Lecture 4. CS118 Term planner. The simple if statement. Control flow: selection. The if... else statement cont... The if... else statement.
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1 1 Lecture 4 CS118 Term planner You should have done, or be working through, problem sheet 2; next is sheet 3 Coursework - common mistakes do while (robot.look(direction) = IRobot.WALL); if (robot.look(irobot.ahead) = IRobot.WALL) You are here If you want to get the coursework running on your home computer and are stuck talk to a friend who has got it working, or mail the CS118 Forum 4 Control flow: selection The simple if statement The order in which instructions are executed is central to imperative programming It is in controlling this order that we manage the order in which the side effects (e.g. assignment) take place This ultimately dictates the result of the code Selection statements in Java: if if switch Syntax: if (booleanexpression) Brackets optional if there is only one statement in the body if (booleanexpression) History: Introduced in Algol 60 If the booleanexpression evaluates to true, then the body of the if statement is executed, otherwise control passes to the next program statement after the if construct 7 8 The if statement The if statement cont Syntax: if (booleanexpression) Note the use of indentation Brackets optional if there is only one statement in the body if (booleanexpression) If the booleanexpression evaluates to true, then the body of the if statement is executed, otherwise control passes to the body of the part. Sequential control resumes at the next program statement after the if construct. Examples if ( robot.look(direction) == IRobot.WALL ) if (!(a>b) & (x>1) ) P 1 P
2 17 Nested if statements Nested if statements It is possible to nest ifs to any level if (percent > 80) if (percent > 70) grade = 'C'; if (percent > 60) grade = 'D'; grade = 'E'; Dangling problem if (x > 5) if ( y > 5) System.out.println( Both x and y are > 5 ); System.out.println( x <= 5 ); Style of indentation if (percent > 80) if It is possible to nest ifs to any level if (percent > 80) if (percent > 70) grade = 'C'; if (percent > 60) grade = 'D'; grade = 'E'; Dangling problem What gets printed when x = 5? if (x > 5) if ( y > 5) System.out.println( Both x and y are > 5 ); System.out.println( x <= 5 ); 19 Example: Truth table Dog s dinner II: Shortcut ifs Truth table for a logical or with two inputs Input A Input B Output false false false false true true true false true true true true Coded using if if (!InputA &!InputB) if (!InputA & InputB) if (InputA &!InputB) if (InputA & InputB) Factoring unnecessary cases if (!InputA &!InputB) Output = false; Output = false; Standard if statement if (x>0) y = 1; y = -1; Shortcut (dog s dinner) if statement y = (x>0)? 1 : -1; Just don t bother The switch statement The switch statement cont Suppose that there are several choices Some syntax rules: if (typesize == b ) store = 1; if (typesize == s ) store = 2; if (typesize == i ) store = 4; if (typesize == l ) store = 8; Java provides an alternative control structure switch (typesize) case b : store = 1; break; case s : store = 2; break; case i : store = 4; break; case l : store = 8; break; default : System.out.println( Invalid typesize ); switch (switchexpression) case value1 : s1; break; case value2 : s2; break; case value3 : s3; break; case valuen : sn; break; default : sd; The switchexpression must yield a value of type char, byte, short or int The value1 valuen must have the same datatype as the switchexpression case statements are evaluated top to bottom; The branch of a case that matches the result of the switchexpression will be evaluated; The break statement is optional; it will terminate the entire switch statement; The default statement is optional; if reached, its body will always be evaluated 25 29
3 40 Programming tricks and commonc mistakes Some languages (Modula 2, Ada, Fortran 90) allow you to compare ranges of values switch ( some value ) case 3..4 : second choice; case 1..2 : first choice; As Java (and C) require each value to have a separate label, you can simulate this as follows switch ( some value ) In this case the program falls through to the next case case 1 : case 2 : first choice; break; case 3 : case 4 : second choice; break; While the use (or not) of breaks (and defaults) seems like a good idea, often this will just lead to errors Consider the following example int i = 1; switch (i) case 0 : System.out.print( zero ); case 1 : System.out.print( one ); case 2 : System.out.print( two ); case 3 : System.out.print( three ); default : System.out.print( done ); This results in: one two three done This weird action is because case 2 onwards are discounted by the fact that the control flow of the program continues though the statements (to the right of the colon) until a break is reached or the switch terminates 46 and commonc mistakes You also need to be careful about the cases which your switch covers switch (typesize) case b : store =1; break; case s : store =2; break; case i : store =4; break; case l : store =8; What happens if typesize is f? Notice that there is no break statement here; this is OK Reasoning with choice statements Under what conditions will p4 be executed? if (a<0) if (b!=0) if (a==b) p3 if (a==0) p4 In some languages (Ada, Pascal) this causes a dynamic semantic error, in Java (and C and Fortran) we can get away with it Add preconditions Add preconditions cont condition : true condition: true.: true & (a<0) if (b!=0) if (a==b) p3 if (a==0) p4.: true & (a<0) if (b!=0).: true & (a>=0) & (b!=0) if (a==b) p3 if (a==0) p
4 53 condition: true condition: true.: true & (a<0) if (b!=0).: true & (a>=0) & (b!=0) if (a==b).: true & (a>=0) & (b==0) & (a==b) p3 if (a==0) p4.: true & (a<0) if (b!=0).: true & (a>=0) & (b!=0) if (a==b).: true & (a>=0) & (b==0) & (a==b) p3 if (a==0).: true & (a>=0) & (b==0) & (a!=b) & (a==0) p4 54 General rule for reasoning about if statements Reasoning about if statements if (guard) & guard S1 Combining the precondition and guard logic allows us to strengthen the precondition of the guarded statements in S1 if (guard) & guard S1 &!guard S Reasoning about switch statements case v1 : & (e==v1) statement1; break; case v2 : & (e!=v1) & (e==v2) statement2; break; case v3 : & (e!=v1) & (e!=v2) & (e==v3) statement3; break; case vn : & (e!=v1..v(n-1)) & (e==vn) statementn; break; default : & (e!=v1..vn) statement for default; Missing break: alters precondition case v1 : & (e==v1) statement1; case v2 : & ((e==v1) (e==v2)) statement2; break; Missing default: pre => (e==v1) (e==v2) / case v1 : & (e==v1) statement1; break; case v2 : & (e!=v1) & (e==v2) statement2; since if neither case were true, Java does nothing 57 58
5 59 Pragmatism This technique will not always be applied When should I use it? When the logic is complicated When there can be no question as to the result When your programs fail
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