;
int *p;
p = #
De-referencing operator * : from addresses to variables
printf( “%d\n”, num );
printf( “%d\n”, *p );
printf( “%d\n”, p ); p
num
2020
2020: 10
Prints: 10
<some address>
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

5. Arrays are pointers
An array name represents the address of the first element of the array
Example:
int scores[3];
scores[0] = 5; scores[1] = 10; scores[2] = 8;
int *p;
p = scores;
printf( “%d”, *p ); /* same as p[0] */
Prints: 5
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

6. What values do we assign to pointer variables?
An address generated through &
An array name
Pointer arithmetic calculation
Through the malloc() function
For char* variables, string literals
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

7. Pointer arithmetic Adding a number to a pointer provides an address
Let p be a pointer value of type T and i is an integer
Suppose the address P contains a sequence of T values
p+i is the address of the ith T data object in memory
In fact, *(p+i) refers to this ith value and is synonymous to p[i]
which is why indexing operations like arr[i] works
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

8. malloc
malloc(n) allocates n bytes of memory and returns the address of the first byte
Example
char *s;
s = (char *) malloc( 10 );
/* allocates 10 characters */
/* can now use s like as if char s[10]; */
Need to use sizeof function/macro for other types
int *p = (int *) malloc( 10*sizeof(int) );
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

9. About malloc and dynamic allocation
malloc() returns NULL if allocation fails
malloc()’s return type is void * which is why a cast is required
To free up memory previously allocated by malloc(), use free
free( s ); free( p );
Multidimensional array allocation provides many options (array of pointers or pointer to arrays?)
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

10. Pointers and the -> operator
Suppose struct stype { … }; struct stype *s;
(*s).field is equivalent to s->field
Useful for deep data structures
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

11. Strings String: sequence of characters (stored in a character array)
The null (‘\0’) terminator
String literals ( e.g., “hello” )
String functions (<string.h>)
strcpy
strlen
strcmp
strcat …
9/18/2018
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved.