COMPSCI 210 Part II Decisions & Iterations

Transcription

1 Agenda COMPSCI 210 Part II Decisions & Iterations Based on McGraw-Hill Agenda: Decision-making statements: he if statement he switch statement Iterations he for statement he while statement he do-while statement Nested Loops break, continue Problem Solving in C extbook: Chapter 13 Control Structures Exercise: Write a program to print all the powers of 2 II-06-2 Control Structures Conditional making a decision about which code to execute, based on evaluated expression if if- switch If if (condition) action; Condition is a C expression, which evaluates to RUE (non-zero) or ALSE (zero). Action is a C statement, which may be simple or compound (a block). condition action Iteration executing code multiple times, ending based on evaluated expression while for do-while if (x <= 10) y = x * x + 5; if (x <= 10) { y = x * x + 5; z = (2 * y) / 3; if (x <= 10) y = x * x + 5; z = (2 * y) / 3; compound statement; both executed if x <= 10 only first statement is conditional; second statement is always executed II-06-3 II

2 More If Examples if (0 <= age && age <= 11) kids += 1; if (month == 4 month == 6 month == 9 month == 11) printf( he month has 30 days.\n ); if (x = 2) y = 5; always true, so action is always executed! his is a common programming error (= instead of ==), not caught by compiler because it s syntactically correct. Generating Code for If Statement ; if (x == 2) y = 5; LC3 Code: NO_RUE into R0 ADD R0, R0, #-2 ; subtract 2 BRnp NO_RUE ; if non-zero, x is not 2 AND R1, R1, #0 ; store 5 to y ADD R1, R1, #5 SR R1, R5, #-1 0x0000 R5 0x y x If s can be nested, if (x == 3) if (y!= 6) { z = z + 1; w = w + 2; is the same as if ((x == 3) && (y!= 6)) { z = z + 1; w = w + 2; II-06-5 II-06-6 If- if (condition) action_if; action_; action_if condition action_ Generating Code for If-Else LC3 Code if (x) { y++; z--; { y--; z++; LDR R0, R5, #0 BRz ELSE ; x is not zero LDR R1, R5, #-1 ; load y ADD R1, R1, #1 ; incr y SR R1, R5, #-1 LDR R1, R5, #-2 ; load z ADD R1, R1, #-1 ; decr z SR R1, R5, #-2 BR ; skip code ; x is zero 0x0000 R5 0x z y x Else allows choice between two mutually exclusive actions without re-testing condition. ELSE LDR R1, R5, #-1 ; decr y ADD R1, R1, #-1 SR R1, R5, #-1 LDR R1, R5, #-2 ; incr z ADD R1, R1, #1 SR R1, R5, #-2 II-06-7 II

3 Matching Else with If Else is always associated with closest unassociated if. if (x!= 10) if (y > 3) z = z / 2; z = z * 2; is the same as if (x!= 10) { if (y > 3) z = z / 2; z = z * 2; is NO the same as if (x!= 10) { if (y > 3) z = z / 2; z = z * 2; Indentation can improve readability o force the clause to match the first if clause, you must add a pair of braces: Chaining If s and Else s he conditions are evaluated in order until one that evaluates to true is found. If none of them are true then the block is executed. he final is optional if (month == 4 month == 6 month == 9 month == 11) printf( Month has 30 days.\n ); if (month == 1 month == 3 month == 5 month == 7 month == 8 month == 10 month == 12) printf( Month has 31 days.\n ); if (month == 2) printf( Month has 28 or 29 days.\n ); printf( Don t know that month.\n ); II-06-9 II Switch Alternative to long if- chain. If break is not used, then case "falls through" to the next. No two case statements can have the same value. Value must be an an integer value: type long, int, or char he default statement is optional switch (expression) { case const1: action1; case const2: action2; default: action3; evaluate expression = const1? = const2? action3 action1 action2 Switch Example /* same as month example for if- */ switch (month) { case 4: case 6: case 9: case 11: printf( Month has 30 days.\n ); case 1: case 3: /* some cases omitted for brevity*/ printf( Month has 31 days.\n ); case 2: printf( Month has 28 or 29 days.\n ); default: printf( Don t know that month.\n ); II II

4 More About Switch Case expressions must be constant. /* illegal if i is a variable */ case i: If no break, then next case is also executed. switch (a) { case 1: printf( A ); case 2: printf( B ); default: printf( C ); If a is 1, prints ABC. If a is 2, prints BC. Otherwise, prints C. Writing Loops Loops Repeated execution of one or more statements until a terminating condition occurs Pre-test and post-test loops ypes of loops: Pre-test loops while for Post-test loop do while II II While Executes loop body as long as test evaluates to RUE (non-zero). while (test) loop_body; test loop_body Generating Code for While x = 0; while (x < 10) { printf( %d, x); x = x + 1; AND R0, R0, #0 SR R0, R5, #0 ; x = 0 ; test Note: est is evaluated before executing loop body When false, the loop body is not executed at all LOOP ADD R0, R0, #-10 BRzp ; loop body <printf> ADD R0, R0, #1 ; incr x SR R0, R5, #0 BR LOOP ; test again II II

5 Do-While Executes loop body as long as test evaluates to RUE (non-zero). loop_body Generating Code for Do-While do { printf( %d, x); x = x + 1; while (x < 10); do loop_body; while (test); Note: est is evaluated after executing loop body he loop body is executed at least once. Note: Don't forget to put a semicolon after the test. test LOOP <printf> ADD R0, R0, #1 ; incr 1 SR R0, R5, #0 ; store x ADD R0, R0, #-10 ;test x BRzp BR LOOP ; test again II II Infinite Loops he following loop will never terminate: x = 0; while (x < 10) printf( %d, x); or Executes loop body as long as test evaluates to RUE (non-zero). for (init; end-test; re-init) statement Loop body does not change condition, so test never fails. his is a common programming error that can be difficult to find. Initialization and re-initialization code included in loop statement. Note: est is evaluated before executing loop body. init test loop_body II re-init II

6 Generating Code for or for (i = 0; i < 10; i++) printf( %d, i); ; init AND R0, R0, #0 SR R0, R5, #0 ; i = 0 ; test his is the same as the while example! LOOP LDR R0, R5, #0 ; load i ADD R0, R0, #-10 BRzp ; loop body LDR R0, R5, #0 ; load i <printf> ; re-init ADD R0, R0, #1 ; incr i SR R0, R5, #0 BR LOOP ; test again Example or Loops /* -- what is the output of this loop? -- */ for (i = 0; i <= 10; i ++) printf("%d ", i); /* -- what does this one output? -- */ letter = 'a'; for (c = 0; c < 26; c++) printf("%c ", letter+c); /* -- what does this loop do? -- */ numberofones = 0; for (bitnum = 0; bitnum < 16; bitnum++) { if (inputvalue & (1 << bitnum)) numberofones++; /* -- what is the output of this loop? -- */ int i = 1; while (i < 3) { II II Nested Loops Loop body can (of course) be another loop. /* print a multiplication table */ for (mp1 = 0; mp1 < 10; mp1++) { for (mp2 = 0; mp2 < 10; mp2++) { printf( %d\t, mp1*mp2); printf( \n ); he test for the inner loop depends on the counter variable of the outer loop. for(i = 0; i < row; i ++) { for (j = 0; j <= i; j++) { printf("*"); printf("\n"); * ** *** for (outer = 1; outer <= input; outer++) { for (inner = 0; inner < outer; inner++) { sum += inner; Braces aren t necessary, but they make the code easier to read. Input an integer: 4 he result is 10 II or vs. While In general: or loop is preferred for counter-based loops. Explicit counter variable Easy to see how counter is modified each loop While loop is preferred for sentinel-based loops. est checks for sentinel value. Either kind of loop can be expressed as the other, so it s really a matter of style and readability. II

7 Break and Continue used only in switch statement or iteration statement passes control out of the smallest (loop or switch) statement containing it to the statement immediately following usually used to exit a loop before terminating condition occurs (or to exit switch statement when case is done) continue; used only in iteration statement terminates the execution of the loop body for this iteration loop expression is evaluated to see whether another iteration should be performed if for loop, also executes the re-initializer II Examples on break/continue for(i = 0; i < row; i ++) { for (j = 0; j < row; j++) { if ( i+j >= row ) printf("*"); printf("\n"); *** ** * for (i= 1; i <= row; i++ ) { if( i % 2 == 0) continue; // Go back to for printf("i = %d\n", i ); i = 1 i = 3 i for output 1 1<=4 i=1 2 2<=4 Continue, jump to next iteration 3 3<=4 i=1 i=3 4 4<=4 Continue, jump to next iteration 5 alse II Example What does the following loop do? for (i = 0; i <= 20; i++) { if (i%2 == 0) continue; printf("%d ", i); How about this one? What happens if break instead of continue? for (i = 0; i <= 20; i++) { if (i%2 == 0) printf("%d ", i); What would be an easier way to write this? II