compare const char * with strcmp

Question

I compare a const char * to a string and for some reason it always succeeds.

    if (std::strcmp(t->detectColor->name, "ghghjg") != 0) {
        printf("XXXXXXXXXXX\n");
        // check if it was allready a sequencer
        if (std::strcmp(t->className, "IM_SURE_IT_CANT_BE_THIS") != 0) {
          printf("what is going on?\n");

The detectColor name is always something like green or blue, and t->className is "ofxDTangibleBase" for example. Still it prints

XXXXXXXXXXX
what is going on?

in the console. How can i get a valid compare?

Did you sleep well? Coming from this post: http://stackoverflow.com/questions/2931704/how-to-compare-string-with-const-char It's quite a easy mistake. — clankill3r, Nov 14 '13 at 13:57

Maxime Chéramy · Accepted Answer · 2013-11-14T13:25:05.070

9

According to cplusplus.com:

Returns an integral value indicating the relationship between the strings: A zero value indicates that both strings are equal. A value greater than zero indicates that the first character that does not match has a greater value in str1 than in str2; And a value less than zero indicates the opposite.

Or said differently by cppreference.com:

Return value

Negative value if lhs is less than rhs.

0 if lhs is equal to rhs.

Positive value if lhs is greater than rhs.

So, in your code, strcmp(t->detectColor->name, "ghghjg") will return something different than 0. As a consequence, "XXXXXXXXXXX" will be printed.

You only have to change:

if (std::strcmp(t->detectColor->name, "ghghjg") != 0)

to

if (std::strcmp(t->detectColor->name, "ghghjg") == 0)

And the same for the other comparison.

edited Nov 14 '13 at 13:25

answered Nov 14 '13 at 13:19

Maxime Chéramy

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use better cppreference for citations – 4pie0 Nov 14 '13 at 13:25
@piotruś: Are you saying this citation is wrong? It looks fine to me, although perhaps it would be better to quote the C standard rather than some website. – Mike Seymour Nov 14 '13 at 13:30
@piotruś I have added your reference. I used cplusplus.com because they are often the first result on Google and do the job. – Maxime Chéramy Nov 14 '13 at 13:30
I just wanted to give an advice, cppreference is better source, in this case however citation is of course correct – 4pie0 Nov 14 '13 at 13:31
Thanks man, didn't except comparing was so advanced in c++ :) – clankill3r Nov 14 '13 at 13:58
2

@clankill3r: In C++, you'd use `std::string` and compare with `==`. It's only complicated because you're using the C library for some reason. – Mike Seymour Nov 14 '13 at 17:07
@Mike Seymour what about comparing a const char * to a string? Also `==`? (I can't test now but i'm curious) – clankill3r Nov 14 '13 at 23:19
@clankill3r: Yes, you can also compare `std::string` with a C-style `const char*` string using `==`. – Mike Seymour Nov 15 '13 at 13:03

4pie0 · Answer 2 · 2013-11-14T16:40:49.623

std::strcmp returns 0 when strings are the same, and value less than 0 or greater than 0 when strings differ. So you might change your code for something like this:

if (std::strcmp(t->className, "IM_SURE_IT_CANT_BE_THIS") != 0) {
  printf("indeed, strings are different\n");

Negative value if lhs is less than rhs. 0 if lhs is equal to rhs. Positive value if lhs is greater than rhs.

http://en.cppreference.com/w/cpp/string/byte/strcmp

How can i get a valid compare?

if (std::strcmp(t->className, "IM_SURE_IT_CANT_BE_THIS") == 0) {
      printf("strings are equal lexicographically\n");

compare const char * with strcmp

2 Answers2