Thread: Issue with pointer math

Hello all.

I am trying to learn C and have a PHP background.

What I am looking to do (to learn C some more) is to read a file line by line and return what items in the list are missing.

Example lines are:

Store 1
Store 2
Store 3
Store 12
Store 13
Store 19
Store 20

I would like to have the application return a list such as

Store 4
Store 5
Store 6
...
Store 14
Store 15
...

Here is what I have thus far and it currently thinks "next" in "*getMissingStores" is 123456789. This program was looking a lot better as far as structure is concerned, but I have been just trying to get it to work so it may look a bit jumbled.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define STORE_FILE "c:/stores.txt"

char *strip_newline(char *szString){
  if(szString[strlen(szString)-1] == '\n')
    szString[strlen(szString)-1] = '\0';
  return szString;
}

int getDigits(char *szString){
  char *tmp = strchr(szString, ' ');
  char *tmp2;
  if(tmp == NULL) return -1;
  tmp++;
  if(tmp != NULL){
    tmp2 = tmp;
    tmp2[strlen(tmp2)+1] = '\0';
    return atoi(tmp2);
  } else {
    return -1;
  }
}

char *getMissingStores(char *szStoreNums){
  int next = getDigits(szStoreNums + 1);
  int curr = getDigits(szStoreNums);
  do {
    while(curr != next && curr < next){
      printf("Store &#37;d is missing.\n", ++curr);
      printf("Curr: %d -> Next: %d", curr, next);
    }
    next = atoi(++szStoreNums);
  } while(szStoreNums != NULL);
  return;
}

int main(int argc, char **argv){
  
  FILE *storeFile = fopen(STORE_FILE, "r");
  /** create space for the store names **/
  char storeList[200][51];
  
  if(storeFile == 0){
    printf("Could not open file %s for reading.\n", STORE_FILE);
    system("pause");
    return -1;
  } else {
    /** begin to read the file **/
    int i = 0;
    while(fgets(storeList[i], 1024, storeFile) != 0){
      strip_newline(storeList[i]);
      printf("Read in: %s\n", storeList[i]);
      printf("Digits are: %d\n\n", getDigits(storeList[i]));
      i++;
    }
    char *szMissingStores = getMissingStores((char *)storeList);
    printf("Total stores on file: %d\n", i);
  }
  
  system("pause");
  
  return 0;
}

Any pointers would be appreciated and I am not against recoding the entire thing.

fgets(storeList[i], 1024, storeFile)

do not lie to fgets - you r buffer is only 50 bytes long asking to write there 1024 byte - it is asking for crash

getMissingStores((char *)storeList);
do not lie to getMissingStores - your 2D array has another type

getMissingStores should be rewritten in full

tmp2[strlen(tmp2)+1] = '\0'; - what do you want to achieve here? strlen already has found last not-nul char - so next char is '\0', why do you need to overwrite with nul one more char?
Why do you need tmp2 at all?

Thanks for your comments. Here is what I've come up with. Unfortuantely I get a invalid pointer type error when calling printStoreList

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define STORE_FILE "c:/stores.txt"

char *strip_newline(char *szString){
  if(szString[strlen(szString)-1] == '\n')
    szString[strlen(szString)-1] = '\0';
  return szString;
}

int getDigits(char *szString){
  char *tmp = strchr(szString, ' ');
  char *tmp2;
  if(tmp == NULL) return -1;
  tmp++;
  if(tmp != NULL){
    tmp2 = tmp;
    tmp2[strlen(tmp2)+1] = '\0';
    return atoi(tmp2);
  } else {
    return -1;
  }
}

void printMissingStores(char **szStoreNums){
  int next = getDigits(*(szStoreNums + 1));
  int curr = getDigits(*(szStoreNums));
  do {
    while(curr != next && curr < next){
      printf("Store &#37;d is missing.\n", curr);
      printf("Curr: %d -> Next: %d", curr, next);
      curr++;
    }
    next = atoi(++(*szStoreNums));
  } while(szStoreNums != NULL);
  return;
}

int main(int argc, char **argv){

  FILE *storeFile = fopen(STORE_FILE, "r");
  /** create space for the store names **/
  char storeList[200][51];

  if(storeFile == 0){
    printf("Could not open file %s for reading.\n", STORE_FILE);
    system("pause");
    return -1;
  } else {
    /** begin to read the file **/
    int i = 0;
    while(fgets(storeList[i], 50, storeFile) != 0){
      strip_newline(storeList[i]);
      printf("Read in: %s\n", storeList[i]);
      printf("Digits are: %d\n\n", getDigits(storeList[i]));
      i++;
    }
    printMissingStores(storeList);
    printf("Total stores on file: %d\n", i);
  }

  system("pause");

  return 0;
}

Can you explain or provide a good link to why you can't have function(char arr[][])?

Is char[][] equal to char**? If not, why?

How can I ditch arrays and use pointers only?

This might be overkill, but would a linked list using structs be better than arrays?

Thanks for your insights.

Is char[][] equal to char**? If not, why

It is not

when you make ptr++; it is advanced on sizeof(*ptr) bytes

because sizeof (char*) is 4 bytes it will not advance the whole "row" and you get the wrong location

Make function prototype like

Code:

void printMissingStores(char szStoreNums[][51], size_t rowNumber);

in this case szStoreNums[i] will be advanced full i rows and be pointing the right point n the 2D array

I got it up and running with this function

And what is your total value? It does nothing to do with the array size.

Mainly because I wan't sure what to do with size_t rowNumber

It should be number of rows in the array you have initialized.
And your loop should look like

Code:

for(i = 0; i < rowNumber ; i++){

Also note that due to [i+1] access inside the loop - it should be actually

Code:

for(i = 0; i < rowNumber-1 ; i++){

to avoid out of bounds access

A quick fix of your print routine, works finely if you pass your 'i' == total stores read as a 2nd parameter.

Code:

void printMissingStores(char szStoreNums[][51], int sz)
{
    int idx = 0;
    int prev;
    int curr;
    do {
        prev = getDigits(szStoreNums[idx]);
        curr = getDigits(szStoreNums[idx + 1]);
        if (curr > prev + 1) {
            for (prev = prev + 1; prev != curr && prev < curr; prev++) {
                printf("Store &#37;d is missing.\n", prev);
                printf("Prev: %d -> Curr: %d\n", prev, curr);
            }
        }
        idx++;
        while (idx < sz && getDigits(szStoreNums[idx]) < prev)
            idx++;
    }
    while (idx < sz - 1);
    return;
}