String comparison for constant string and a string element in C++ using strcmp

Question

I am trying to convert III stored in string s in Roman numerals to 3. Here is a code snippet:

int i = 0;
int num = 0;
while (i < s.size()){
    if (strcmp(s[i], "I") == 0){
        num = num + 1;
        i = i + 1;
    }
    else{
        continue;
    }
}

return num;

I am facing issues with using the strcmp() function. How can I use it successfully?

Here is the error:

Line 18: Char 17: error: no matching function for call to 'strcmp'
            if (strcmp(s[i], "I") == 0){
                ^~~~~~
/usr/include/string.h:137:12: note: candidate function not viable: no known conversion 
from '__gnu_cxx::__alloc_traits<std::allocator<char>, 
char>::value_type' (aka 'char') to 'const char *' for 1st argument; 
take the address of the argument with &
extern int strcmp (const char *__s1, const char *__s2)
           ^

Usage of `s.size()` indicates this is not `C`. Tag the language that you're using. Also as an aside, this: `IIIIIIIIIIIIIIIIIIIII` is not a valid Roman numeral, but your program will process it as if it is one. — PaulMcKenzie, Jun 23 '21 at 02:14
`s[i] == 'I'` ... you're comparing characters, not c strings. — ChrisMM, Jun 23 '21 at 02:14

score 2 · Answer 1 · answered Jun 23 '21 at 02:39

2

You are comparing s[i] of type char(not a string) with "I" of type const char*(is string).

In this case, you just need to compare s[i] == 'I'

answered Jun 23 '21 at 02:39

toan

31
4

score 0 · Answer 2 · answered Jun 23 '21 at 02:18

Since you're using s.size(), it seems s is std::string and s[i] would be a character at index i. It's not a char* so obviously you can't compare that with "I" which is a const char[2]. To compare characters you do that directly: s[i] == 'I'

If you really want to do a string compare then you must get a const char* from s:

if (strncmp(s.c_str() + i, "I", 1) == 0){

String comparison for constant string and a string element in C++ using strcmp

2 Answers2