C programming 3

Question 1

can one of the expressions in for loops be omitted ?

Answer 1

you can omit any of them , but keep the semicolons .

Question 2

what happens if the second expression is missed in for loop ?

Answer 2

it defaults to true (one) .

Question 3

can the first expression of if statement be replaced with a declaration in same location ?

Answer 3

yes

Question 4

what happens if we declare a variable that already exists , as the first expression of if statement ?

Answer 4

another new version of it is created to be used solely inside the if statement , it won’t be visible outside it.

Question 5

are variables declared by a for statement (as it’s first expression) visible outside it or not ?

Answer 5

not visible , they cannot be accessed outside the body of the loop .

Question 6

can we declare multiple variables inside the if statement (as its first expression) ?

Answer 6

yes . provided they are of the same type .

Question 7

what is comma expression for ?

Answer 7

it is for gluing two expressions together , possibly to use them where only one expression is allowed normally.

Question 8

what to do if we want to write a for statement with two or more initialization expressions or ones that increment or decrement several variables in each iteration ?

Answer 8

we use comma operator as much as required .

Question 9

describe the form a comma operator takes .

Answer 9

expression1 , expression2

bascially two expressions glued with a comma .

Question 10

describe how comma operator is evaluated .

Answer 10

first expression is evaluated then discarded then the second expression is evaluated and it’s value becomes the value of the whole expression .

so there is no purpose of the first expression unless it has side effects .

Question 11

is the comma operator left or associative ?

Answer 11

left associative .

Question 12

if comma operators glue expressions together then how to glue statements together ?

Answer 12

we can glue statements together using a compound statement .

Question 13

where can you find comma operators often ?

Answer 13

with for statements and certain macro definitions .

Question 14

can we determine the order expressions separated by comma operators will be evaluated in ?

Answer 14

yes , from left to right .

Question 15

must the 3 expressions in for statement be related ?

Answer 15

no , C puts no restrictions on the type of expressions used with for statement .

Question 15

what to do if we need to make an exit point in the middle of the loop or more than one exit point ?

Answer 15

use break statement .

Question 16

where can we use break statements ?

Answer 16

with :

for loop , while loop , do while loop , and switch statement .

Question 17

why is goto not widely used ?

Answer 17

because of the availability of break and continue statements .

Question 18

how many levels of nesting can the break statement escape ?

Answer 18

only one level .

Question 19

where can continue statement be used ?

Answer 19

it is limited to loops .

Question 20

to where do break statements transfer control to ?

Answer 20

to the end point just past the loop or switch .

Question 21

to where does continue statement transfer control to ?

Answer 21

to the point just before the end of the loop .

Question 22

to where can goto jump statement transfer control ?

Answer 22

to any statement that follows s label , within a function , given that the statement has a label .
C99 adds a restriction that goto cannot be used to bypass the declaration of a variable-length array .

Question 23

what is a label ?

Answer 23

it is an identifier placed at the beginning of a statement .

Question 24

what form do labels take ?

Answer 24

identifier : statement

they must be separated by a colon.

Question 25

what form does goto statement take (syntax) ?

Answer 25

goto identifier ;

Question 26

what purpose goto statement may serve given that the other jump statements are available ?

Answer 26

goto statement is useful for exiting nested loops , or exiting a loop from within a statement .

Question 27

what is a null statement syntax and what is it good for ?

Answer 27

it is written devoid of all symbols except the terminating semicolon .

it is used to create loops with empty bodies , which can be handy for reading character data .

Question 28

how to use null statement to rewrite this part of the prime finding loop : ?

for (d = 2 ; d < n ; d++)
if (n % d == 0)
break ;

Answer 28

for (d = 2 ; d < n && n % d != 0 ; d++ )
;

/* we put the null statement on it’s own line otherwise readers may get confused,we can also add the following : */

if ( d < n)
printf( “ %d is divisible by %d \n” , n , d )
else
printf(“ %d is prime \n” , n)

Question 28

how to use null statement to rewrite this part of the prime finding loop : ?

for (d = 2 ; d < n ; d++)
if (n % d == 0)
break ;

Answer 28

for (d = 2 ; d < n && n % d != 0 ; d++ )
;

/* we put the null statement on it’s own line otherwise readers may get confused ,we can also add the following : */

if ( d < n)
printf( “ %d is divisible by %d \n” , n , d )
else
printf(“ %d is prime \n” , n)

Question 29

why do programmers put null statement on it’s own line ?

Answer 29

to avoid confusing the reader who may think that the next statement may be actually the loop’s body .

Question 30

what is the main effect of putting a semicolon after while , for and if statement’s control expression’s parentheses ?

Answer 30

it makes a null statement and thus terminates the loop or if statement prematurely .

Question 31

what is the result of the following code ?

if ( d == 0) ;
printf(“ ERROR : Division by zero \n”)

Answer 31

the if statement is followed by a null statement because it’s followed by a semicolon , thus it will has no effect and the next statement will print the division by zero error no matter what’s the value of “d” variable ,zero or not .

Question 32

what is the result of the following code ?

i = 10 ;
while (i > 0) ;
{
    printf(" %d and counting \n" , i) ;
    --i ;
 }

Answer 32

it is an infinite loop because “i” value will not be decremented and it will be stuck at 10 and doing nothing as a result of the null statement as it’s body .

Question 33

what is the result of the following code ?

i = 11 ;
while(–i > 0) ;
printf(“%d and counting down \n” , i) ;

Answer 33

it would run the printf function once because the while loop has null statement only .

Question 34

what is the result of the following code ?

for (i = 10 ; i > 0 ; i–) ;
printf(“%d and counting down \n” , i ) ;

Answer 34

due to null statement , it will exit the loop when “i” reaches value zero then it will print once only .

Question 35

name a rare case where while loop cannot be converted to a for loop in a standard pattern .

Answer 35

when the control variable is incremented at the end of while loop ,but there is a continue statement before that that will execute when one specific value is entered .

when we try to convert it to for loop using standard pattern it will be different because it would still increment even if the specific value is entered , because the incrementing was moved inside for loop’s third expression .

Question 36

which form of infinite loop is preferable while(1) or for( ; ; )?

Answer 36

older compilers used to check the infinite while loop’s control expression every time , making it more efficient , but now newer compilers does optimization and therefore there is no difference in performance .

Question 37

what is so bad about goto statement ?

Answer 37

it is not inherently bad , but there are better alternatives that are more readable and easier to modify.
programs that use more than a few goto statements can quickly degenerate into spaghetti code (with control jumping from here to there all time)

goto statements are usually harder to read because they jump back and forth through the program unlike continue and break which only jump forward .

and they make programs harder to modify because the statements that follow the label may have more than one purpose , and maybe reached in more than one way either by normal flow from previous statement or by a jump from goto statement.

Question 38

does the null statement have any other uses beside making a loop statement with an empty body ?

Answer 38

it has potential uses , though practically the only other use is rare and it is used for putting a label at the end of a compound statement by adding a null statement after the label ,as it requires one statement at least .

Question 39

are there any other ways to make an empty loop besides putting a null statement on it’s own line ?

Answer 39

yes , using continue statement as the only statement in for loop’s body , or using an empty compound statement , both are ways for clarity .