Sorting strings with C

Question

Hey Guys

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.

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

Without re-writing your code from scratch, it is difficult to to know where to start. I'm afraid it is all wrong. In order to get a good understanding of pointer and character string use in C, you must read a good, authoritative book on the language, Luckily, C has one of the best such books in the world - The C Programming Language. If you haven't already got a copy, get one, and if you have, re-read the chapters on pointers and strings.

Answer 2

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.

---

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.

---

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.

Answer 3

Thank you very much Blindy for your hints. I tried to implement your hints. Now the program doesn't throw errors but the problem is that i can't check if the operation is done correctly with the programm-environment i've to work with. Do you think the code is correct as it is shown below? Also the time it takes is quite less: 11ms. Do I calc this right?

Neil Butterworth,... well your're probably right, but I had to start somehow... and I tried my best to do so. Vinko Vrsalovic,... well you're not right ;) I did it step by step but I thought that its better to show you the whole program and all errors at one time.

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

long start, finish;
static int ARRAY_LENGTH = 500;
static int WORD_LENGTH = 4;
static int LOOPS = 10;
char *values[1000];

static long getTime(void){
    struct timeval  now;
    gettimeofday(&now, NULL);
    return (long)(now.tv_sec*1000 + now.tv_usec/1000);
}

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 strdup(randStr);
}


void fillStringArray(void)
{
  char *allowedChars = "abcdefghijklmnopqrstuvwxyz";
  int k;
  for(k = 0; k < ARRAY_LENGTH; k++){
     char *randStr = generateRandomChar(allowedChars);
     values[k] = randStr;
  }
}

jlong
Java_com_example_hellojni_HelloJni_processStringSort( JNIEnv* env, jobject thiz)
{
    start = getTime();
    int i;
    for(i = 0; i < LOOPS; i++){
      fillStringArray();
      sortStringArray();
    }
    finish = getTime();
    return (finish - start);
}