arrays, strings, & pointerscsce.uark.edu/~ahnelson/csce4114/lectures/lecture4.pdf · arrays,...

Arrays, Strings, & Pointers

Alexander Nelson

September 9, 2019

University of Arkansas - Department of Computer Science and Computer Engineering

Arrays, Strings, & Pointers

Arrays, Strings, & Pointers are all connected in C

• Pointer – Variable storing a memory location

• Array – Block of memory storing associated variables

Array identifier is a pointer to location of first element

• String – Array of character variables

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Example

#include <stdio.h>

int main(){

char* myString1 = "Hello!\n";

char myString2[] = "Hello!\n";

printf("%s",myString1);

printf("%s",myString2);

return 0;

}

myString1 and myString2 are equivalent statements!

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Pointers

Pointers

Pointer – Variable that points to a memory location

Requires:

• Name – Name of the pointer variable

• Type – Type of the variable that is being addressed, with

appended asterisk

e.g. int* myPointer;

Variable size in memory is not based on type, but on memory size

• Why?

3

Dereferencing Pointers

Pointer stores memory location – not value

Dereferencing – Obtaining value at a memory location

Dereference Operators:

• Unary Operator (*)

e.g. *myPtr = 5; //Dereferences myPtr and sets value to 5

• Bracket offset

e.g. myPtr[0] = 5; //Dereferences to value at

// myPtr+offset*sizeof(objective type)

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Address of

Variable address accessed through ampersand (&) operator

Literally the “address of” the variable

Example:

int val = 5;

int* myPtr = &val;

//myPtr now is equal to memory location of val

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Pointer Example

int val = 10;

int* myPtr = &val; //declare pointer to val

*myPtr = 5; //dereference pointer and set

//that location = 5

printf("val =%d",val);//print val=(value of val)

Output = ?

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Objective Type

Objective Type – type of variable pointed at

e.g.

• int* p; //objective type = int

• int a[10]; //objective type = int

• int**p; //root objective type is int, objective type is int*

Incrementing pointer variable increments by size of objective type

e.g.

• Incrementing an int* increments value by sizeof(int)

• Incrementing an int** increments value by sizeof(int*)

7

Arrays

Arrays

Array – “an ordered series or arrangement”

C Array – Block of memory storing associated variables

• Accessed by same variable name

• Same data type

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Declaration of Arrays

Arrays must be declared before use

Format:

type variable name[length]

• type – variable type of element to be stored

• variable name – identifier of the array

• length – number of elements to be stored

Declaration and definition of array reserves a block of memory

space according to type and length

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Example Arrays

double height[10];

• type: double

• name: height

• length: 10 (memory size = 10×sizeof(double))

float width[30]; int c[9]; char name[20]; //string array

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Array Implementation

Array identifier stores address of first element

Array identifier dereferenced by bracket operator obtains value

1st element stored at position 0, 2nd at 1, nth at (n-1)th element

e.g. a[n] = (n+1)th element

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Array Initialization

Arrays should be initialized to some value

Uninitialized arrays are able to be accessed, & contain garbage

memory contents

Examples:

• int a[] = {10,20,30,40};• int a[5] = {1,2,3};

If array size > num initialized, all others initialized to 0

• int a[5] = {0};Shorthand to initialize all to 0

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Accessing Array Elements

Accessed with a dereference operator and offset with [] operator

Can be read & modified like a normal variable

Valid Examples:

• c[0] = 3;

• c[3] += 5;

• y = c[x+1];

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Multidimensional Arrays

Multidimensional arrays declared with each dimension

e.g. int array[3][2];

Initialize each column through comma separated curly brace

statements

e.g. int array[3][2] = {{1,2},{3,4},{5,6}};

Memory-wise, the above is equivalent to:

int array[6] = {1,2,3,4,5,6}

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Shorthand multi-dimensional initialization

Rules:

Unspecified elements in given row – initialized to 0

Unspecified rows – initialized to 0

Examples:

• int array[3][2] = {{1},{3,4}} // == {1,0,3,4,0,0}• int array[3][2] = {{0}} // == {0,0,0,0,0,0}

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Passing Arrays to Functions

Array identifier = pointer to first element

Identifier (memory address) is passed by value = reference to array

Array elements may be accessed by offset (known type)

Length of array is not known!

Use delimiter or pass length parameter(s)

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Passing Array Elements

Array elements are passed by value!

Original memory location will not be modified

Example:

void myFunction(int a){

a = a+1;

return a;

}

int main(){

int array[5] = {0};

myFunction(array[0]);

printf("%d",array[0]); //Will print 0!

}

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Function with Const Array

For arrays passed to functions with no intent to modify the array

const keyword before array parameter

e.g. int sum array(const int array[], int n);

Compiler will throw error for any element modification operations

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What is the difference?

Identifier name points to first element

Pointer points to any element

Arithmetic operations on array name are disallowed

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Example Difference

int g, grades[ ] = {10, 20, 30, 40 }, myGrade = 100;

int yourGrade = 85, *pGrades;

/* grades can be (and usually is) used as array name */

for (g = 0; g < 4; g++)

printf("%d\n",grades[g]);

/* grades can be used as a pointer to its array if it doesn’t change*/

for (g = 0; g < 4; g++)

printf(%d\n", *(grades + g));

/* but grades can’t point anywhere else */

grades = &myGrade; /* compiler error */

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Example Continued

/* pGrades can be an alias for grades and used like an array name */

pGrades = grades; /* or pGrades = &(grades[0]); */

for( g = 0; g < 4; g++)

printf( "%d\n", pGrades[g]);

/* pGrades can be an alias for grades and be used like a pointer that changes */

for (g = 0; g < 4; g++)

printf("%d\n",*(pGrades++));

/* BUT, pGrades can point to something else other than the grades array */

pGrades = &myGrade;

printf( "%d\n", &pGrades);

pGrades = &yourGrade;

printf( "%d\n", &pGrades);

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Array of Pointers

Pointers are a data type & can be placed in an array

e.g. ArrayList

Especially useful for array of variable string sizes to reduce wasted

bytes

e.g. char* myStringArray[] or char** myStringArray

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Strings

Strings/Char Arrays

C doesn’t have a “string” type

Implemented using char arrays & “string literals”

To be a “string”, a char array must be null terminated

Example:

• char string1[] = “hello world!”;

• char string2[] = {’h’,’e’,’l’,’l’,’o’,’ ’,’w’,’o’,’r’,’l’,’d’,’ !’,’\0’};• char string3[5] = “hello”; //Error: 6 characters in string literal

• char string4[10] = “hello”; // “hello\0\0\0\0\0”

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ASCII

American Standard Code for Information Interchange (ASCII)

• Encodes the Roman alphabet used in English/West European

languages

• Has 128 characters

• Encoded as leading 0 and 7 bit value

• Chars 0-31 represent control characters (e.g. “\n”)

C was designed to work with ASCII

The char data type is used to store ASCII characters in C

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ASCII Table

Decimal Hex Char

Decimal Hex Char Decimal Hex Char Decimal Hex Char

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Special Characters

Backslash character (“\”) used to indicate special character

e.g.

• \n – newline

• \t – tab

• \” – double quote

• \’ – single quote

• \\– backslash (since backslash has special meaning)

• \a – beep (unprintable)

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Character Library

ctype.h – C library of char functions

Examples:

• int isdigit (int c) – Is c a decimal digit (‘0’ - ‘9’)?

• int isdigit(int c) – Is c a hexadecimal digit (‘0’ - ’9’, ‘a’ - f’, or

‘A’ - ‘F’)?

• int isalpha (int c) – Is c an alphabetic character (‘a’ - ‘z’ or

‘A- ‘Z’)?

• int isspace (int c) – Is c a whitespace character (space, tab,

etc)?

• int isprint (int c) – Is c a printable character?

• int tolower (int c) – Change c to lower case if alpha

• int toupper (int c) – Change c to upper case if alpha

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String Output

Use %s in printf() or fprintf() to print string

Will print until NULL is seen

Can provide field width and justification

e.g.

char book1[] = “Dune”;

char book2[] = “Brave New World”;

printf(“Favorite Book: %12s”, book1);

printf(“Other Book: %12s”,book2);

What will the above print?

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scanf()

scanf() – stdio function for taking input from stdin

Format: scanf(“%s”,string1);

• Reads characters from stdin until whitespace encountered

• Can write beyond end of array!

• Best case – “stack smashing detected”

• Worst case – Overwrites other memory

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Example Program

#include <stdio.h>

int main(){

char string1[20], string2[]="string";

int i;

printf("Enter a string: ");

scanf("%s", string1);

printf("string1: %s\nstring2: %s\n",string1,string2);

for(i=0;string1[i]!=’\0’;i++)

printf("%c",string1[i]); // Print by iteration

printf("\n");

return 0;

}

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scanf(“%#s”)

To avoid overflow, specify the number of characters!

char buffer[100] = ””;

scanf(”%99s”,buffer);

Why 99?

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gets()/fgets()

gets(char* buffer) – read a line of input until ‘\n’

Newline then replaced with null character

Can overflow buffer!

fgets(char* buffer, int len, FILE *stream) – read a line of input

from file until n-1 characters or ‘\n’

Newline then replaced with null character

Safer, as max length can be specified! File can be STDIN

Returns NULL when EOF detected!

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fgets() Example

FILE *inFile;

inFile = fopen( "myfile", "r" );

/* check that the file was opened */

char buffer[120];

while ( fgets(buffer, 120, inFile ) != NULL )

printf( "%s\n", string );

fclose( inFile );

Will read line-by-line up to 120 characters until EOF detected

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sprintf()

Format a string in an array of characters

Akin to toString() in Java

sprintf() works like printf() but output goes to character array

e.g.

char buffer[100] = 0; int numStudents = 33;

sprintf(buffer,“There are %d students\n”,numStudents);

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C String Library

string.h – C String library

Common string manipulation functions

e.g.

• strlen( const char string[ ] ) – Number of characters in string

(not including “null”)

• strcpy( char s1[ ], const char s2[ ] ) – Copies s2 on top of s1

• strcmp ( const char s1[ ] , const char s2[ ] ) – Returns < 0, 0,

> 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically

• strcat( char s1[ ] , const char s2[ ]) – Appends (concatenates)

s2 to s1

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C String Library

Some functions have size parameter

e.g.

• strncpy( char s1[ ], const char s2[ ], int n ) – Copies at most n

characters of s2 on top of s1

• strncmp ( const char s1[ ] , const char s2[ ], int n ) –

Compares up to n characters of s1 with s2 Returns < 0, 0, >

0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically

• strncat( char s1[ ], const char s2[ ] , int n) – Appends at most

n characters of s2 to s1

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Array of Strings

Strings are arrays – Array of Strings == Multi-dimensional array

e.g.

char months[10][] = {“January”, “February”, “March”, “April”,

“May”, “June”, “July”, “August”, “September”, “October”,

“November”, “December”}; //120 byte array w/ 2 indices

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Command Line Arguments

Command line arguments are passed to main as an array of strings

int main(int argc, char* argv[])

• argc – number of arguments

• argv – array of strings

• argv[0] – always the name of the executable

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Pointer Arithmetic

Pointer Arithmetic

Incrementing a pointer increments by sizeof(objectiveType)

e.g.

char c, *cPtr = &c;

int i, *iPtr = &i;

float f, *fPtr = &f;

printf(“%p,%p%p”,cPtr++,iPtr++,fPtr++);

printf(“%p,%p%p”,cPtr++,iPtr++,fPtr++);

Outputs:

0x00, 0x10, 0x20

0x01, 0x14, 0x24

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void Pointers

Void pointers are special case

C Standard does not allow arithmetic or dereferencing of void

pointers

However:

GCC has an extension that allows arithmetic

To dereference a void pointer, first cast to typed pointer, then

dereference

e.g. *(int *)myVoidPointer; (int)*myVoidPointer does not work!

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Function Pointers

Function identifiers are a pointer to instruction memory

Can create a pointer to the instruction

Type == Return type of function

Also declared with parameters of function

e.g.

int myFunction(int param1);

int (*myFunctionPtr)(int) = &myFunction;

int ret = (*myFunctionPointer)(10);

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