ECE 103 Engineering Programming Chapter 22 Selection

1. ECE 103 Engineering ProgrammingChapter 22Selection Herbert G. Mayer, PSU CS Status 6/22/2014 Initial content copied verbatim from ECE 103 material developed by Professor Phillip Wong @ PSU ECE

2. Syllabus • If • if-else • if-else if • Switch • Triadic Conditional Expression • Nested Selection Statements

3. Making Decisions in Programs A selection statement uses a conditional test to control the flow of execution in a program “conditional test” is AKA “conditional expression” or “boolean expression” A conditional test contains relational and/or logical expressions Conditional test evaluates to “true” or “false”: If test is “true” (non-zero), one group of statements is executed, known as the “Then Clause” If test is “false” (zero), a different group of statements is executed

4. Decisions can be documented by a flowchart or by pseudocode: expression branch A branch B IF expression is true THEN Execute branch A ELSE Execute branch B END IF true false Example of 2-way true or false decision

5. Selection Statements in C In C, logic states have these definitions: “false” is the zero value (e.g., 0, 0.0, '\0') “true” is any non-zero value (e.g., -1, 5, 12.4, 'A’) C has three types of selection statements based on the if keyword: if if–else if–else if any number of times C has an additional selection called switch

6. if pre-code; if (expression) Statement; post-code; Statement post-code pre-code T expression F

7. expression must be inside parentheses ( ). Statement can be a single statement or a block: if (expression) Statement; /* Single statement */ if (expression) { Statement_1; /* Statement block */  Statement_k; } All statements in the block are executed if the expression is true.

8. Example: #include <stdio.h> #define TRUE 1 #define FALSE 0 #define LO_LIMIT 20 #define HI_LIMIT 100 int main( void ) { // main int N; /* Input value */ int low = FALSE, high = FALSE; printf( "Enter N: ” ); scanf( "%d", &N ); /* Check input range */ if( N < LO_LIMIT ) low = TRUE; if( N > HI_LIMIT ) high = TRUE; if( low || high ) printf("Out of range.\n"); if( !low && !high ) printf("Within range.\n"); return 0; } if( temperature <= 65 ) turn_heat_on = 1; if( temperature > 65 ) turn_heat_on = 0; if( x < 0 ) { printf("x is negative.\n”); } //end if if( x < 0 ) { printf("x is negative.\n”); y = abs(x); } //end if Note: Indentation improves clarity. 7

9. if–else pre-code; if (expression) Statement_T; else Statement_F; post-code; post-code pre-code T F expression Statement_T Statement_F

10. Example: #include <stdio.h> #define TRUE 1 #define FALSE 0 #define LO_LIMIT 20 #define HI_LIMIT 100 int main( void ) { // main int N; /* Input value */ printf("Enter N: "); scanf("%d", &N); /* Check input range */ if( N < LO_LIMIT || N > HI_LIMIT ) printf("Out of range.\n"); else printf("Within range.\n"); // end if return 0; } if( temperature <= 65 ) turn_heat_on = 1; else turn_heat_on = 0; if( age >= 18 ) { printf( "Adult\n” ); allow_vote = 1; }else{ printf( "Pre-Adult\n” ); allow_vote = 0; } //end if 9

11. Example: /* This is wrong! */ if( y ) m = 1; printf( "Yes!” ); else m = -1; is actually seen by the compiler as if( y ) m = 1; printf( "Yes!” ); else m = -1; Write code that does this: if y is true, then set m to 1 and display "Yes!". Otherwise, set m to -1 and display nothing. /* This is correct */ if( y ) { m = 1; printf( "Yes!” ); }else{ m = -1; } //end if 10

12. if–else if pre-code; if (expression_1) Statement_1; else if (expression_2) Statement_2; else Statement_3; post-code; post-code pre-code expression_1 Statement_1 Statement_2 Statement_3 expression_2 T F T F Note: It is else if, not elseif.

13. Example: #include <stdio.h> int main( void ) { // main int score; /* Numeric grade */ char grade; /* Letter grade */ printf("Enter score: "); scanf("%d", &score); /* Check grade brackets */ if (score >= 90) grade = 'A'; else if (score >= 80) grade = 'B'; else if (score >= 70) grade = 'C'; else if (score >= 60) grade = 'D'; else grade = 'F'; printf("Grade = %c\n", grade); return 0; } //end main if( x > 0 ) { printf("Positive\n"); p_count++; }else if( x < 0 ) { printf( "Negative\n” ); n_count++; }else{ printf( "Zero\n” ); z_count++; } //end ifs 12

14. switch switch (expression) if (expression==const_1) { { caseconst_1: Statements_1; Statements_1; } break; else if (expression==const_2) caseconst_2:{ Statements_2; Statements_2; break; } … … default: else Statements_d; { break; Statements_d; } //end switch } //end if

15. expression must be inside parentheses and evaluate to a single char or integer value A case value must evaluate to a single char or integer constant that is known at compile-time A case value cannot be a string or floating-point number If expression matches a case value, then its associated statements are executed until a break is encountered 14

16. When break is encountered, the switch exits and execution continues at post-code If no break exists in a case block, execution “falls through” to the following case default handles the “none of the above” case.It can be omitted if not needed 15

17. Example: #include <stdio.h> #include "myfunctions.h" #define WARNING 1 #define DANGER 2 #define PANIC 3 int main (void) { int status, num_warnings = 0; scanf("%d", &status); switch (status) { case WARNING: printf("What?\n"); num_warnings++; break; case DANGER: printf("Leave now!\n"); notify_friends(); break; case PANIC: printf("HELP ME!!!!!\n"); freak_out(10); break; default: printf("All is fine.\n"); } return 0; } switch(experience) { case 'n': printf("Newbie\n"); N++; break; case 'a': disp('Amateur'); A++; break; case 'p': case 'P': printf("Professional\n"); P++; break; } 16

18. Tip:Selection Ordering To improve the performance of a multi-conditional selection, arrange it in order of most likely occurrence. Example: Use an if-else if selection to test a character variable. The character value can be 'a', 'b', 'c', 'd', or 'e'. They are not all equally likely. Sorted from most likely to occur to least likely to occur: 'c', 'd' 'b' 'a' Assume c and d are equally likely. Inefficient ordering: if (ch == 'a') {…} else if (ch == 'b') {…} else if (ch == 'c') {…} else if (ch == 'd‘) {…} Better arrangement: if (ch=='c' || ch=='d') {…} else if (ch == 'b') {…} else if (ch == 'a') {…} MOST LEAST 17

19. Triadic Conditional Expression var = (condition) ? expr1 : expr2; is equivalent to if (condition) var = expr1; else var = expr2; Both expressions should evaluate to the same data type as var. 18

20. Example: /* If voltage > 2, then set state to high (1) else set state to low (0) */ state = (voltage > 2.0) ? 1 : 0; Example: /* Sinc calculation handles x == 0 case */ y = (fabs(x) < 1e-8) ? 1.0 : sin(x)/x; 19

21. Nested Selection Statements The body of a selection statement can contain another selection statement within it. Each else is matched to the nearest if or else if. 20

22. Example: Convert the if-else if to a nested if. /* Nested if version */ if (x == 0) t++; else { if (x==1 || x==2) printf("zen state\n"); else { if (x < 0) { m = sin(Q); f = 3 * m * sqrt(m); } else printf("Moo\n"); } } /* if-else if version */ if (x == 0) t++; else if (x==1 || x==2) printf("zen state\n"); else if (x < 0) { m = sin(Q); f = 3 * m * sqrt(m); } else printf("Moo\n"); 21

23. Example: if (t >= 0 && t < 100) if (F < 1500e3) flim = 143.5; else { flim = 500.0; overload = 1; if (cps == 'x') cnt++; } else if (t2 > 750) { printf("D22 max\n"); if (!mox) mox = 1; else pc += 10; } else done = 1; if (t >= 0 && t < 100) if (F < 1500e3) flim = 143.5; else { flim = 500.0; overload = 1; if (cps == 'x') cnt++; } else if (t2 > 750) { printf("D22 max\n"); if (!mox) mox = 1; else pc += 10; } else done = 1; Use indenting to make it clear what you really mean! 22

24. Nesting can lead to ambiguous situations. If necessary, use braces to clarify your intentions. Example: if (x >= 0) if (y > 5) printf("Hi!\n"); else printf("Goodbye!\n"); Is the "else" associated with the first or second "if"? if (x >= 0) if (y > 5) printf("Hi!\n"); else printf("Goodbye!\n"); if (x >= 0) { /* Braces clarify */ if (y > 5) printf("Hi!\n"); else printf("Goodbye!\n"); } 23