View Compilation Error

0_0_15494694_20233.cpp: In function 'int main()':
0_0_15494694_20233.cpp:18:31: error: expected ';' before '[' token
        if(isalnum(a[j])==0)&&a[j]!=95)
                               ^
0_0_15494694_20233.cpp:18:30: error: label 'a' used but not defined
        if(isalnum(a[j])==0)&&a[j]!=95)
                              ^