Question

My aim is to write an app which generates an char – array (each should be random-filled with strings of the length 4) and sorts this array. The time this process takes should be measured. I coded the following:

#include <string.h>
#include <jni.h>
#include <time.h>
#include <math.h>

clock_t start, finish;
static int ARRAY_LENGTH = 200;
static int WORD_LENGTH = 4;
char values[200];

void sortStringArray(void){
    int i, j;
    for(i = 0; i < ARRAY_LENGTH; i++){
        for(j = 0; j < ARRAY_LENGTH-1; j++){
          if(strcmp(values[j], values[j+1]) > 0) {
            char holder = values[j+1];
            values[j+1] = values[j];
            values[j] = holder;
          } 
        }
    }   
}
char generateRandomChar(char aC[]){
   int length = strlen(aC);
   char randStr[WORD_LENGTH];
   int m;
   for(m = 0; m <WORD_LENGTH; m++){
     int randNr = rand()%length;
     randStr[m] = aC[randNr];
  }
  return randStr;
}

void fillStringArray(void)
{
  char allowedChars[] = "abcdefghijklmnopqrstuvwxyz";
  int k;
  for(k = 0; k < ARRAY_LENGTH; k++){
     char randStr = generateRandomChar(allowedChars);
     values[k] = randStr;
  }
}
double
Java_com_example_hellojni_HelloJni_processStringSort( JNIEnv* env, jobject thiz)
{
    start = clock();
    fillStringArray();
    sortStringArray();
    finish = clock();
    return ((double)(finish - start));
}

Since I am pretty new to coding C, i am not that fimilar with the concept of pointers, and therefore i recieve some mistakes.

alt text http://img38.imageshack.us/img38/2894/androidndkdebugc.jpg

It would be helpful if sb could explain me where it would be useful to use a pointer in this code. Some help with the errors would be very appreciated.
Thanks! Ripei

Answer 1

Well for one thing you seem to think that char means string…. sometimes? char means a character, a number between 0 and 255. As the warnings on line 15 say, values[j] and values[j+1] are not strings (char *), they are characters (char). You probably want to make values an array of strings, ie an array of arrays of characters.

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The 2nd set of warnings you’re getting are related to line 31, where you’re returning an array of characters (a pointer) from a function that states that it returns a character. The compiler silently casts the pointer to a character (since a pointer is a number) and returns that. You’ll end up with a random number, which is probably not what you want.

To fix this you’ll have to make the function return a char *, but there’s a catch. randStr is gone as soon as you get out of the function, thus making it impossible to return. You could use strdup to duplicate the string and, after you’re done using it in your main function, you call free to get rid of it.

While we’re on this function, the parameter to it should be a char *, not a char[]. They have different meanings.

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The last message (the only error reported as such apparently) is because you didn’t define rand(). Adding a #include <stdlib.h> at the beginning of the program should fix it.