1. Chapter 3 Control Flow Ku-Yaw Chang canseco@mail.dyu.edu.tw Assistant Professor, Department of Computer Science and Information Engineering Da-Yeh University

2. 2Ku-Yaw ChangControl Flow 3.1 Statements and Blocks An expression such as x = 0 or x = 0 or i++ or i++ or printf(…) printf(…) becomes a statement when it is followed by a semicolon x = 0; x = 0; i++; i++; prinft(...); prinft(...); In C, the semicolon is a statement terminator.

3. 3Ku-Yaw ChangControl Flow 3.1 Statements and Blocks Braces { and } are used to group declarations and statements together into a compound statement, or block. Syntactically equivalent to a single statement Syntactically equivalent to a single statement No semicolon after the right brace that ends a block No semicolon after the right brace that ends a block

4. 4Ku-Yaw ChangControl Flow 3.2 If-Else The if-else statement is used to express decisions. if (expression) statement 1 else statement 2 The expression is evaluated The expression is evaluated True : expression has a non-zero value False: expression is zero The else part is optional The else part is optional

5. 5Ku-Yaw ChangControl Flow 3.2 If-Else Coding shortcuts if (expression) if (expression != 0) Ambiguity when an else is omitted from a nested if sequence if(n > 0) if (a > b) z = a; else z = b; if(n > 0) { if (a > b) z = a; }else z = b;

6. 6Ku-Yaw ChangControl Flow 3.3 Else-If A multi-way decision if (expression) statement else if (expression) statement statement statementelsestatement The last else part Non of the above or default case Can be omitted To catch an impossible condition

7. 7Ku-Yaw ChangControl Flow 3.4 Switch A multi-way decision Test whether an expression matches one of a number of constant integer values, and branches accordingly. switch (expression) { case const-expr: statements case const-expr: statements default: statements }

8. 8Ku-Yaw ChangControl Flow 3.4 Switch switch (n) { case 1: printf(“The number is 1\n.”); break; case 2: printf(“The number is 2\n.”); break;default: printf(“The number is not 1 or 2\n.”); break;}

9. 9Ku-Yaw ChangControl Flow Exercise 3-1 Write a program to count the occurrences of each digits, while space, and all other characters.

10. 10Ku-Yaw ChangControl Flow 3.5 Loops – While and For While while (expression) statement The expression is evaluated The expression is evaluated Non-zero: statement is executed and expression is re- evaluated The cycle continues until expression becomes zero The cycle continues until expression becomes zero

11. 11Ku-Yaw ChangControl Flow 3.5 Loops – While and For For for (expr 1 ; expr 2 ; expr 3 ) statement is equivalent to expr 1 ; while (expr 2 ) { statement expr 3 ; } Any of the three parts can be omitted Semicolons must remain An infinite loop: for (;;) { … } break or return

12. 12Ku-Yaw ChangControl Flow Exercise 3-2 Write a program that calculates and prints the sum of the even integers from 2 to 30.

13. 13Ku-Yaw ChangControl Flow Exercise 3-3 Write a function reverse(s) that reverses the string s in place. void reverse(char s[]) void reverse(char s[])

14. 14Ku-Yaw ChangControl Flow 3-6 Loops – Do-while The syntax of the do is dostatement while (expression); The body is always executed at least once. The body is always executed at least once. do-while is much less used than while and for. do-while is much less used than while and for.

15. 15Ku-Yaw ChangControl Flow 3.6 Loops – Do-while /* itoa: convert n to characters in s */ void itoa(int n, char s[]) { int i, sign; if ((sign=n) < 0)/* record sign */ n = -n;/* make n positive */ n = -n;/* make n positive */ i = 0; do {/* generate digits in reverse order */ s[i++] = n % 10 + ‘0’;/* get next digit */ } while ((n /= 10) > 0);/* delete it */ if (sign < 0)s[i++] = ‘-’; s[i] = ‘\0’; reverse(s);}

16. 16Ku-Yaw ChangControl Flow 3.7 Break and Continue The break statement provides an early exit from for, while, and do, just as from switch. A break causes the innermost enclosing loop or switch to be exited immediately.

17. 17Ku-Yaw ChangControl Flow 3.7 Break and Continue /* trim: remove trailing blanks, tabs, newlines */ int trim(char s[]) { int n; for (n = strlen(s)-1; n >= 0; n--) if (s[n] != ‘ ‘ && s[n] != ‘\t’ && s[n] != ‘\n’) break; s[n+1] = ‘\0’; return n; }

18. 18Ku-Yaw ChangControl Flow 3.7 Break and Continue The continue statement causes the next iteration of the enclosing for, while, or do loop to begin. An example: skip negative values for ( i = 0; i < n; i++ ) { if (a[i] < 0) /* skip negative elements */ continue; continue; … /* do positive elements */ }

19. The End