C programming 2

Question 1

What is an Lvalue?

Answer 1

The assignment operator requires an Lvalue as it’s left operand ,a variable for example.

An Lvalue is an object stored in computer memory not a constant or a result of computation.

Question 2

What are some tips to avoid undefined behavior?

Answer 2

Avoiding writing expressions that access the value of a variable and also modify it in same expression.
Some compilers may produce a warning.

It’s also a good practice to avoid using assignment operators in subexpressions , and using a series of separate assignments instead.

Question 3

Name the operators that modify their operands.

Answer 3

Assignment operators, decrement and increment are the only operators that modify their operands.
And care must be taken to not write an Expression that depend on a particular order of evaluation.

Question 4

What is fetching in C programming and how does it function?

Answer 4

Fetching a variable means to retrieve the value of the variable from memory, a later change to the variable won’t affect fetched variable which is typically stored in register inside the CPU.

Question 5

What is undefined behavior in C ?

Answer 5

When a program ventures into the realm of undefined behavior all bets are off , it may behave differently when compiled with different compilers, or might not compile at all , or may compile but not run , or runs but then crashes , behave erratically or produce meaningless results.

Question 6

What is an Expression statement in C ?

Answer 6

C has an unusual rule that any expression can be used as a statement , It’s done by turning an expression into a statement by adding a semicolon at the end.

Which makes sense only if it has side effects.

Question 7

How do C compilers deal with expression statements that do nothing?

Answer 7

Some of them may show a warning of “statement with no effect”.

Question 8

How to raise a number to a power “exponentiation” in C language?

Answer 8

It’s best done by repeated multiplication ,for noninteger power use pow function.

Question 9

How to apply the % operator to a floating-point operand ?

Answer 9

The % operand requires integer operands ,try the fmod function instead.

Question 10

In C Standard ,What is the guaranteed result of : (a / b )* b + (a%b) ?

Answer 10

a

Question 11

Why does C provide the ++ and – operators, are they faster or just more convenient?

Answer 11

Historically C inherited it from B language , Thompson apparently created these operators because his B compiler could generate a more compact translation for ++i than for i = i + 1.

But it’s continued popularity is due to it’s brevity. And convenience.

Question 12

Do ++ and – work with floats ?

Answer 12

Yes.

Question 13

When using postfix ++ or – when do the increment or decrement be performed?

Answer 13

Unfortunately it’s difficult to answer , C standard introduce the concept of “sequence point” and says that “updating the stores value of the operand shall occur between the previous and next sequence point”.

Question 14

What are some kinds of sequence points in C? explain them briefly and give situations that impose them .

Answer 14

One kind is the end of an expression statement , by the end of it all increments and decrements within the statement must have been performed (after the previous sequence point and before the next one), the next statement can’t begin until this condition has been met.

Certain operators like logical and , logical or , conditional and comma also impose sequence points.

Also function calls does , because it’s arguments must be evaluated before the function is called ( they may contain an increment or decrement)

Question 15

What’s meant by that statements like ( i = 1; )is discarded?

Answer 15

They modify i , but the Expression is considered discarded because the assignment is an operator that produces a value , it’s not a great loss at all.

Question 16

If n is an integer , what’s remarkable about the result of n % 10 ,and n / 10 ?

Answer 16

n % 10 equals the last digit in n .

And n / 10 is n with the last digit removed.

Question 17

Does C have relatively many or few statements?

Answer 17

Few ( relatively).

Question 18

Other than expression statement and return statement , what are the 3 main categories that most statements fall into ?

Answer 18

Selection statements.
Iteration statements.
Jump statements.

Question 19

Name two selection statements.

Answer 19

1 - if .

2 - switch .

Question 20

Name three iteration statements.

Answer 20

1- while .
2- do .
3 - for .

Question 21

Name 4 jump statements.

Answer 21

1- break.
2- continue.
3- goto.
4 - return.

Question 22

What do selection statements do ?

Answer 22

They allow a program to select a particular execution path from a set of alternatives .

Examples : if and switch statements.

Question 23

What are iteration statements for? ( Briefly)

Answer 23

They are for looping.

Examples: while , do and for statements.

Question 24

What are jump statements for ?

Answer 24

They cause unconditional jump to some other place in the program.
Examples : break , continue , goto and return statements.

Question 25

What statements are there in C other than selection, iteration,jump and expression statements?

Answer 25

Compound statement and null statement left , there's no more.

Question 26

What does a compound statement do ? (Briefly)

Answer 26

They group several statements into a single statement.

Question 27

What does a null statement do ?

Answer 27

It performs no action.

Question 28

What does a comparison yield in C language?

Answer 28

It yields an integer: 0 to indicate it's false or 1 to mark it as true , unlike other languages that have Boolean or logical type.

Question 29

Do relational operators work with floats and mixed types in C ?

Answer 29

Yes they do.

Question 30

How do relational operators precedence compare to arithmetic operators?

Answer 30

They have lower precedence than arithmetic ones.

Question 31

Are relational operators left or right associative?

Answer 31

Left associative.

Question 32

What does the expression i < j < k do?

Answer 32

Because the relational operators are left associative the expression equals : ( i < j ) < k. The expression first evaluates if i < j is true and if it's it yields 1 or 0 if false and evaluates that the value 1 or the value 0 is lower than the value of k . It doesn't test whether j lies between i and k .

Question 33

Name 4 relational operators in C language.

Answer 33

1- less than: < 2- greater than: > 3-less than or equal to: <= 4-greater than or equal to: >=

Question 34

Name 2 equality operators in C language and explain what they do .

Answer 34

Equality operator is very different from the assignment operator "=" in it's function but it appears a bit similar which is "==" for equal to , and "!=" For not equal to . They evaluates the two sides similarly to the relational operators.

Question 35

Are equality operators left or right associative in C?

Answer 35

They are left associative just like relational operators.

Question 36

How does the precedence of equality operators compare to the precedence of relational operators?

Answer 36

They have lower precedence than the relational operators.

Question 37

Name 3 logical operators.

Answer 37

Logical negation : ! Logical and : && Logical or : ||

Question 38

Is the logical operator "!" Unary or binary?

Answer 38

It's unary.

Question 39

What do logical operators produce ? Explain.

Answer 39

Either 1 or 0 , any nonzero operand will be treated as true and any zero operand will be treated as false

Question 40

In the following expression is j incremented or not? i > 0 && ++j > 0

Answer 40

If i > 0 is false then the part after the && isn't evaluated then no incrementing happens due to short-circuit evaluation.

Question 41

What is the precedence of the ! Operator?

Answer 41

Same as unary plus and unary minus

Question 42

What is the precedence of && and || operators?

Answer 42

Lower than that of relational and equality operators

Question 43

What is the associativity of logical operators : ! , && , || ?

Answer 43

! Is right associative. | The && and || are left associative.

Question 44

Are parentheses around the expression of an if statement optional or mandatory?

Answer 44

Mandatory, as a part of the if statement not the expression.

Question 45

When is if statement executed?

Answer 45

When the expression inside the parentheses evaluates to a nonzero.

Question 46

How to be able to make if statement control more than one statement?

Answer 46

By using compound statements , putting the statements inside curly brackets -braces- to form a compound statement. With each statement ending in a semicolon but not the compound statement itself ,hence no semicolon after the curly brackets.

Question 47

Does a compound statement end with a semicolon?

Answer 47

No ,but every statement within it will end in a semicolon including the last statement before enclosing the curly brackets.

Question 48

When does the statement after else clause execute?

Answer 48

If the expression of the if statement (inside the parentheses) evaluates to zero.

Question 49

Layout issue: how to position the statements after if statement and after the else clause?

Answer 49

We either indent them in a new line after " if" and then after "else"and end them both with semicolons. Or if they're short we can keep them on same line as "if" and "else" and end both with semicolons.

Question 50

Are there restrictions on what kind of statements can appear inside an if statement?

Answer 50

No , there's not any, they can even be nested!

Question 51

How should else clause be aligned as a convention for readability?

Answer 51

It should be aligned with the matching if statement. | Curly brackets can be added to eliminate confusion or even a possibility of compiling issue.

Question 52

How to style the nested if statements and their else clauses?

Answer 52

We can position the starting curly bracket at the end of if statement's expression (at the end of the line). And then we align the closing curly bracket with the matching if statement. We also indent else clauses and put them on same line with matching if statement's closing bracket . We put the starting curly brackets of else clause in same line as well , but we align their closing curly brackets with the matching if statement.

Question 53

Name 2 advantages for using curly brackets/ braces even when they're not required.

Answer 53

1- it becomes easier to modify. 2- it helps avoiding errors that can result from forgetting to use braces when adding statements to an if or else clause.

Question 54

How to style cascaded if statements with their else clause included?

Answer 54

We can either match else with it's if statement and indent the next if in the next line . Or as most programmers and the usual we align every "else if " and "else" with the first original "if" ,if this is followed.

Question 55

Is else statement always present at the end of cascaded if statements?

Answer 55

No ,not always.

Question 56

Name two benefits of using cascaded if statements.

Answer 56

1- it avoid the problem of excessive indentation. When the number of tests is large . 2- it assures the reader that the statements are just a series of tests.

Question 57

What are cascaded if statements?

Answer 57

A cascaded if isn't some new type of statements,they are simply if statements that happen to have an else clause with another if statement in it (ad infinitum).

Question 58

What is the dangling else problem? Explain

Answer 58

When if statements are nested and there's an else clause ,it belongs to the nearest if statement that has not been paired with an else ,even if indentation suggests it's paired with another . To change that use indentation on the outer "if" and keep else outside it. (Inner if enclosed in brackets along with everything that should be considered inside the outer if statement)

Question 59

How many operands the conditional operator requires?

Answer 59

3 operators, unlike others which require 2 or 1 and that makes it unique and sometimes it's called a ternary operator.

Question 60

What does the conditional operator consist of?

Answer 60

Two symbols and 3 operands. Which must be used together in the following way: expr1 ? expr2 : expr3 It actually means : if expression 1 is true then expression 2 else expression 3

Question 61

How does the conditional expression work in the following code: Int i , j ,k ; ``` i = 1 j = 2 k = i > j ? i : j ; ```

Answer 61

its layout is : k = expression 1 ? expression 2 : expression 3 I > j Is evaluated first if it's value is nonzero then 2 is evaluated and it's value is the value of the entire conditional expression therefore it's assigned to k , else expression 3 is evaluated as the value of the entire conditional expression.

Question 62

When are parentheses necessary in the conditional expression?

Answer 62

It's necessary if there's at least one expression or number that is outside the conditional expression but is processed with it , for example added to it as a whole after it's evaluation.

Question 63

What is the precedence of conditional operator?

Answer 63

It's less than most others including relational and logical operators , but is higher than that of assignment.

Question 64

Is it possible to use conditional expressions in certain kinds of macro definitions?

Answer 64

Yes ,they are common in some.

Question 65

what are logical expressions built of besides operands?

Answer 65

operators , more specifically relational ,equality and logical operators.

Question 66

what is a short-circuit evaluation?

Answer 66

its performed when the value of an expression as a whole can be determined before evaluating the rest of the expression, a logical expression for example.

Question 67

does C89 have a boolean type built in?

Answer 67

no.

Question 68

how to work around the lack of boolean type in C89?

Answer 68

we can assign 0 or 1 to a flag if some program needs a variable with assigned boolean type, it would let logical expressions evaluate flag as true or false by the 1 and 0 assignment respectively. a better way would be to define macros with the names TRUE and FALSE . and we can even define our own integer type to indicate its a boolean but it will behave as integer : #define BOOL int

Question 69

what is stdbool header used for ?

Answer 69

it is a header in C99 that make it easier to work with boolean types , and it supplies the program with the macro Bool to stand for _Bool type , and true and false to stand for 1 and 0 respectively .

Question 70

what is a good alternative for cascaded if statements?

Answer 70

the switch statement

Question 71

what are some advantages of using switch statements instead of cascaded if statements?

Answer 71

it is usually easier to read and is often faster especially when there is more than a handful of cases.

Question 72

what are the components of a switch statement ?

Answer 72

controlling expression : the switch statement must be followed by an integer expression inside parentheses , characters are treated as integers in C and therefore can be tested in switch statements , floating-point numbers and strings do not qualify. case labels : each case label starts with the keyword case or the keyword default and followed by a constant expression directly which must evaluate into an integer (characters are accepted) and then a colon . after that statements are written (no braces are required) and the last statement in each group of statements is normally the break statement , but several cases can precede the same group of statements . several case labels can be written on same line too.

Question 73

is it possible to write a case label that specifies a range of values in C language?

Answer 73

no it is not possible at least until C99.

Question 74

is default case label required in switch statement?

Answer 74

no it is not.

Question 75

must the default label be the last one in switch statements?

Answer 75

no , it is not required that the cases should be ordered.

Question 76

what happens if you omit the break statement in case labels of switch statement?

Answer 76

it would flow to the first statement of the next case without checking its case label.

Question 77

what is a case label in switch statement ?

Answer 77

it is a marker that indicates a position within the switch , as switch is a form of computed jump .

Question 78

what convention you have to do when you have to omit a break statement in a switch statement's case label ?

Answer 78

you should write a comment indicating that it is done deliberately to share code between some case labels .

Question 79

is n + 1 a constant expression ?

Answer 79

no unless n is a macro defined constant .

Question 80

how to force a compiler to notice a suspicious use of = instead of a == ?

Answer 80

some programmers put the integer value where an lvalue would normally come , so it throws an error if it is supposed to be an equality check instead of an assignment . some compilers are able to do checks if enabled . for example with GCC compiler if the -Wparentheses option is used or -Wall (all warnings).

Question 81

how to make the compiler ignore a particular case in if statement's expression when checking for suspicious ( = instead of == ) use , is enabled ?

Answer 81

by putting the expression inside an additional set of parentheses .

Question 82

what styles are there for braces and indentation for compound statements ?

Answer 82

4 of them : K&R . allman. whitesmiths. GNU.

Question 83

what is K&R style in indentation and braces placement ?

Answer 83

it puts the left brace on the same line as "if" , and align the other brace with if's start . its most often used in JAVA. if (line_num == MAX_LINES ) { line_num = 0 ; page_num++ ; }

Question 84

what is the style of allman in indentation and braces placement ?

Answer 84

it puts the first brace on a separate line : ``` if (line_num == MAX_LINES) { line_num = 0 ; page_num++ ; } ```

Question 85

what is whitesmiths style in indentation and braces placement ?

Answer 85

it indents the braces and align them with the start of statements . ``` if (line_num == MAX_LINES) { line_nums = 0 ; page_num++ ; } ```

Question 86

what is the GNU style in indentation and braces placement ?

Answer 86

it indents the braces and further indents the statements more than the braces. its used in GNU project. ``` if (line_num == MAX_LINE) { line_num = 0 ; page_num++ ; } ```

Question 87

if i is an integer value and f is a float value , what type is the value of the conditional expression (i > 0 ? i : f) ?

Answer 87

if conditional expressions are of mixed type (float and integer) , the expression is of type float , so if i > 0 is true then i is the value of the expression after its converted into float .

Question 88

why does not C99 have a better name for its _Bool type ?

Answer 88

because bool or boolean may break some old programs as programmeers may have used them in their programs before adding _Bool , _Bool would not break the code as C89 standard say that names starting with underscore then an uppercase letter are reserved for future use .

Question 89

what are the 2 most common methods of indentation inside the switch statement ? and which one is the best ?

Answer 89

the first is to indent the case labels and put the statements in same line as the case label and the break statement at next line aligned with the beginning of the first statement above it . a variation of it is to put the break statement as the same line as the other statements if there is only one statement before break. the second is to put statements under case label and in one variation of it each case label is aligned under the word switch so they end at same level.

Question 90

what is a loop in C language ? describe what it does.

Answer 90

a loop is a statement that its job is to execute other statements (loop body ) each time (iteration of the loop) as long as its controlling expression evaluates to true which means to 1 . if that expression is still true then it keeps executing the loop body until controlling expression evaluates to false.

Question 91

what is the difference between "do" ,"while" and "for" loops ?

Answer 91

while : is used if we want the controlling expression to be evaluated before each iteration. do : used if we want that expression to be evaluated after each iteration ,which means it runs once at least. for : it is used when the loop increment or decrement a counting variable to control when it runs or stop.

Question 92

what does the break statement do in loops ?

Answer 92

they stop the loop (break it) and move to the next statement after the loop .

Question 93

what does the continue statement do in loops ?

Answer 93

it skips the rest of the current iteration .

Question 94

what does the goto statement do ?

Answer 94

it jumps to any statement within a function.

Question 95

what can a null statement do ?

Answer 95

it can create a loop with empty body .

Question 96

when does a loop terminate ?

Answer 96

when the control expression evaluates to false (zero) .

Question 97

does the while loop's body have to be a single statement ?

Answer 97

yes but it could be a one compound statement that consist of many sub-statements . we do that by using braces .

Question 98

can variables be incremented inside printf function?

Answer 98

yes .

Question 99

what is the widely used idiomatic expression used while making an infinite loop ?

Answer 99

while (1) { insert statements here} a while statement written with the above syntax will execute infinitely unless we use a statement inside it that transfers control out of the loop , such as return ,break , or goto or some function that does it .

Question 100

what is the syntax for "for" statement ?

Answer 100

for (expression1 ; expression2 ; expression3) expression1 is for initialization ,expression2 is for comparison ,and the last is for incrementing or decrementing .