Program Control

1. Program Control • Selection • Repetition • Altering the flow of control • Unconditional branch • More operators

2. Selection – 3 Ways • If structure (single selection) • If/else structure (double, multiple selection) if(condition1) statement1; else if(condition2) statement2; . . . . . .; else statementN; • Switch structure (multiple selection)

3. If Structure • If no parenthesis, then only the 1st statement following the if statement is associated with it • if(a>b) c=3; d=4; • So d=4; will always be evaluated

4. If Structure • To make your code clear (to yourself and others), you should always use brackets after your if-statements • if(a>b){ c=3;} d=4; • if(a>b){ c=3; d=4;}

5. If/else Ambiguity • If one if statement doesn’t have an ending else & another one does, may be ambiguous • Resolved by associating the else with the closest if if(a>b) if(c>d) e=3; else d=4; • Must use braces to associate with the outside if if(a>b){ if(c>d) e=3; }else d=4;

6. Switch Structure • Handles a series of decisions • Must include a break statement • If not, each case statement will be executed until encounter another break statement or reach end • Several cases can execute the same statements • By listing case statements together • Can only test constant integer expressions

7. /* Counting A,B,C or a,b,c (counting.c)*/ #include <stdio.h> int main(){ int a=0,b=0,c=0,e=0,ch=0; while((ch=getchar()) != EOF){ switch(ch){ case 'A': case 'a': a++; break; case 'B': case 'b': b++; break; case 'C': case 'c': c++; break; default: e++; break;/*defensive programming*/ } } printf("\na=%d\nb=%d\nc=%d\nextra=%d",a,b,c,e); return 0; }

8. Character I/O • I/O = Input/Output • Text input or output = stream of characters • Standard library functions for char I/O char c; c = getchar(); /*input*/ putchar(c); /*output*/ • Will get or put one character at a time from the input or output text stream

9. End-of-File Marker • EOF is a character used to mark the end of a file or the end of input • EOF key combinations on different systems: • UNIX & Mac: Ctrl-d • PCs: Ctrl-z

10. I/O Redirection in UNIX • Instead of typing input for a program, you can use I/O redirection in UNIX to use a file as input or output to your program • < (less-than sign) is for input • > (greater-than sign) is for output • For example: • File input: ./program < input.txt • File output: ./program > output.txt • Both: ./program < input.txt > output.txt • Use EOF (end of file) to end the program • Try it: counting < electricity.txt

11. Repetition – 3 Ways • while (condition) {statements;} • while(1+2*5){ printf(“infinite loop ”); } • do{statements} while(condition) • do{printf(“one”); } while(1 < 0); • for(initialize, condition, increment) • for(i=0; i<5; i=i+1) {printf(“five”);} • Note:For conditions in C, • false = 0 & true = nonzero (anything else)

12. for = = while • for(expr1;expr2;expr3) statement; • Is equivalent to: • expr1; while(expr2){ statement; expr3; } • This will create an infinite loop: • for(;;){ . . .}

13. Comma in For Loops • Can put commas in for loops • Evaluated left to right #include <stdio.h> int main() { int a=0,b=0; for(a=0, b=10; a<b; a++, b--) printf("a=%d b=%d\n",a,b); return 0; }/*see comma.c*/

14. Alter Flow of Control • break; • Immediately exit from while, for, do/while, or switch statements • continue; • Skips the remaining statements • Performs the next iteration of while, for, or do/while loop

15. Unconditional Branch #include <stdio.h> int main(){ start: printf("hello"); gotostart; return 0; } /*see goto.c*/

16. Goto • As a general rule, not a good idea to use goto • Might use when need to escape from a deeply nested structure (since break will only exit from innermost loop) for(. . .) { for(. . .){ if(need_to_stop) goto stop_loop; }} stop_loop: Continue with the program. . .

17. ASCII Character Codes Character Decimal Hex Binary • '0' 48 dec 0x30 0011 0000 • '9' 57 dec 0x39 0011 1001 • 'A' 65 dec 0x41 0100 0001 • 'Z' 90 dec 0x5A 0101 1010 • 'a' 97 dec 0x61 0110 0001 • 'z' 122 dec 0x7A 0111 1010

18. Formatted Output • Conversion specifiers • %c (char), %d (dec), %x (hex), %f (float) char c = 'A'; int i = 66; printf("c=%c i=%c\n", c, i); //c=A i=B printf("c=%d i=%d\n", c, i); //c=65 i=66 printf("c=%x i=%x\n", c, i); //c=41 i=42 • What’s the output if we set i = 66 + 1024? • See format.c

19. Relational & Logical Operators • Relational: >, >=, <, <=, ==, != • Logical: &&, ||, ! • Evaluated left to right • Evaluation stops as soon as true or false is determined • Ex: if(a && b && c) • Will stop if “a && b” is false • Probably should write as: if((a && b) && (b && c)) • Often see if(!x), which means if(x==0)

20. Type Conversions • Implicit conversion – if an operation has operands of different types, the “narrower” one will be converted to the “wider” one • 2.0 / 5 evaluates to 0.4 (a float) • Explicit conversion – can force a conversion with a cast • (type-name) expression • ((int)2.0) / 5 evaluates to 0 (an integer) • a = sqrt((double) x); (a = double, x = integer)

21. Increment & Decrement Operators • Preincrement • Add 1, then assign new value • For example, if a = 3 b = ++a; /*(b=4, a=4)*/ • Postincrement • Assign old value, then add 1 • For example, if a = 3 b = a++; /*(b=3, a=4)*/ • Example: increment.c

22. Assignment Operators x = x + 5; • Can also be written as x += 5; a *= b + 5; • Evaluates to a = a *(b + 5); • And not a = a * b + 5;

23. Conditional Expressions if (x>y) a=0; else a =1; • Can also be written as x>y ? a = 0 : a = 1;

24. Precedence • Precedence & of operators • See table in Dietel & Dietel, Appendix A • If you are not sure of operator precedence, or if you want to make your code more readable • Use parenthesis ()