C error Expected expression before int

Question

When I tried the following code I get the error mentioned   if a  1    int b  10    But the following is syntactically correct  if a  1       int b   10      Why is this

User · Answer

--   defines scope  so if a  1    int b   10    says  you are defining int b  for   - this scope  For   if a  1    int b  10    there is no scope  And you will not be able to use b anywhere

User · Answer

By C89  variable can only be defined at the top of a block   if  a    1      int b   10       it s just a statement  syntacitially error   if  a    1                        refer to the beginning of a local block      int b   10        at the top of the local block  syntacitially correct                       refer to the end of a local block  if  a    1        func        int b   10        not at the top of the local block  syntacitially error  I guess

User · Answer

This is actually a fairly interesting question   It s not as simple as it looks at first   For reference  I m going to be basing this off of the latest C11 language grammar defined in N1570  I guess the counter-intuitive part of the question is  if this is correct C   if  a    1      int b   10      then why is this not also correct C   if  a    1    int b   10    I mean  a one-line conditional if statement should be fine either with or without braces  right   The answer lies in the grammar of the if statement  as defined by the C standard   The relevant parts of the grammar I ve quoted below   Succinctly  the int b   10 line is a declaration  not a statement  and the grammar for the if statement requires a statement after the conditional that it s testing   But if you enclose the declaration in braces  it becomes a statement and everything s well   And just for the sake of answering the question completely -- this has nothing to do with scope   The b variable that exists inside that scope will be inaccessible from outside of it  but the program is still syntactically correct   Strictly speaking  the compiler shouldn t throw an error on it   Of course  you should be building with -Wall -Werror anyways  -     6 7  declaration              declaration-speci ers init-declarator-listopt               static assert-declaration   6 7  init-declarator-list              init-declarator             init-declarator-list   init-declarator   6 7  init-declarator              declarator             declarator   initializer   6 8  statement              labeled-statement             compound-statement             expression-statement             selection-statement             iteration-statement             jump-statement   6 8 2  compound-statement                block-item-listopt     6 8 4  selection-statement              if   expression   statement             if   expression   statement else statement             switch   expression   statement

[c] C error: Expected expression before int

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