String equality

This should be easy. I'm admitting my newness by posting which for most will be, I hope, a simple question.

I'm having trouble testing a string for equality.

Code:

---

double myfunc(const char* z) {
    const char* mystr = "Test";
    double retVal = 100;
    if ( z == mystr) retVal = retVal * 2;
    return retVal;
}

---

This comes back with 100 and I'm looking for 200. Can somebody spot the error?

Please don't make suggestions that involve changing the "const char* z" portion of line 1. I'm stuck with that.

Thanks

John

They may be "C-style strings", but the variables themselves are just pointers. Therefore, if ( z == mystr) is asking if the address stored in z is equal to the address stored in mystr. Unless the caller of myfunc() happens to pass that value in somehow, your if() is going to be false.

If you are stuck with const char* z then I would suggest reading up on strcmp().

Then I shall read up on strcmp(). Thanks.

John

Thanks. It worked like a charm. Way cool.

Use std::string or std::vector(preferable)
Or compare them index by index.

As a noob to c++ I wouldn't have the foggiest notion of how to implement the "preferred" approach of using std::vector. Since I went to the bother of reducing my code to the smallest possible block that demonstrated the issue, if you could modify that block to show an example of how the std::vector approach would work that would be most appreciated.

I think Mr. 777 is incorrect. There is no need to use std::vector here, nor is it preferable, since you are comparing strings. std::string should suffice.

Thanks Elysia. You're a programming master! How the hell do you know every thing?

std:string worked just fine, too.

Which is more efficient, std::string or using strcmp?

For comparison, I doubt you'll see a difference. Nevertheless, the general rule is that unless your program is running slowly and you can pinpoint the slowdown to a specific code that is attributed to std::string, there is no reason not to use std::string since it's the C++ alternative of C-style strings.

I suppose the fact that I'm stuck with the pointer as an input means that I should go with the original suggestion (strcmp()) and only if I've got some other reason to do string manipulation, which would cause me to assign both of the values to c-style strings should I go with std::string.

You can wrap your C-style string inside a std::string object.
std::string str(my_c_string);

I don't think both variables need to be std::string for == to work. This should work just as well:

Code:

---

double myfunc(const char* z) {
    const std::string mystr = "Test";
    double retVal = 100;
    if ( mystr == z) retVal = retVal * 2;
    return retVal;
}

---

Indeed it does. Would that it were that were the first suggestion.

Many thanks. Learning, learning, learning.........