Compilers

Question 1

Which one of the following statements is FALSE?

A
Context-free grammar can be used to specify both lexical and syntax rules.
B
Type checking is done before parsing.
C
High-level language programs can be translated to different Intermediate Representations.
D
Arguments to a function can be passed using the program stack.
Question 1 Explanation: 
Type checking is done in semantic analysis phase after syntax analysis phase (i.e., after parsing).
Question 2
A lexical analyzer uses the following patterns to recognize three tokens T1, T2, and T3 over the alphabet {a,b,c}.
1: a?(b∣c)*aT
2: b?(a∣c)*bT
3: c?(b∣a)*c
Note that ‘x?’ means 0 or 1 occurrence of the symbol x. Note also that the analyzer outputs the token that matches the longest possible prefix.
If the string bbaacabc is processes by the analyzer, which one of the following is the sequence of tokens it outputs?
A
T1T2T3
B
T1T1T3
C
T2T1T3
D
T3T3
Question 2 Explanation: 
a? means either 0 or 1 occurrence of “a”, so we can write T1, T2 and T3 as:
T1 : (b+c)*a + a(b+c)*a
T2 : (a+c)*b + b(a+c)*b
T3 : (b+a)*c + c(b+a)*c
Now the string is: bbaacabc
Please NOTE:
Token that matches the longest possible prefix
We can observe that the longest possible prefix in string is : bbaac which can be generated by T3.
After prefix we left with “abc” which is again generated by T3 (as longest possible prefix).
So, answer is T3T3.
Question 3

Type checking is normally done during

A
lexical analysis
B
syntax analysis
C
syntax directed translation
D
code optimization
Question 3 Explanation: 
Type checking is normally done during syntax directed translation.
Question 4

The number of tokens in the Fortran statement DO 10 I = 1.25 is

A
3
B
4
C
5
D
None of the above
Question 4 Explanation: 
DO → 1
10 → 2
I → 3
= → 4
1.25 → 5
Question 5

The number of tokens in the following C statement.

printf("i = %d, &i = %x", i, &i); 

is

A
3
B
26
C
10
D
21
Question 5 Explanation: 
We have six different types of tokens are available
(i) Keyword
(ii) Identifier
(iii) Constant
(iv) Variable
(v) String
(vi) Operator
Print = Token 1
( = Token 2
"i=%d%x" = Token 3 [Anything inside " " is one Token]
, = Token 4
i = Token 5
, = Token 6
& = Token 7
i = Token 8
) = Token 9
; = Token 10
Here, totally 10 Tokens are present in the equation.
Question 6

Consider a program P that consists of two source modules M1 and M2 contained in two different files. If M1 contains a reference to a function defined in M2, the reference will be resolved at

A
Edit time
B
Compile time
C
Link time
D
Load time
Question 6 Explanation: 
The link time can gives the reference to the executable file when the functions are present in the other modules.
Question 7

Generation of intermediate code based on an abstract machine model is useful in compilers because

A
it makes implementation of lexical analysis and syntax analysis easier
B
syntax-directed translations can be written for intermediate code generation
C
it enhances the portability of the front end of the compiler
D
it is not possible to generate code for real machines directly from high level language programs
Question 7 Explanation: 
In Intermediate code optimizations can also enhances the probability of optimizer.
Question 8

For the program segment given below, which of the following are true?

 program main (output);
 type link = ^data;
      data = record
         d : real;
         n : link
         end;
 var ptr : link;
 begin
    new (ptr);
    ptr:=nil;
    .ptr^.d:=5.2;
    write ln(ptr)
 end. 
A
The program leads to compile time error
B
The program leads to run time error
C
The program outputs 5.2
D
The program produces error relating to nil pointer dereferencing
E
None of the above
Question 8 Explanation: 
Note: Out of syllabus.
Question 9

In a compiler the module the checks every character of the source text is called:

A
The code generator.
B
The code optimizer.
C
The lexical analyser.
D
The syntax analyser.
Question 9 Explanation: 
Lexical analyzer phase checks every character of text to identify tokens.
Question 10

Using longer identifiers in a program will necessarily lead to:

A
Somewhat slower compilation
B
A program that is easier to understand
C
An incorrect program
D
None of the above
Question 10 Explanation: 
Lexical analyzer will take more time to recognize the longer identifiers.
Question 11

Consider line number 3 of the following C-program.

int main ( ) {                   /* Line 1 */
  int I, N;                      /* Line 2 */
  fro (I = 0, I < N, I++);       /* Line 3 */
} 

Identify the compiler's response about this line while creating the object-module:

A
No compilation error
B
Only a lexical error
C
Only syntactic errors
D
Both lexical and syntactic errors
Question 11 Explanation: 
There is no error in the above code. Actually it is a link error. Here compiler fro is a function which is not declared. Hence, it will not produce any error. It will only throw a warning in C.
Question 12

Match the following according to input (from the left column) to the compiler phase (in the right column) that processes it:

A
P→(ii), Q→(iii), R→(iv), S→(i)
B
P→(ii), Q→(i), R→(iii), S→(iv)
C
P→(iii), Q→(iv), R→(i), S→(ii)
D
P→(i), Q→(iv), R→(ii), S→(iii)
Question 12 Explanation: 
Character stream is input to lexical analyzer which produces tokens as output. So Q → (iv).
Token stream is forwarded as input to Syntax analyzer which produces syntax tree as output. So, S → (ii).
Syntax tree is the input for the semantic analyzer, So P → (iii).
Intermediate representation is input for Code generator. So R → (i).
There are 12 questions to complete.

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