Lecture #12: Statements 3

Friday, October 29, 1999

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

the if statement

•      Only allows 2 choices: true and false

•      Can be extended using if/else if/…/else

there is a conditional expression, too

•      A ternary expression (3 operands)

•      Allows you to embed a simple decision inside an expression

•      No side-effects

•      A convenient shorthand for simple if statements.

Text Box: /* * z = max (a, b); */ if (a > b) z = a; else z = b;
 


conditional expression

expr1 ? expr2 : expr3

•      expr1 is evaluated first

•      If expr1 is non-zero (true), then the value of the overall expression is the value of expr2

•       If expr2 is zero (false), then the value of the overall expression is the value of expr3

Text Box: /* * z = max (a, b); */ z = (a > b) ? a : b;
 


conditional expression details

expr1 ? expr2 : expr3

•      Only one of the two expressions after the ? is evaluated.

–   this means if one of those expressions has a side-effect, it only happens if that’s the selected expression

Text Box: z = (a > b) ? a++ : b--;
 


the switch statement

•      A multiway conditional statement

–   similar to the if-else if-else “statement”

–   allows the selection of an arbitrary number of choices based on an integer value

    switch (expression) {

        case const-expr: statements

        case const-expr: statements

        …

        default: statements

    }

the switch statement

 switch (expression) {

        case const-expr: statements

        case const-expr: statements

        …

        default: statements

    }

•      expression must have an integral value

•      When the switch statement is executed:

–   the expression is evaluated

–   if a case matches the value of the expression, the program jumps to the first statement after that case label

–   otherwise, the default case is selected

switch example

Text Box: switch (grade/10) { case 10: case 9: printf (“A\n”); break; case 8: printf (“B\n”); break; case 7: printf (“C\n”); break; case 6: printf (“D\n”); break; default: printf (“F\n”); break; }
 


what’s that “break;”?

Text Box: case 8: printf (“B\n”); break;
 


•      When the switch transfers to the chosen case, it starts executing statements at that point

•      It will keep going into the next case unless you do something

–   break; is that something

•   it causes the program to immediately jump to the next statement after the switch statement

•   “breaking” out of the switch statement

falling through

•      Without the break statement, execution would keep going into the next case

•      We call this “falling through”

Text Box: switch (grade/10) { case 10: case 9: printf (“A\n”); case 8: printf (“B\n”); case 7: printf (“C\n”); case 6: printf (“D\n”); default: printf (“F\n”); }
 


falling through

•      We can sometimes make use of this:

Text Box: switch (c) { case ‘a’: case ‘A’: a_count++; break; case ‘b’: case ‘B’: b_count++; break; }
 


switch vs. if

•      In many cases, you can rewrite one using the other

–   but often one is easier and more readable

iteration statements

•      Also called “loops”

•      We have three “flavors”:

–   while

–   for

–   do-while

•      All do similar things

–   repeatedly execute a statement until some condition is not met

iteration statements

•      Like if, all iteration statements have one or more control expressions

–   these control the execution of the loop

•      …and a loop body

–   this is the statement (or block) that gets repeatedly executed

while loops

while ( expression ) statement

•      As with the if statement, expression is evaluated as true (non-zero) or false (zero)

•      When this loop is executed:

–   the expression is evaluated

–   if the expression is true the statement is executed

–   then the expression is evaluated again

–   if the expression is true the statement is executed again

–   then the expression is evaluated again

–   etc.

while loops

while ( expression ) statement

•      If the expression evaluates to false, execution transfers to the next statement after the while

•      The statement may, of course, be a compound statement

while loop examples

Text Box: /* compute n factorial */ i = 0; factorial = 1; while (i < n) { i++; factorial *= i; }
 

Text Box: /* print powers of 2 that are less than 1000 */ i = 1; while (i < 1000) { printf (“%d\n”, i); i *= 2; }
 


while loop examples

Text Box: while ((c = getchar()) != EOF) { if (c == ‘ ‘) blank_count++; else if (c >= ‘0’ && c <= ‘9’) digit_count++; else if (c >= ‘a’ && c <= ‘z’ || c >= ‘A’ && c <= ‘Z’) letter_count++; else if (c == ‘\n’) newline_count++; }
 


while loop examples

Text Box: while (fahr <= upper) { celsius = 5 * (fahr - 32) / 9; printf ("%d\t%d\n", fahr, celsius); fahr = fahr + step; }
 


for loops

for ( expr1; expr2; expr3 ) statement

•      This is (almost) equivalent to writing:

          expr1;

          while (expr2) {

              statement

              expr3;

          }

•      This is easiest to see by example.

for loops

for ( expr1; expr2; expr3 ) statement

•      In an early lecture, we saw a program with this for loop:

Text Box: for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i);
 


•      In this case:

–   expr1 is i = 0

–   expr2 is i < 10

–   expr3 is i = i + 1

–   statement is printf (“i is %d\n”, i);

for loops

•      And we could have written this for loop

Text Box: for (i = 0; i < 10; i = i + 1) printf (“i is %d\n”, i);
 


using while instead

Text Box: i = 0; while (i < 10) { printf (“i is %d\n”, i); i = i + 1; }
 


for loop examples

Text Box: /* compute n factorial */ i = 0; factorial = 1; while (i < n) { i++; factorial *= i; }
 

Text Box: /* compute n factorial */ factorial = 1; for (i = 1; i <= n; ++i) { factorial *= i; }
 


for loop examples

Text Box: /* print powers of 2 that are less than 1000 */ i = 1; while (i < 1000) { printf (“%d\n”, i); i *= 2; }
 

Text Box: /* print powers of 2 that are less than 1000 */ i = 1; while (i < 1000) { printf (“%d\n”, i); i *= 2; }