CS 1400 Chapters 9, 10 C-strings and Pointers

A few C-string library functions… #include int strcmp (char a[ ], char b[ ]); 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 characters char 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 3 rd name) names[2][1] = ‘x’; –changes the 2 nd 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 3 rd 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. Fred93.4 John87.1 Cheryl96.9 Shannon81.2 George77.2 ……

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-strings void 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-strings void 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… 1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz FizzBuzz …

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-spelled A 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 help\0 word (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 xyxy 0x200 0x202 0x204 0x300 0x

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 * symbol float *fptr;// may hold an address of a float short *iptr;// may hold an address of a short The address of a variable or cell may be determined using the & operator float b = 3.14; short c = 567; fptr = &b; iptr = &c;

fptr iptr x400 0x or more commonly diagrammed using arrows… fptr iptr x400 0x a b a b

Using pointer variables A cell value pointed to by a pointer variable may be referenced using the * operator cout << *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]; namesnames[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

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

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; }