C programming 1

Question 1

which system is C language a product of?

Answer 1

UNIX operating system

Question 2

what was UNIX programmed in originally? explain the history behind the changes

Answer 2

it was originally programmed in Assembly like the other operating systems of that time.
then thompson decided that a higher level language is required for further development of UNIX so he designed a small language named B which was based on BCPL.

after Ritchie joined the UNIX project he started working on an extended version of B and named it “NB” or new B , later as the language diverged more from B he changed the name to “C” , they rewrote the UNIX in C .

Question 3

what language was B based on ?

Answer 3

it was based on BCPL

Question 4

what was the old name for C language?

Answer 4

it was “NB” or New B ,at first

Question 5

who made the C language?

Answer 5

Dennis Ritchie

Question 6

where was UNIX made and by whom ?

Answer 6

at Bell Laboratories by Ken Thompson , Dennis Ritchie and others.

Question 7

name some 4 C based programming languages and how they relate to C.

Answer 7

C++ : includes all C features of C but adds others like classes and more features to support OOP .

Java : is based on C++ and therefore inherits many of C features.

C# :is a more recent language, derived from C++ and Java .

Perl : was a fairly simple language but as it has grown it adopted many features from C language.

Question 8

name 3 philosophies of the C language design.

Answer 8

1: it is a low level language.
2: it is a small language.
3: it is a permissive language.

Question 9

what is meant that C language is low level? how is that important for systems programming?

Answer 9

it provides access to low level concepts like addresses and bytes which other languages try to hide.

it also provides operations that correspond closely to machines built-in instructions. so it can be fast as systems cannot afford to be slow.

Question 10

how was C languages features kept small?

Answer 10

it was kept small by relying on a library of standard functions.

Question 11

name 6 strengths of C language.

Answer 11

1: its efficient , regarding speed and running with limited memory.
2: its portable .
3: its powerful , it has a large collections of datatypes and operators which gives it power and it can do more with fewer lines.
4: its flexible , as it imposes few restrictions on the use of its features.

5 :has a standard library which has many useful functions.

6: integrated with UNIX and UNIX-like operating systems.

Question 12

how did C become portable?

Answer 12

1 - it was associated with UNIX and ISO/ANSI early which prevented it from splintering into numerous dialects.

2 - its compliers are small and easily written which makes them widely available.

3- has many features that support portability.

Question 13

name 3 weaknesses for C langauge

Answer 13

1 : its programs can be error prone.

2 : its programs can be hard to understand.

3 : its programs can be hard to modify.

Question 14

name one reason C programs can be error prone.

Answer 14

its flexibility can make it error prone

Question 15

name 6 ways you can ensure more effective use of C language .

Answer 15

1 : learn its pitfalls and avoid them.

2 : use software tools that make programs more reliable , like Lint for error analysis.

3 : take advantage of existing code libraries.

4 : adopt a sensible set of code conventions.

5 : avoid tricks and overly complex code.

6 : stick to standard.

Question 16

what does Lint do?

Answer 16

Lint is a tool to check for potential errors .

Question 17

how to get “Lint” tool ?

Answer 17

it is included as Splint in many linux distributions “secure programming lint” , and it can be download from splint.org for free.

Question 18

what steps the C program takes to become executable ?

Answer 18

preprocessing : the program is given first to the preprocessor which obeys commands preceded by “#” which are known as directives , and makes modifications.

compiling ; the compiler translates to the machine instructions making “object code” .

linking : .the linker includes additional code to make the program executable .

Question 19

in gnu/linux how to make the executable version of the file “pun.c” get named “pun” ?

Answer 19

we enter the following command in the command line while in the same directory as the file:

gcc -o pun pun.c

Question 20

what is an IDE and what can it do?

Answer 20

IDE = integrated development environment.

it can edit , compile , link ,execute and debug a program without leaving the environment .

Question 21

what are functions in C language , generally?

Answer 21

they are like building blocks of programs , known in other languages as subroutines and procedures .

Question 22

what are the two types of functions in C based on their source?

Answer 22

there are two :

those written by the programmer

and

those provided as a part of C implementation which are known as library functions.

Question 23

what is a statement in C ?

Answer 23

its a command to be run when the program is run .

Question 24

what are storage locations called in C ?

Answer 24

Variables

Question 25

what is the difference between different variable types in general ?

Answer 25

they are stored differently.

they have different operations that can be done on them depending on the type.

they can store different sizes or lengths depending on the variable type .

they can be slower or faster to work with , and appropriate or not .

Question 26

where to put declarations and statements in main function in different C versions?

Answer 26

declarations should come first before statements inside main function.

but in C99 its allowed to not declare until before its needed , just like in C++ and Java .

Question 27

what is it called when a variable is given a value?

Answer 27

assignment .

Question 28

what happens when you divide two integers in C if the result has a number after the decimal point ?

Answer 28

the result / output gets truncated or “ rounded down” , it means any numbers after decimal point is lost.

Question 29

what is an uninitialized variable in C and what happens if you try to access it ?

Answer 29

an uninitialized variable is a one that is not given an initial value yet

if you try accessing it you may get a random meaningless number or a program crash.

Question 30

how to give a constant a name before main function?

Answer 30

by using macro definition through the directive “#define” .

Question 31

what conventions are there for macro definitions in C?

Answer 31

it must be all in CAPS , and should be inside parentheses if it contains an operator in its value.

just for conventional purposes .

Question 32

what is meant by identifiers in C ?

Answer 32

they are names given to Variables , Functions , Macros and other entities.

Question 33

what are the rules for identifiers names in C ?

Answer 33

they can only contain letters ,numbers and underscores , and should start with a letter or an underscore but not a number.

they are case sensitive.

in C99 they can contain certain “universal characters names” as well .

Question 34

what are some conventions for identifiers in C language ?

Answer 34

even if C language is case sensitive , variables should be different unless related.

one should either use all small letters and separate words with underscores , or use no underscores and capitalize words after the first word in .

unless they are macros then use all in Caps.

Question 35

can keywords be used as identifiers ?

Answer 35

no , they are reserved .

Question 36

what is meant by tokens in C , and what are some of them ?

Answer 36

they are basic units that cant be split inside them (using spaces or whitespace characters like tabs and newlines ) without changing their meanings , but you can usually insert whitespace characters between them safely and without limits.

some of them are :

keywords , identifiers , punctuation marks , string literals .

Question 37

is splitting a string into lines without tricks legal in C ?

Answer 37

no , its illegal .

Question 38

what does GCC stand for ?

Answer 38

it used to stand for GNU C compiler . but it was extended to include many other languages and now means : (GNU compilers collection )

Question 39

what does GNU stand for ?

Answer 39

it stands for “GNU is Not UNIX” .

Question 40

are : “exit (0) ;” and ‘return 0 ;” equivalents when they are inside main ?

Answer 40

yes

Question 41

does the compiler remove the comment entirely or replace it with a blank space ?

Answer 41

some old compliers did remove the characters entirely but the current standards replace each comment with a single space characters .

Question 42

how to nest a comment inside another in C ?

Answer 42

it can only be done since C99 , by nesting // comments inside old style comments /* */

Question 43

why do floating point constants need to end with f letter ?

Answer 43

because otherwise they would be of type double which can be larger than float ,and therefore may sometimes end up exceeding the float capacity .

Question 44

explain the length limits for identifiers in C language .

Answer 44

they can be arbitrarily long but compilers are only required to read the 31 first characters ( 63 in C99).

BUT if they have external linkage they may only be required to check the first six characters (31 in C99) .

most compilers and linkers are more generous than the standard .

Question 45

how to spot a format specifier inside printf and what does it do ?

Answer 45

it starts with “%” and is used to specify how the value is converted from its internal form “binary” to its printed form such as strings of a decimal digits .

Question 46

explain the requirements to write a format specifier inside printf ?

Answer 46

it should start with “%” .

should contain a letter after “%” either directly or after following % with an optional minus for left justification then the optional values of : minimum field width , or a dot then the value of precision , or both of them (separated by a dot) .

Question 47

what is minimum field width in format specifiers in printf ?

Answer 47

its the minimum number of characters to print , if more is required its automatically expanded , if less were to be printed then blank spaces will precede the field , unless its left justified then spaces will follow it .

Question 48

what is meant by precision in format specifier : %d ?

Answer 48

its minimum number of digits to be printed , if there is less , zeroes will precede the value .

Question 49

what is the default value of precision in format specifier %d ?

Answer 49

its 1 .

Question 50

what does the format specifier %d do ?

Answer 50

it displays an integer in decimal format

(base 10 ) .

Question 51

what does the format specifier %e do ?

Answer 51

it displays a floating point number in exponential format .

Question 52

what does precision mean in the format specifier %e ?

Answer 52

it specifies how many digits to be displayed after decimal point .

Question 53

what is the default precision value for the format specifier %e?

Answer 53

its 6 .

Question 54

what happens if you set precision to 0 for the format specifier %e ?

Answer 54

it displays no decimal point.

Question 55

what does the format specifier %f do ?

Answer 55

it displays a floating point number with a fixed decimal format , without an exponent .

Question 56

what does precision mean in the format specifier %f ?

Answer 56

it means the number of digits to display after the decimal point .

Question 57

what is the default value of precision for the format specifier %f ?

Answer 57

its 6 .

Question 58

what does the format specifier % g mean ?

Answer 58

displays a floating number in either fixed decimal format or exponential format depending on the numbers size , and g will not show trailing zeroes.

Question 59

what is the meaning of precision for the format specifier %g ?

Answer 59

it means maximum number of significant digits .

Question 60

name 4 numerical format specifiers .

Answer 60

%d
%e
%f
%g

Question 61

what is used to mark an end of a statement in C ?

Answer 61

we use the statement terminator which is a semicolon “ ; “ .

Question 62

why we need to terminate statements in C ?

Answer 62

to help the parsers identifying the end of each statement since C statements can be multilinear .

but preprocessing directives do not end with semicolons.

Question 63

how to separate variables if you declare them in one line ?

Answer 63

with a comma ,and the last one ends with a semicolon .

Question 64

what does an escape sequence do?

Answer 64

an escape sequence enables strings to contain characters that would otherwise cause an error for the compiler.

Question 65

name 7 escape sequences .

Answer 65

alert (bell) :  \a
backspace :  \b
newline : \n
horizontal tab : \t
\\
\"
\'

Question 66

what is “&” used for in scanf ?

Answer 66

its used to create a pointer to a variable .

which evaluates to the address of the variable that is its operand.

Question 67

what should the programmer check for when using scanf?

Answer 67

the programmer must check that the number of conversion specifications matches the number of input variables.

and should check that each specification is appropriate for the corresponding input variable , as the compiler is not required to check for possible mismatches.

Question 68

what does scanf do when it encounters a character in the input that cannot be a part of the current item ?

Answer 68

it puts it back to be read again for the next call or the scanning of the next input item

Question 69

what does scanf do if it encounters one or more whitespace characters in the input.

Answer 69

it repeatedly reads whitespace characters until it reaches non white space character which is then put back .

Question 70

how does scanf handle the format string?

Answer 70

it tries to locate items of the appropriate type in the input for each conversion specifier in the format string.

it will skip blank spaces if necessary and reads the items until it reaches a character that cannot possibly belong to the item then if its reads successfully it will continue processing the rest of the format string , if not successfully then it will return immediately without looking at the rest of the format string or the input data ..

Question 71

what does scanf do if it encounters one or more ordinary characters or whitespace characters in the format string?

Answer 71

if the non-whitespace character matches that in the input it discards it from input and continues processing the format string , if the two do not match , it puts the offending character back into the input and aborts without further processing format string or reading more input .

If it’s a whitespace it will match any number of whitespace characters in input.

Question 72

what is the bad practice in this code :

scanf(“%d , %d” , &i , &y) ;

Answer 72

it uses a comma inside the format string which is usually a bad idea.

Question 73

what is the difference between %i and %d ?

Answer 73

in printf format string there is no difference ,however in scanf format string %d will match integers written in decimal only , while %i will match decimal ,octal or hexadecimal integers.

if a number has 0 as prefix , %i will treat it as an octal ,if it has 0X or 0x prefix it will be treated as a hexadecimal .

Question 74

how to print the % character inside the printf format string ?

Answer 74

by using two consecutive % characters “%%”

Question 75

how do we know how far apart tab stops are in C language ?

Answer 75

its not possible , it depends on the operating system ,while its typically 8 characters long , C makes no guarantee .

Question 76

what does scanf do if it is asked to scan for a numeric input but the user enters nonnumeric characters ?

Answer 76

if the nonnumeric characters appear after the numeric as in “29foo” it will store the numbers in the variable and the nonnumeric “foo” will be left to be read in the next call of scanf .

if the nonnumeric is at the start as in “foo29” , it will quits without storing anything in the variable , and will read “foo’ in the next call .

Question 77

how does scanf “put back” characters and read them again later ?

Answer 77

characters are not read as they are written ,instead they are stored in a hidden buffer , to which scanf has access .

Question 78

is C language more known for its emphasis on expressions or statements ?

Answer 78

expressions

Question 79

what is meant by expressions in C language ?

Answer 79

expressions are formulas that show how to compute a value , they are legal combinations of symbols ., they consist of one operand at least and may contain operators too .

Question 80

what are the simplest expressions in C and how to link them ?

Answer 80

the simplest are variables and constants , variables are values that are computed as the program runs , constants are values that do not change .

we use operators to link the basic expressions .

Question 81

what happens if the % operator is used with non integer numbers ?

Answer 81

the program wouldn’t compile .

Question 82

what happens if zero is used as the right operand of % or / ?

Answer 82

it would cause undefined behaviour .

Question 83

describe the result when / or % are used with a negative operand in C language .

Answer 83

if one of two operands used with / is negative they will either be rounded up or down in C89 but in C99 they will always be truncated towards zero .

for % the sign of the result depends on the implementation in C89 , but in C99 it will have the sign of the first operand .

Question 84

how does C deal with implementation defined behaviour ?

Answer 84

it is left out for the “implementation” , the software used to compile , link and execute the program on a particular platform to fill in the unspecified parts of behaviour .

Question 85

what results from the existence of implementation defined behaviours in C ?

Answer 85

the behaviour of the program may vary somewhat between platforms .

Question 86

why does C have implementation defined behaviours ?

Answer 86

for efficiency ,it will need to match the way the hardware behaves .

Question 87

how to take implementation defined behaviour into consideration ?

Answer 87

we need to try to avoid creating programs that rely on expecting a certain behaviour from implementation defined behaviours .
if its not possible then we need to check the manual .

Question 88

what are associativity and precedence rules used for in C programming ?

Answer 88

precedence rules are used to resolve ambiguity in expressions ,
associativity rules have the same purpose when two operators have the same level of precedence , they either have left associativity or right associativity .

Question 89

what is the order of precedence for unary “+” , “-“ , and binary “+” , “-“ ,”/” , “*” , “%” ?

Answer 89

from high level to low level :
1- unary + and - .
2 - * and / and % .
3- binary + and - .

Question 90

how is e treated in the expression v = e if they have different types?

Answer 90

the value of e is converted to the type of v and then its assigned to it .

Question 91

do operators have side effect ?

Answer 91

some do like the assignment operator , but most do not.

Question 92

how can several assignments be chained together ?

Answer 92

a = b = c = 0 ;

Question 93

is the assignment operator right associative or left associative ?
rewrite the following example to illustrate that :
a = b = c = 0;

Answer 93

right associative

a = (b = (c = 0))

Question 94

what does ++i and i++ do ? and what is the difference between them?

Answer 94

++i increments i by 1 immediately , i++ increments later and will use the old value by now .

Question 95

what are the final values for i , j and k after this code runs:
int i = 1 , j = 2 ;
int k = 0 ;

k = ++i + j++ ;

Answer 95

they are equivalent to
i += 1 ;
k = i + j ;
j = j + 1 ;

which will make i , j and k values 2 , 3 and 4 respectively

Question 96

what is the precedence level and associativity for prefix and postfix ++ and - - ?

Answer 96

for postfix they have higher precedence than unary minus and plus and are left associative , for prefix version they have same precedence as unary plus and minus and are right associative .

Question 97

order the following operators according to precedence and mention their associativity
: unary + and minus , additive plus and minus , increment and decrement postfixes and prefixes , assignment operators = , += , *= , /= , -= , %= , and multiplicative operators * , / , % .

Answer 97

1 : increment and decrement postfixes ++ , - - , and they are left associative .

2 : increment and decrement prefixes and unary plus and minus , ++ , - - , + , - , and they all are right associative .

3 : multiplicative operators , * , / , % , and they all are left associative .

4 : additive plus and minus , + , - , and they are left associative .

5 : assignment operators , = , *= , -= , /= , %= , += , and they are right associative .

Question 98

what does knowing order of precedence and associativity allow us to know about the expressions and what it doesn’t allow us to know ?

Answer 98

it allows us to determine uniquely where the parenthesise would go in the expressions if they were all parenthesized .

but they do not determine that we know what the values will be which may depend on what order the values will be evaluated in some cases .

C does not define in what order the subexpressions will be evaluated , except for some involving logical and , logical or , conditional and comma operators .

most expressions have the same value regardless of what order the subexpressions are evaluated in , but not always when a subexpression modifies one of it’s operands .

Question 99

why doesn’t knowing the precedence order and association determine the values of all expressions ?

Answer 99

because C does not determine in what order the subexpressions are evaluated in , if there is some subexpression that modifies one of it’s operands , the values may differ depending on the order of evaluation.

Question 100

does C determine in what order subexpressions are evaluated ?

Answer 100

no , except for some operators like logical or ,logical and , conditional and comma .