CODEWITHSUNDEEP
androidc++matrixjava-native-interfacelogcat
p7130
p7130

Reputation: 63

JNI: Printing Matrix to logcat doesn't work

I've been trying to give out a matrix via logcat while using c++ and JNI. I'm totally new to this stuff and so after some research I tried it with the following code:

for(int i = 0; i<4; i++){
  for (int j = 0;j<4; j++){
    float v = Matrix[i][j];
    __android_log_write(ANDROID_LOG_INFO , "Matrix: ", (char*)&v);
  }
}

but this approach just gives me :

07-22 09:21:56.517  14487-14582/name.example I/Matrix:﹕ [ 07-22 09:21:56.517 14487:14582 I/Matrix:    ]

How can I show what's inside the matrix?

Upvotes: 3

Views: 457

Answers (1)

hlt
hlt

Reputation: 6317

Your problem is in the following line of code:

__android_log_write(ANDROID_LOG_INFO , "Matrix: ", (char*)&v);

(char*)&v reinterprets the byte pattern of the float (v) as a C string, which doesn't work (and is also only occasionally allowed). Instead, to convert the float to a string, use sprintf or snprintf or, if possible, std::to_string (this requires C++11):

char str[buffer_size];
snprintf(str, buffer_size, "%f", Matrix[i][j]);
__android_log_write(ANDROID_LOG_INFO, "Matrix: ", str);

or

__android_log_write(ANDROID_LOG_INFO, "Matrix: ", std::to_string(Matrix[i][j]).c_str());

As pointed out in the comments, there is also __android_log_print, which integrates printf syntax already:

__android_log_print(ANDROID_LOG_INFO, "Matrix: ", "%f", Matrix[i][j]);

Upvotes: 1

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