#CD Doubt

Question

What is the diffrence between Lexical error , Syntax error and semantic error..?

Please explain with this example

void main()
{
int total-value, Num=2,sum=5,var1;
5=Num;
var1=2;
sum=num*1;
if(sum=var1)
{
sum=sum+1
}
}

Comments

void main()
{
int ${\color{Red}total-value}$, Num=2,sum=5,var1;
5=Num;
var1=2;
sum=${\color{Green} num}$*1;
if(sum=var1)
{
sum=sum+1
}
}

${\color{Red} red}$ -> lexical errors

${\color{Blue} blue}$-> syntax errors

${\color{Green} green}$-> semantic errors

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lexical errors : which comes while design DFA.

syntax errors : while creating while drawing parse tree from grammer.

semantic errors: which  created while semantic is wrong parse tree.

syntax errors : here 5= num is not according to grammar i,e. variable stored constant not reverse + sum=sum+1   since it does not end with a semicolon.

semantic errors : sum=num*1; since in declaration Num is used i.e. num is not declared (is not not syntax error since syntax tree is possible + DFA also for token)

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nitish why lexical error.

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@Anu sir,

total-value is also there which is lexical error since identifiers are char(char + digit)*

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total-value : No token will be generated as there is no such pattern or RE or DFA which can
recognize this lexeme, hence lexical error.

sum= sum +1 : Every statement should be terminated by semicolon (;) which is
not present here, syntax violated, hence syntax error.

5=num : Syntax voilated, hence Syntax error

sum = num*1; : No lexical error, no syntax error, but num is used which is not yet defined, hence
semantic error

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yes nitish :)

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@Anu  total−value since we cannot use hyphen in variable declaration i think so thatz y iot will be lexical error...we need to use underscore _

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@Anu007 @Nitish

but the solution which i have saying   5=Num;  // semantic error not syntax error..please confirm on this....?

void main()
{
int total-value, Num=2,sum=5,var1; // total-value Lexical error as hyphen is not allowed
  5=Num;  // semantic error
  var1=2;
sum=num*1;  // num is undeclared variable as C is case sensitive, so Num and num is different (semantic error)
if(sum=var1)  // no error
{
sum=sum+1     // semicolon missing syntax error
}

---

5=var; is syntax error, which is known as L-value error.

Answer 1

'total-value' is a lexical error. Lexical analyzer won't be able to identify the token.

 

'5=Num;' and 'sum=sum+1' are syntax errors. These tokens don't qualify as valid syntax.

 

'sum=num*1;' is a semantic error as only semantic analyzer will be able to check that 'num' is an undeclared variable.