07-ConditionalStatements.pdf

Transcript

1. ICS 212 Program Structure William McDaniel Albritton M.S.
2. Memory Allocation  Relational Operators  Logical Operators  If Statements  Switch Statements
4. Relational Operators
5. Relational Operators  Relational operators  > (greater than)  < (less than)  >= (greater than or equal)  <= (less than or equal)  Will return a 0 (false) or 1 (true)
6. Relational Operators int num3 = 3, num5 = 5; printf(“%d\n”,num3<num5); //1 printf(“%d\n”,num3>num5); //0
7. Equality Operators  Equality operators  == (equal)  != (not equal)  Will return a 0 (false) or 1 (true) int num3 = 3, num5 = 5; printf(“%d\n”,num3==num5); //0
8. Common Error  WARNING: a common error is to use the assignment operator (=), instead of the equality operator (==)  Still compiles, so it is a difficult bug to find!  And it will change the value of the 1st operand int num3 = 3, num5 = 5; printf(“%d\n”,num3=num5); //5
9. Boolean Operator not (!)  !x is the same as 0==x  For the C language, zero is false  And anything non-zero is true  Determines if x is equal to zero, or not equal to zero int zero = 0, num5 = 5; printf(“%d\n”,!zero); //1 printf(“%d\n”,!num5); //0
10. Example Program  See example code at: operators.c
12. Logical Operators
13. Logical Operators Logical Operator Symbol AND && OR | | NOT !
14. Logical Operators  AND (&&), OR (||), NOT (!)  Evaluated left to right  Returns true (1) or false (0)
15. AND (&&) Operator  Has 2 operands  Returns true if both operands are true, otherwise false X Y X && Y True True True True False False False True False False False False
16. AND (&&) Operator  A && B  People who like cats AND People who like dogs A (like cats) B (like dogs)
17. OR (||) Operator  Has 2 operands  Returns true if at least one operands is true, otherwise false X Y X || Y True True True True False True False True True False False False
18. OR (||) Operator  A || B  People who like cats OR People who like dogs A (like cats) B (like dogs)
19. NOT (!) Operator  Has 1 operand  If false (0), returns true (1) and if true (not zero) returns false (1) x ! x false true true false
20. Do not like cats NOT (!) Operator  !A  People who do NOT like cats A (like cats)
21. Example Program  See example code at: relations.c
23. If Statements
24. Simple if Statements true false if(condition) statement; condition statement
25. if(condition){ statement1; statement2; statement3; …. statement n; } Simple if Statements true false condition statement statement statement statement
26. if(condition) { statement1; } else { statement2; } if/else Statement true false condition statement 1 statement 2
27. Empty Statement if(x<y) ; else count++; if(x>=y) count++; Should be rewritten!!
28. Comparing Numbers with Decimals  Because of precision and rounding differences in floating point numbers, do not use the equality operator (==) to compare two floating points double threshold = 0.00001; if(fabs(number) < threshold){ printf(“Close to zero”); }
29. if Statements are for Ranges  if (x > 10)  if (a>0) && (a <10) 0 10 -10 0 10 -10
30. Take No Shortcuts  In C, it is better to not take any shortcuts to writing relational and logical statements  Do not write  If a<b is false, the condition becomes: if( 0 < c)  If a<b is true, the condition becomes: if( 1 < c) if(a < b < c) if((a < b)&&(b < c))
31. Logical Operator Evaluation  C uses shortcuts with logical operators AND, OR, NOT (&&, | |, !), which are evaluated left to right
32. Logical Operator Evaluation  Evaluation stops as soon as true or false is determined  Example:  Will stop if “a && b” is false.  Rewrite: if(a && b && c) if((a && b) && (b && c))
33. Example Program  See example code at: if.c
35. Switch Statements
36. switch Statements  For discrete values  Work only with characters and integer numbers
37. Diagram for a switch Statement case 1 Expression statement list 1 statement list 2 statement list 3 statement list N case 2 case 3 default
38. switch Statements Syntax switch(expression) { case label1: //instructions case label2: //instructions case label3: //instructions default: //instructions }
39. The break Statement  The break statement transfers control outside the structure  If no break statement is present ALL the code within the switch statement will execute, beginning in the matching case and ending whenever a break statement is found or the switch statement ends
40. switch vs. if  Use switch statements for discrete values  Use if statements for ranges  Switch functionality can be duplicated with if statements, many of them…
41. Example Program  See example code at: switch.c
42. Memory Management  Relational Operators  Logical Operators  If Statements  Switch Statements