C Programming Lecture 12 The Compound Statement b

C Programming Lecture 12

The Compound Statement b. A compound statement is a series of declarations and statements surrounded by braces. b A compound statement is itself a statement • In C, wherever it is correct to place a statement, you may also place a compound statement.

Example of a Compound Statement { } int a = 1, sum = 0; while (a <= 10) { sum = sum + a; a = a + 1; printf(“%dn”, a); } printf(“%dn”, sum); /* Note there is a compound statement within a compound statement. */

The Use of Compound Statements b To group statements into an executable unit. b To achieve the desired flow of control in if, if-else, while, for, do, and switch statements. • We are about to study these statements in greater detail.

The Empty Statement b The empty statement is written as a single semicolon. • It is useful where a statement is needed syntactically but no action is required semantically.

Delay Loop with Empty Statement i = 0; . . . printf(“Error: reenter data. n”); while (++i < 10000) /* delay */ ; system(“clear”); . . . /* while loop keeps message on */ /* the screen momentarily. */

The while Statement b b b General Form of while Statement while (expr) statement next statement First expr is evaluated. If it is nonzero (true), then statement is executed, and control is passed back to the beginning of the while loop. The body of the while loop, namely statement, is executed repeatedly until expr is zero (false). At that point control passes to next statement.

Example of while Statement i = 1, sum = 0; . . . while (i <= 10) { sum += i; ++i; } printf(“%dn”, sum); Number of times through loop first second third Value of sum 1 (0 + 1) 3 (1 + 2) 6 (3 + 3)

The for Statement b General Form of the for Statement for (expr 1; expr 2; expr 3) statement next statement b First expr 1 is evaluated; typically expr 1 is used to initialize the loop. Then expr 2 is evaluated; if it is nonzero (true) then statement is executed, expr 3 is evaluated, and control passes back to the beginning of the for loop again, except that evaluation of expr 1 is skipped. The process continues until expr 2 is zero (false), at which point control passes to next statement. b

Typical Roles of for Loop expr 1, expr 2, & expr 3 b b General Form of the for Statement for (expr 1; expr 2; expr 3) statement next statement expr 1 is used to initialize the loop. expr 2 is a logical expression controlling the iteration. The loop exits when expr 2 becomes false. expr 3 typically modifies a variable in expr 2 eventually causing expr 2 to become false.

/* Printing Random Numbers */ #include <stdio. h> #include <stdlib. h> int main(void) { int i, n; } i=0 printf(“n%s”, “Some randomly distributed integers will be printed. ”, “How many do you want to see? “); scanf(“%d”, &n); for (i = 0; i < n; ++i) { while (i++ < n) { if (i % 6 == 0). . . printf(“n”); . . . printf(“%9 d”, rand()); . . . } } printf(“n”); equivalent code return 0; using while

Equivalence of for and while Loops for (expr 1; expr 2; expr 3) statement Is Equivalent to: expr 1; while (expr 2) { statement expr 3; } next statement The for loop has the advantage of keeping both the control (expr 2) and the indexing (expr 3) all in one place at the top of the loop.

Missing Expressions in the for Loop b Any for the or all of the expressions in a statement can be missing, but two semicolons must remain. i = 1; sum = 0; for ( ; i <= 10; ) sum += i++; printf(“%dn”, sum); }

Embedded for Statements b b A for statement can be used as the statement part of an if, if-else, while, or another for statement. See example “Combinatorics” on page 108. • Combinatorics involves combinations and permutations. This program lists all triples of nonnegative numbers that add up to a given number N.

The Comma Operator b The comma operator has the lowest precedence of any operator in C. • It is a binary operator that associates from left to right. b General Form expr 1, expr 2

Value and Type of the Comma Operator b The comma expression as a whole has the value and type of its right operand. b Example: a = 0, b = 1 If b has been declared as an int, this expression has a value of 1 and a type of int.

Use of the comma Operator in a for Statement b b You can use the comma operator in a for statement to do multiple initializations and multiple processing of indices. Examples for (sum = 0, i = 1; i <= n; ++i) sum += i; for (sum = 0, i = 1; i <= n; sum += i, ++i) ;

Example of comma Expressions Declarations and Initializations int double i, j, k = 3; x = 3. 3; Expression: Equiv Expression: Value: i = 1, j = 2, ++k + 1 ((i = 1), (j = 2)), ((++k) + 1) 5 Expression: Equiv Expression: Value: k != 7, ++ x * 2. 0 + 1 (k != 7), (((++ x) * 2. 0) + 1) 9. 6

Use of Commas in Your Code b Most commas in programs do not represent comma operators. • Commas used to separate expressions in argument lists of functions or within initializer lists are not comma operators. • If a comma operator is to be used in these places, the comma expression in which it occurs must be enclosed within parentheses.

The do Statement b The do statement is a variant of the while statement that tests its condition at the bottom of the loop. General Form of the do Statement do statement while (expr); next statement

Examples of a do Statement do { sum += i; scanf(“%d”, &i); } while (i > 0); do { printf(“Input feet: “); scanf(“%d”, &feet); if (feet > 2) printf(“n. Enter a 1 or 2: nn”); } while (feet > 2);

The goto Statement b goto causes an unconditional jump to a labeled statement somewhere in the current function. b Form of a labeled statement. label: statement Where label is an identifier.

Example of goto and label while (scanf(“%lf”, &x) == 1) { if (x < 0. 0) goto negative_alert; printf(“%f %f %f”, x, sqrt(x), sqrt(2 * x)); } negative_alert: if (x < 0. 0) printf(“Negative value encountered!n”);

Same Example Without goto while(scanf(“%lf”, &x) == 1 && x >= 0. 0) printf(“%f %f %fn”, x, sqrt(x), sqrt(2 * x)); if (x < 0. 0) printf(“Negative value encountered!n); && x >= 0. 0 accomplishes what the goto accomplished, namely the printf() will not be executed if a negative number is entered.

When Should goto Be Used? b Almost never (it is never needed). b • gotos can make a program very difficult to read and understand. In rare instances such as exiting from a deeply nested inner loop to the outermost level, a goto can make a program execute with significantly greater efficiency.

The break and continue Statements b b The break statement causes an exit from the innermost enclosing loop or switch statement. The continue statement causes the current iteration of a loop to stop and the next iteration to begin immediately.

Example Using a break Statement while (1) { scanf(“%lf”, &x); if (x < 0. 0) break; printf(“%fn”, sqrt(x)); } /* break causes jump to next statement */ /* after the while loop. */

Example Using a continue Statement while (cnt < n) { scanf(“%lf”, &x); if (x > -0. 01 && x < +0. 01) continue; /* disregard small values */ ++cnt; sum += x; /* continue transfers control to */ /* here to immediately begin the */ /* next iteration. */ }