CS 1400

Chapters 9, 10

C-strings and Pointers

A few C-string library functions…

#include <cstring>

int strcmp (char a[ ], char b[ ]);returns <0 if string a is less than string b, returns 0 if the strings are the same, and returns >0 if string a is greater than string b

int strlen (char a[ ]);returns the number of characters in string a

void strcpy (char dest[ ], char source[ ]);copies the source string into the destination array (including the null), but does no bounds checking.

More C-string functions…

void strncpy (char d[ ], char s[ ], int length);

copy exactly length characters from string s into string d.

char* strstr (char s[ ], char p[ ]);

return a reference (pointer) to the first occurence of string p in string s – or NULL if not found.

Arrays of C-strings

• Arrays of C-strings are just 2-D arrays of characterschar names[10][30]; // 10 names of up to 29 chars ea.

• Rules of thumb:• names refers to the entire array of names and is

used to pass the array to a function.• names[n] refers to a single name and can be used

to pass a single name to a function or for input and ouput.

• names[n][m] refers to a single character.

Examples• cin >> names[2];

– inputs a name into row 2 (the 3rd name)

• names[2][1] = ‘x’;– changes the 2nd character in row 2 to ‘x’

• MyFunc (names);– passes the entire list of names to a function with this

prototype;void MyFunc (char thesenames[ ] [30]);

• MyFunc2 (names[2]);– passes the 3rd name to a function with this prototype;void MyFunc2 (char thisname[ ]);

Example…

Consider a file of 100 student names and grades called “students.txt”

Write a program to accept a name from the user and then output this student’s grade.

Fred 93.4

John 87.1

Cheryl 96.9

Shannon 81.2

George 77.2

… …

Pseudocode…

• read in database from a file into two parallel arrays

• get a particular name from the user

• do a linear search for the name

• output the corresponding grade

int main ( ){ char names[100][30], thisname[30]; float grades[100]; ifstream fin; fin.open (“students.txt”); for (int n=0; n<100; n++)

fin>>names[n] >> grades[n]; cout << “enter a name: “; cin >> thisname; cout << “the grade is: “ <<

grades[ LinearSearch(names, 100, thisname) ] << endl;

return 0;}

LinearSearch() for C-strings…int LinearSearch (char list[ ][30], int size, char value[ ]){ bool found = false; int position = -1; for (int n=0; n<size && !found; n++) { if (strcmp(list[n], value) == 0) { position = n; found = true; } } return position;}

SelectionSort() for C-stringsvoid SelectionSort (char array[ ][30], int size){ int smallest_position; for (int n=0; n<size-1; n++)

smallest_position = FindSmallest (array, n, size);Swap (array, n, smallest_position);

}

Swap() for C-stringsvoid Swap (char a[ ][30], int n, int k) { char temp[30]; strcpy (temp, a[n]); strcpy (a[n], a[k]); strcpy (a[k], temp);}

FindSmallest() for C-strings

int FindSmallest (char array[ ][30], int start, int size){ int position; char smallest[30]; strcpy (smallest, array[start]); // first assumption for (int k=start+1; k<size; k++)

if (strcmp (array[k], smallest) < 0){ strcpy (smallest, array[k]); // change your mind position = k; // and remember pos.}

return position;}

Common interview question…

Write a program that prints out the numbers 1 to 100 on their own line; except if the number is a multiple of 3, write only the word “Fizz” on its own line, if the word is a multiple of 5, write only the word “Buzz” on its own line, and if the word is a multiple of both, write only the word “FizzBuzz” on its own line.

Example output lines…12Fizz4 BuzzFizz78FizzBuzz11Fizz1314FizzBuzz…

int main(){ for (int n=1; n<=100; n++) { if ((n%3 == 0) && (n%5 == 0))

cout << “FizzBuzz\n”;else if (n%3 == 0) cout << “Fizz\n”;

else if (n%5 == 0) cout << “Buzz\n”;else cout << n << endl;

}}

Example…

• Write a program that will accept a textual file name and then output all the words in that file that are miss-spelledA spell-checker!

Enter filename: myletter.txt

Miss-spelled words:

ardvarc

reciept

requiam

Pointers, values, and names

• Each memory cell has– a value– an address

char word[5] = “HELP”; // each cell is one byte

0x100 0x101 0x102 0x103 0x104

h e l p \0word

(Addresses are typically given in hexadecimal notation)

Different data types my require different sized cells

short x[3] = {100, 200, 300}; // each cell is two bytes

float y[2] = {1.2, 3.4}; // each cell is four bytes

x

y

0x200 0x202 0x204

0x300 0x304

100 200 300

1.20000000 3.40000000

sizeof() function

• The number of bytes utilized by a particular system for a variable may be determined by the sizeof() function;

• The argument to sizeof() may be a type or a variable name

cout << sizeof(short) << sizeof(float);

cout << sizeof(x) << sizeof(y);

Pointers vs. References

• A reference and a pointer both refer or point to a memory location.– A reference is managed by the system– A pointer is managed by the programmer

Pointer variables

• A variable may be declared to hold an address using the * symbolfloat *fptr; // may hold an address of a floatshort *iptr; // may hold an address of a short

• The address of a variable or cell may be determined using the & operatorfloat b = 3.14; short c = 567;fptr = &b;iptr = &c;

fptr

iptr

3.1400000

0x400

0x400

0x500

0x500

567

or more commonly diagrammed using arrows…

fptr

iptr

3.1400000

0x400

0x500

567

a

b

a

b

Using pointer variables

• A cell value pointed to by a pointer variable may be referenced using the * operatorcout << *iptr; // output value pointed to by iptr

cout << *fptr; // output value pointed to by fptr

So, the * operator means “value pointed to by”

Confusing use of *

• Be careful!– When used in a declaration, * creates a

pointer variable

int *ptr;– When used as a unary operator, * resolves or

dereferences (uses) a pointer.

cout << *ptr;– When used between two values, * means

multiply

cout << cost * rate;

Confusing use of &

• Be careful!– When used in a declaration, & creates a

reference variable

void MyFunc (int &ptr);– When used as a unary operator, & determines

a pointer.

cout << &ptr;

Pointer arithmetic

• Pointer variables may be incremented, decremented, or have constants added or subtracted.

• When a pointer variable is incremented, the computer will automatically increment the address by the appropriate number of cells – regardless of their size

Examples…

float x[4] = {1.2, 3.4, 5.6, 7.8};

float *fptr = &x[0];

cout << *fptr << endl;

fptr++;

cout << *fptr << endl;

fptr += 2;

cout << *fptr << endl;

The * and & operators have high precedence…

int cost, *value, temp, rate;

value = &rate;

cin >> cost >> rate;

temp = cost * * value;

What would be stored in temp if the user enters 2.5 and 2?

Pointers may be passed as function parameters…

void MyFunc (int *p1, int *p2);

int main(){ int x = 55, y = 77; MyFunc (&x, &y); cout << x << “, “, << y << endl;}

void MyFunc (int *p1, int *p2){ int temp; temp = *p1; *p1 = *p2; *p2 = temp;}

So… what does this function do?

Arrays and pointers

• When an array is declared, the system creates the array cells and a pointer variable to the array:

char letters[5];

letters

Examples…char letters[5] = “HELP”;

cout << *letters << endl;cout << letters[0] << endl;cout << *(letters+2) << endl;cout << letters[2] << endl;

letters++; // illegal – letters is a constant // pointer (a reference managed // by the system)!

Passing arrays…

• What is being passed?MyFunc (letters);

• What should the function prototype be?void MyFunc (char array[ ]); // ??void MyFunc (char *array); // ??

Either! These are equivalent: The parameter array is a pointer to a char array.

Equivalent notation…

Pointer notation Array notation

void MyFunc (char *array); void MyFunc (char array[ ]);

*(array+1) = ‘x’; array[1] = ‘x’;

MyFunc (letters); MyFunc (&letters[0]);

float *fptr; float fptr[ ];

MyFunc (letters+n); MyFunc (&letters[n]);

2-D arrays and pointers

• You may visualize a 2-D array as follows;

char names[5][10];

names names[n] names[n][m]

Dynamically allocated memory

• Using pointers, array space can be allocated manually to a size determined at run-time!

int size;char *docptr;cout << “Enter document size: “;cin >> size;docptr = new char[size] ;

Deallocating…

• Memory allocated using new can be manually deallocated using delete

delete [ ] docptr;

• Memory allocated using new that is not manually deallocated is automatically deallocated when a program exits

Sorting arrays with pointers

• Consider a table of long names;

John Jacob Jinglehiemer Schmidt

Alloitious Promethius Quoroyo Gildersleve

Mahatma Moriajib Mattesumium Matilda

Maria Cecila Prozelli Motsorelli Puchini

Alfred Erasmus Nobelius Scarlotta Newman

char names[N][200]; // names is a reference

char *nameptrs[N]; // nameptrs is a pointer array

John Jacob Jinglehiemer Schmidt

Alloitious Promethius Quoroyo Gildersleve

Mahatma Moriajib Mattesumium Matilda

Maria Cecila Prozelli Motsorelli Puchini

Alfred Erasmus Nobelius Scarlotta Newman

nameptr names

0

1

2

3

4

After sorting…

John Jacob Jinglehiemer Schmidt

Alloitious Promethius Quoroyo Gildersleve

Mahatma Moriajib Mattesumium Matilda

Maria Cecila Prozelli Motsorelli Puchini

Alfred Erasmus Nobelius Scarlotta Newman

nameptrs names

0

1

2

3

4

Program…

char names[N][200];char *nameptrs[N];

for (int n=0; n<N; n++){ cin >> names[n]; nameptrs[n] = names[n];}

SortByPointer (names, nameptrs);

for (int n=0; n<N; n++) cout << *(nameptrs[n]) << endl;

void SortByPointer (char names[ ][100], char *nameptrs[ ]){ int smallest_position; for (int n=0; n<N-1; n++) { smallest_position = FindSmallest (nameptrs, n, N);

Swap (nameptrs, n, smallest_position); }}

We’ll use a Selection Sort…

int FindSmallest (char *list[ ], int start, int size){ int position; char *smallest = list[start]; for (int k=start+1; k<size; k++)

if (strcmp(list[k], smallest) < 0){ smallest = list[k]; position = k;}

return position;}

FindSmallest()

Swap()

void Swap (char *list[ ], int j, int k){ char *temp; temp = list[j]; list[j] = list[k]; list[k] = temp;}