Study Note of K. N. King(2008) ‘C Programming: A Modern Approach, 2nd Edition’

1. SC, Chen
Ch6 Loops
Self Study Note of K. N. King(2008) ‘C Programming A Modern Approach, 2nd Edition’

2. Ch6 Loops
6.1 The while Statement
6.2 The do Statement
6.3 The for Statement
6.4 Exiting from a Loop
6.5 The NULL Statement
• A loop is a statement whose job is to repeatedly execute some other
statement (the loop body).
• Every loop has a controlling expression in C
• An iteration is one time the loop body is executed

3. 6.1 The while Statement
• The while statement is the simplest and most fundamental of the
loops in C
• The parentheses are required
• The braces is allowed, but not required
• The controlling expression is evaluated first
1. If its value is nonzero(true), the loop body is executed and the expression is
tested again
2. Until the controlling expression has the value zero(false)
while ( expression ) statementControlling Expression
Loop Body
Compound
Statement
Ch 5.2
i = 10;
while (i > 0) {
printf(“T minus %d and countingn”, i);
i--;
}
i = 10;
while (i > 0) {
printf(“T minus %d and countingn”, i--);
}
When a loop controlled by the
expression i>0 terminates, i must
be less than or equal to 0.
The body of a while
loop may not be
executed at all

4. 6.1 The while Statement: Infinite Loops
• A while statement won’t terminate if the controlling expression
always has a nonzero value, which called infinite loop
• C programmers sometimes deliberately create an infinite loop by
using a nonzero constant as the controlling expression
Will execute forever
Until its body contains a statement that transfers control out of the loop
(break, goto, return) or calls a function that causes the program to terminate
while (1) …

5. 6.1 The while Statement: Printing a Table of
Squares

6. 6.1 The while Statement: Summing a Series of
Numbers

7. 6.2 The do Statement
• The do statement is essentially a while statement whose controlling
expression is tested after each execution of the loop body
• The loop body is executed first
Then the controlling expression is evaluated
1. If its value is nonzero(true), the loop body is executed again and then the expression is
evaluated once more
2. Until the controlling expression has the value zero(false) after the loop body has been
executed
• It’s a good idea to use braces in all do statements
A do statement without braces can easily be mistaken for a while statement
do statement while ( expression ) ;
i = 10;
do {
printf(“T minus %d and countingn”, i);
i--;
} while (i > 0);
The body of a do loop
is always executed at
least once
When a loop controlled by the
expression i>0 terminates, i must
be less than or equal to 0.
i = 10;
do
printf(“T minus %d and countingn”, i--);
while (i > 0);

8. 6.2 The do Statement: Calculating the Number
of Digits in an Integer
while( n > 0 ){
n /= 10;
digits ++;
}
Won’t execute if n is 0

9. 6.3 The for Statement
• The for statement is ideal for loops that have a counting variable
Except in a few rare cases, a for loop can always be replaced by an equivalent
while loop
for ( expr1 ; expr2 ; expr3 ) statement
expr1;
while ( expr2 ) {
statement
expr3;
}
expr1:
An initialization step, which is
performed only once before the
loop begins to execute
expr2:
Controls loop termination
expr3:
An operation to be performed at
the end of each loop iteration
for (i = 10; i > 0; i --) {
printf(“T minus %d and countingn”, i);
}
i = 10;
while (i > 0) {
printf(“T minus %d and countingn”, i);
i--;
}

10. When will for cannot be converted to while?
• When the body of a for loop contains a continue statement
n = 0;
sum = 0;
while (n < 10) {
scanf(“%d”, &i);
if (i == 0)
continue;
sum += i;
n++;
}
sum = 0;
for (n= 0; n < 10; n++) {
scanf(“%d”, &i);
if (i == 0)
continue;
sum += i;
}
When i is equal to 0:
1. The while loop doesn’t increment n
2. The for loop does increment n
expr1;
while ( expr2 ) {
statement
expr3;
}
for ( expr1 ; expr2 ; expr3 ) statement

11. 6.3 The for Statement: for Statement Idioms
• The for statement is ideal for loops that have a counting variable
1. Counting Up
Use < or <= as controlling expression
2. Counting Down
Use > or >= as controlling expression
• “Off-by-one”
for (i = 0; i < n; i++) … for (i = 1; i <= n; i++) …
for (i = n - 1; i >= 0; i--) … for (i = n; i > 0; i--) …

12. 6.3 The for Statement: Omitting Expressions in a
for Statement
• C allows to omit any or all of the expressions
 The two semicolons must always be present, even when some of the expressions are omitted
• If the first expression is omitted
 No initialization is performed before the loop is executed
• If the second expression is omitted
 It defaults to a true value, so the for statement doesn't terminate: infinite loop
• If the third expression is omitted
 The loop body is responsible for ensuring that the value of the second expression eventually becomes false
• When the first and third statements are both omitted
 The resulting loop is nothing more than a while statement
i = 10;
for (; i > 0; i --) {
printf(“T minus %d and countingn”, i);
}
for (i = 10; i > 0; ) {
printf(“T minus %d and countingn”, i--);
}
for (; i > 0; ) {
printf(“T minus %d and countingn”, i--);
}
while (i > 0) {
printf(“T minus %d and countingn”, i--);
}
for (; i > 0; i --) …

13. Infinite Loops
• C programmers have traditionally preferred for(; ;) than while(1)
Efficiency
− Older compiler would often force programs to test the 1 condition each time through the
while loop
− There should be no difference in performance with modern compilers

14. 6.3 The for Statement: for Statements in C99
• In C99, the first expression in a for statement can be replaced by a
declaration
A variable declared by a for statement is only visible inside the loop
− If a declaration of i already exists, this statement creates a new version of i that will be
used solely within the loop
Pros
1. Convenient
2. Make programs easier to understand
A for statement may declare more than one variable
− Provide all variables have the same type
for (int i = 0; i < n; i ++) …
for (int i = 0, j = 0; i < n; i ++) …

15. 6.3 The for Statement: The Comma Operator
• Using a comma expression to write more than one expressions in the for
statement
1. expr1 is evaluated and its value discarded
2. expr2 is evaluated
Its value is the value of the entire expression
• Can chain together a series of comma expressions
Left associative
• Needed when there is a single expression, but we’d like to have two or
more expressions
Certain macro definitions can benefit from the comma operator
The for statement is the only other place where the comma operator is to be found
expr1, expr2
Macro definitions
Ch 14.3
i = 1, j = 2, k = i + j
((i = 1), (j = 2)), (k = (i + j))

16. 6.3 The for Statement: Printing a Table of
Squares (Revisited) (1/2)

17. 6.3 The for Statement: Printing a Table of
Squares (Revisited) (2/2)
Allows the program to compute consecutive
squares without performing any multiplications

18. 6.4 Exiting from a Loop
• Loops have an exit point
1. Before the loop body
 while
 for
2. After the loop body
 do
3. In the middle of the loop body
 break
• continue
Skip part of a loop iteration without jumping out of the loop
• goto
Allow a program to jump from one statement to another

19. 6.4 Exiting from a Loop: The break Statement
• break
Transfer control out of a switch statement
Also can be used to jump out of a while, do,
or for loop
• The break statement is particularly
useful for writing loops in which the exit
point is in the middle of the body rather
than at the beginning or end
• A break statement transfers control out
of the innermost enclosing while, do ,
for, or switch statement
while(…) {
switch (…) {
…
break;
…
}
}
for(;;) {
printf(“Enter a number (enter 0 to stop): ”);
scanf(“%d”, &n);
if (n == 0)
break;
printf(“%d cubed is %dn”, n, n * n * n);
}

20. 6.4 Exiting from a Loop: The continue Statement
• break transfers control just past the end of a loop
Control leaves the loop
Can be used in switch statements and loops
• continue transfers control to a point just before the end of the loop
body
Control remains inside the loop
Limited to loops
n = 0;
sum = 0;
while (n < 10) {
scanf(“%d”, &i);
if (i == 0)
continue;
sum += i;
n++;
/* continue jumps to here */
}
n = 0;
sum = 0;
while (n < 10) {
scanf(“%d”, &i);
if (i != 0) {
sum += i;
n++;
}
}

21. continue Statement
• Should newer use the continue statement?
continue statements are rare
continue can be helpful when there are a number of validity tests (or they are
complex)
for (; ;) {
read data
if (data fails first test)
continue;
if (data fails second test)
continue;
.
if (data fails last test)
continue;
process data
}

22. 6.4 Exiting from a Loop: The goto Statement (1/2)
• break and continue statements are both restricted
The target of a break is a point just beyond the end of the enclosing loop
The target of a continue is a point just before the end of the loop
• The goto statement can jump to any statement in a function
Can’t be used to bypass the declaration of a variable-length array in C99
The label must be in the same function as the goto statement itself
• A label is just an identifier placed at the beginning of a statement
A statement may have more than one label
Identifier : statement
goto identifier;
Variable-
length arrays
Ch 8.3

23. 6.4 Exiting from a Loop: The goto Statement (2/2)
• The goto statement is rarely needed in everyday C programming
The break, continue, and return statements are essentially restricted goto
statements
The exit function are sufficient to handle most situations that might require a goto
in other languages
• The goto statement is useful for exiting from nested loops
exit function
Ch 9.5
for (d = 2; d < n; d++) {
if (n % d == 0 )
goto done;
}
done:
if (d < n)
printf (“%d is divisible by %dn”, n, d);
else
printf(“%d is primen”);
while(…) {
switch (…) {
…
goto loop_done;
…
}
}
loop_done: …

24. Spaghetti Code
• Unstructured and difficult to maintain source code, broadly construed
Code that overuses GOTO statements rather than structured programming
constructs, resulting in convoluted and unmaintainable programs
Such code has a complex and tangled control structure
• Describe older programs having fragmented and scattered files

25. 6.4 Exiting from a Loop: Balancing a Checkbook

26. 6.5 The NULL Statement
• A statement can be NULL
Loop body can be NULL, too
• More concise but usually no more efficient
A loop with an empty body is handy for reading character data
for (d = 2; d < n; d++) {
if (n % d == 0 )
break;
}
for (d = 2; d < n && d != 0; d++)
/* empty loop body */ ;
Reading
characters
Ch 7.3
for (d = 2; d < n && d != 0; d++) ;
if (d < n)
printf(“%d is divisible by %dn”, n, d);
Accidentally putting a semicolon after the parentheses in an if,
while, or for statements creates a null statement
• Ending the if, while, or for prematurely
1. In an if statement
• Create an if statement that apparently performs the
same action regardless of the value of its controlling
expression
2. In an while statement
• May create an infinite loop
• The statement that should be the loop body is executed
only once, after the loop has terminated
3. In an for statement
• Cause the statement that should be the loop body to be
executed only once

27. NULL statement
• The usage of NULL statement is very rare
Put a label at the end of a compound statement
{
…
goto end_of_statement;
…
end_of_statement: ;
}
A label cannot stand alone, must always
be followed by a statement.
Putting a null statement after the label
solves the problem

28. Make An Empty Loop Body Stand Out
1. Put the null statement on a line by itself
2. Use a dummy continue statement
3. Use an empty compound statement
for (d = 2; d < n && d != 0; d++)
/* empty loop body */ ;
for (d = 2; d < n && d != 0; d++)
continue;
for (d = 2; d < n && d != 0; d++)
{ }