1 CS 192 Lecture 9 Winter 2003 December 19, 2003 Dr. Shafay Shamail

2 The for Loop Repetition –construct to execute a section of code a specified number of times for (initialization; test-condition; increment) { statement(s) } (1) Set initial value of counter. (2) Perform test to see if loop should continue. (3) Execute statements. (4) Update counter. (5) Repeat from (2). #include int main() { int count;//loop control variable for (count=1; count<=100; count++) cout << count << " "; return 0; }

3 The for Loop Output? for (count=10; count < 5; count++) cout << count; A square root calculating program #include #include // for older compilers, use int main() { int num; for(num=1; num < 100; num++) { cout << num << " " << sqrt((double) num) << '\n'; } return 0; }

4 Variations of the for Loop int main() { for(int i=100; i >= -100; i = i-5) cout << i << ' '; return 0; } ================================ for(x=0, y=10; x<=10; ++x, --y) cout << x << ' ' << y << '\n'; ================================ #include int main() { int i; // print numbers until a key is pressed for(i=0; !kbhit(); i++) cout << i << ' '; return 0; }

5 Variations of the for Loop int x; for(x=0; x != 123; ) { cout << "Enter a number: "; cin >> x; } ================================ int x = 0; for( ; x<10; ) { cout << x << ' '; ++x; } ================================ for(;;) { body of loop }

6 The while Loop Repetition construct –it’s a for loop stripped of the initialization and update parts while (test-condition) { statement(s) }  for ( ;test-condition; ) { statement(s) }  initialize counter while (test-condition) {statement(s); update counter; }

7 The while Loop Might use it if do not know beforehand when to stop repeating int n = 99; //why initialize? cout << “Guess my fav number”; while(n != 0) cin >> n; cout << “Yes, I was looking for 0 \n”; ================================ unsigned char ch; ch = 1; while(ch) {//when does it terminate? cout << ch; ch++; }//displays what?

8 The do-while Loop Similar to while, except that tests condition after the body of the loop i.e. statements executed at least once do { statement(s); } while (test-condition); ================================ int num; do { cout << "Enter a number (100 to stop): "; cin >> num; } while (num != 100);

9 Which Loop to Use? if (number of repetitions known) use for; else if (body to be executed once at least) use do while; else {use while;} No hard rule; all interchangeable mostly; matter of style

10 The continue Statement continue, break and goto are jump statements. continue used in loops and causes program to skip rest of loop’s body and start the next iteration immediately. Actually takes you to the closing brace of the loop body long dividend, divisor; char ch; do { cout > dividend; cout > divisor; if (divisor == 0) //divide by zero error { cout << "Illegal divisor\n"; continue; } cout << "Quotient is " << dividend/divisor; cout << ", remainder is " << dividend%divisor; cout << "\nDo another? (y/n): "; cin >> ch; } while (ch != 'n');

11 The continue Statement Output of following? #include int main() { int x; for(x=0; x<=100; x++) { if(x%2) continue; cout << x << ' '; } return 0; }

12 The break Statement Causes exit from loop or switch #include int main() { double x; while (1) { cout << "Enter number for square root: "; cin >> x; if (x <0.0) break; //exit loop if x is -ve cout << sqrt(x) << endl; }// break jumps to here cout << "Sorry, can't do that\n"; return 0; }

13 The break Statement int t; for(t=0; t<100; t++) { if(t==10) break; cout << t << ' '; } ================================ for(i=0; i<1000; i++) { // do something if(kbhit()) break; }// keypress stops execution break causes exit from the innermost loop or switch, and not from the enclosing loops or switches

14 The goto Statement Unconditional branch statement that jumps to a label (identifier) goto label; … label: statement(s); ================================ int x = 1; loop1: cout << x << endl; x++; if(x < 100) goto loop1; ================================ goto end; int i = 10; //error: goto skips initialization end: ;

15 Example Read and understand the magic number program given at the end of the chapter 4

16 while #include int main() { int count_down; cout << "How many greetings do you want? "; cin >> count_down; while (count_down > 0) { cout << "Hello "; count_down = count_down - 1; } cout << endl; cout << "That's all!\n"; return 0; }

17 #include int main() { char ans; do { cout << "Hello\n"; cout << "Do you want another greeting?\n" << "Press y for yes, n for no,\n" << "and then press return: "; cin >> ans; } while (ans == 'y' || ans == 'Y'); cout << "Good-Bye\n"; return 0; } do.. while

18 Counting Space Characters (while) void countSpaces1(void) { int ch = 0; int numofspaces = 0; printf("Enter a sentence: "); ch = getchar(); while ( ch != '\n' ) { if ( ' ' == ch ) numofspaces++; ch = getchar(); } cout << "\nThe number of spaces is: " << numofspaces << endl; }

19 Counting Space Characters (do..while) #include // getchar() void countSpaces2(void) { int ch = 0; int numofspaces = 0; printf("Enter a sentence: "); do { ch = getchar(); if ( ' ' == ch ) numofspaces++; } while ( ch != '\n' ); cout << "\nThe number of spaces is: " << numofspaces << endl; }

20 Make Number from Digits #include // getchar() #include // isdigit() void makeInteger(void) { int num=0; int digit=0; printf("Enter a number: "); digit = getchar(); for( ; isdigit(digit) ; digit=getchar() ) { num = num * 10; num = num + (digit - '0'); } cout << "The number is: " << num << endl; }

21 Skip Spaces #include // header file for isspace() for (int = getchar(); isspace( c ) ; c = getchar() ) ; ungetc( c, stdin );// put the non-space char // back in the input buffer

22 Skip Spaces #include // getchar() #include // isdigit(), isspace() void skipSpaces(void) { int c = 0; int count = 0; cout << "\nSkipping Spaces\n\n\n"; cout.flush(); for ( c=getchar(); isspace(c); c=getchar() ) { count++; } ungetc(c, stdin); cout << count << " spaces skipped\n"; } What is the behavior of this program What is the function of cout.flush() What will happen if it is removed What does ungetc() do