Lexical, Syntax and Semantic error

Question

int main()
{
    int 1a, b;
    Printf("\nGate 2018");
    Printf("%d",x);
}

How many types of error are there in this code?

Comments

yes

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what about

int a,b,a;

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it results in predefined error type

Answer 1

int main()
{
    int 1a, b; //syntax error
    Printf("\nGate 2018");
    Printf("%d",x); //x is not defined hence semantic error
}

1.  A lexical error at line 3 "1a" because in C, the first character of a variable name should be either a letter or an underscore.
2.  variable x is not defined anywhere in the program so it will give a semantic error

 

Comments

For example : int a1bc ; 
This line contains 5 tokens {int, a, 1, bc, ;}

there are no commas present (int a1bc)

how can we add commas (int, a, 1, bc, ;)

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MIRIYALA JEEVAN KUMA that was a set representation. 
check now

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@Utkarsh , Lexical Analysis phase uses "Lookahead" to decide where one token ends and next token begins. so 1 and a will not be considered as 2 tokens here.

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can you provide a good source for this?

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please check it :- https://lagunita.stanford.edu/courses/Engineering/Compilers/Fall2014/courseware/708cb92d25c24fbdad592929b4a80917/65c4655d659641518df2c983e9946aeb/

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ankitgupta.1729 i watched the video thanks for the reference. updated the answer :)

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:)

Answer 2

int main()
{
    int 1a, b;
    Printf("\nGate 2018");
    Printf("%d",x);
}

 

int 1a, b; $\Rightarrow$ error: invalid suffix "a" on integer constant

This gives a $lexical\space error$. This is because in C, the first character of a variable name should be either a letter or an underscore.

 

Printf("\nGate 2018"); $\Rightarrow$ warning: implicit declaration of function 'Printf'

$Semantic\space error$ at Printf. C is a case sensitive language and function used to print is printf. The program identifies 'Printf' as a new function and gives 'function not declared' error.

 

Printf("%d",x); $\Rightarrow$ error: 'x' undeclared (first use in this function)

This is a $semantic\space error$ again, as x is used here before declaring.

 

So, the $number\space of\space types$ of errors in this code is $2$. (Lexical error and Semantic error)

Comments

@arun bro , I think by mistake you have written syntax error for int 1a; . it should be lexical error.

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@ankitgupa.1729 Oh  yes, i also think it is lexical error. will correct it.

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want to add some points to clear why 1a is a single invalid token and not 2 valid tokens as 1 and a ..

Lexical analyzer uses lookahead or some different strategy to decide whether one token ends and next token begins and to check whether it is a keyword and identifier...

In C , whitespace is not considered as a token class . it is a separator and whitespaces are removed after lexical analysis phase.

here, lexical analyzer scan 1a from left to right using lookahead till white space comes. when it reaches 1 then it is considered as a valid token  but when it reaches 'a' then it will throw lexical error because token class of identifier starts with a letter or underscore and not a digit. So, 1a will be considered as an invalid identifier. So, it is not a valid token.

 1111 , 1(whitespace)a  and while1 are  valid tokens. here while and 1 should not be considered as 2 different tokens of while and 1...

how lexical analyzer consider "else" as a  keyword not  4 tokens as e,l,s,e and how == is considered as a single token not  2 tokens as = and =  ..it is explained here 

 

Answer 3

Considering the language to be C , there are 4 errors :

1. Identifier "1a" is not a valid declaration.

2. The 'P' in printf function should be in small letter.

3. Variable "x" should be declared first.

4. The function should return a value. So there must be a return statement.

Answer 4

1a -> 1 Lexical Error
Printf -> 2 Syntax Error
x -> 1 Semantic Error

Answer 5

int main()
{
    int 1a, b;/lexical error
    Printf("\nGate 2018");
    Printf("%d",x); //x is not defined hence semantic error
}

1.  A lexical error at line 3 "1a" because in C, the first character of a variable name should be either a letter or an underscore.
2.  variable x is not defined anywhere in the program so it will give a semantic error

 

Answer 6

there will be two types of errors-  lexical and syntax error

error 1  LEXICAL ERROR-    tokens are meaningful lexemes, so while making tokens the analyzer won’t be able to make 

“1a”  as a single token. Hence ERROR.

error 2 SYNTAX ERROR-  as undeclared variable “x” is used so syntactically incorrect.