CS31: Introduction to Computer Science I

1. CS31: Introduction to Computer Science I Discussion 1A 5/7/2010 Sungwon Yang swyang@cs.ucla.edu www.cs.ucla.edu/~swyang

2. Quick Review • What did we learn last week? • Arrays • create the same type of multiple variables at one time • index numbers start from 0 • each item of an array is a regular variable • array declaration • specify the length of the array • the number must be a constant positive integer • value itself or const type of int variable • initialization of an array • values enclosed in curly braces • number of values can be less than the length of an array • if omit the length of an array, it is determined based on the number of values • out-of-bound index • c++ compiler does not check the bound of index • accessing out-of-bound items does not cause a compile-error • be careful not to access out-of-bound items in loop • arrays in functions • can pass individual items either by value or reference • can pass the entire array • can be regarded as passing by reference • const keyword • protects the items of the array from accidentally being changed

3. C-strings • C++ is enhanced from C Language • The C Language is a proper subset of C++ • C does not have string type • it has its own way of representing strings • we can still use the older way in C++ • called C-strings

4. C-strings • Can recall what string is? • a sequence of zero or more characters • we have learned arrays last week • a sequence of variables of the type we define • a string can be represented by an array of characters!

5. C-strings • let’s define an array of characters • char s[10]; • 10 character variables created. • Now, which strings can be stored in the array? • “” : empty string • “abcde” : the length is 5 • “abcdefghi” : the length is 9 • “abcdefghij” : the length is 10 “abcdefghijklmnopqrstuvwxyz” : the length is 26 (X) (X)

6. zero byte/null character • C-string must end with a marker • the marker indicates “this is the end of the string” • without this marker, it is a character array, not a string • this marker is called zero byte or null character • ‘\0’ (ASCII number is 0) • the null character is automatically filled in the last slot

7. \0 a a \0 b ? ? ? c ? ? ? 1 ? 2 ? ? ? ? 3 ? \0 ? ? ? ? ? ? ? 0 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 9 9 9 C-strings C String Examples char st1[10] = ""; char st2[10] = "a"; char st3[10] = "abc 123";

8. a a b b c c d d e e f \0 g h i \0 0 0 1 1 2 2 3 3 4 4 5 5 6 7 8 9 C-strings More C String Examples char st1[] = "abcde"; char st2[] = "abcdefghi";

9. c-string output • can use cout << to print out • will print characters until it meets ‘\0’ int main () { char s[] = "Hello, how are you?"; cout << s <<endl; } int main () { char s[] = "Hello, how are you?"; s[5] = '\0'; cout << s <<endl; } Hello, how are you? Hello

10. c-string output • cout works with character arrays • for other array types, it will print something different int main () { char s1[10]="abcde"; char s2[10]="12345"; char c[10]={'1' ,'2','3','4','5'}; int i[10]={1 , 2 , 3 , 4 , 5}; double d[10]={1.0 , 2.0 , 3.0 , 4.0 , 5.0}; bool b[10]={true,false,true,false,true}; cout << s1 << endl; cout << s2 << endl; cout << c << endl; cout << i << endl; cout << d << endl; cout << b << endl; } abcde 12345 12345 0028F810 0028F838 0028F88C

11. C-string input • cin • read one word • cin.getline(C-string variable, max) • read up to (max number-1) of characters in one line • ‘\0’ is stored in the last Hi Sungwon Hi Sungwon int main () { char s1[20]; char s2[20]; cin >> s1; cin.ignore(10000,'\n'); cin.getline(s2,10); cout << s1 << endl; cout << s2 << endl; } Hi Hi Sungwo

12. initialization vs. assignment int main () { char s[] = "Hello"; cout << s <<endl; } int main () { char s[10]; s = "Hello"; cout << s <<endl; } OK! Compile Error

13. Quick question • what will be displayed? int main () { char s[10] = {'a','b','c','d','e','f','g','h','i'}; cout << s << endl; } int main () { char s[10] = {'a','b','c','d','e','f','g','h','i', 'j'}; cout << s << endl; } abcdefghi abcdefghijts잔O憬7

14. More quick question • what will be displayed? int main () { char s[10]; s[0] = 'a'; s[1] = 'b'; s[2] = 'c'; s[3] = 'd'; s[4] = 'e'; cout << s << endl; } int main () { char s[10]; s[0] = 'a'; s[1] = 'b'; s[2] = 'c'; s[3] = 'd'; s[4] = 'e'; s[5] = '\0'; cout << s << endl; } abcde05"ts?5 abcde

15. C-string to C++ strings • convert C-string into C++ string type • The C++ string operator = works • but, not vice versa int main () { char s[] = "How are you?"; string str = s; cout << str << endl; } int main () { string str = "How are you?"; char s[] = str; cout << s << endl; } How are you? Compile Error

16. Functions for C-strings • basically, C-string is an array of characters • can manipulate characters with index number • There are pre-defined functions for C-strings • #include <cstring> required

17. C-string libraries • strlen(s) • returns the length of s, not counting ‘\0’ int main () { char s[] = "How are you?"; cout << strlen(s) << endl; } 12

18. C-string libraries • strcpy(t, s) • copies the string s to t • t = s does not work • does not check the array size • make sure there is enough space in t int main () { char s[10] = "Hello"; char t[10]; strcpy(t, s); cout << t << endl; } Hello

19. C-string libraries • strcat(t, s) • concatenate (append) s to the end of t • does not check the array size • make sure there is enough space in t int main () { char s[10] = "Hello"; char t[20] = "Sungwon,"; strcat(t, s); cout << t << endl; } Sungwon,Hello

20. C-string libraries • strcmp(t, s) • compare t and s • return 0 if they are equal • return something greater than 0 if t > s • return something less than 0 if t < s int main () { char s[10] = "abcde"; char t[10] = "abcde"; cout << strcmp(t,s) << endl; } int main () { char s[10] = "abcde"; char t[10] = "abced"; cout << strcmp(t,s) << endl; } int main () { char s[10] = "abcde"; char t[10] = "abc"; cout << strcmp(t,s) << endl; } 0 1 -1

21. array of C-strings • A C string is essentially an array of characters. • an array of C strings is an array of arrays of characters. An array of arrays is simply a 2D array • char s[10][20]; • s[3] : can be used to refer to the string in position 3 • s[3][5] : can be used to refer to the letter in position 5 of the string in position 3. hello apple r int main () { char s[3][6]; strcpy(s[0], "hello"); strcpy(s[1], "apple"); strcpy(s[2], "world"); cout << s[0] << endl; cout << s[1] << endl; cout << s[2][2] << endl; }