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Date: Sun, 16 Jun 2024 11:52:30 -0400
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Subject: Re: Whaddaya think?
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 <87sexdx3s1.fsf@bsb.me.uk>
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On 6/15/2024 6:03 PM, Ben Bacarisse wrote:
> DFS <nospam@dfs.com> writes:
> 
>> I want to read numbers in from a file, say:
>>
>> 47 185 99 74 202 118 78 203 264 207 19 17 34 167 148 54 297 271 118 245 294
>> 188 140 134 251 188 236 160 48 189 228 94 74 27 168 275 144 245 178 108 152
>> 197 125 185 63 272 239 60 242 56 4 235 244 144 69 195 32 4 54 79 193 282
>> 173 267 8 40 241 152 285 119 259 136 15 83 21 78 55 259 137 297 15 141 232
>> 259 285 300 153 16 4 207 95 197 188 267 164 195 7 104 47 291
>>
>>
>> This code:
>> 1 opens the file
>> 2 fscanf thru the file to count the number of data points
>> 3 allocate memory
>> 4 rewind and fscanf again to add the data to the int array
>>
>> Any issues with this method?
> 
> There are two issues: (1) you end up with a program that can't be
> "piped" to (because the input can't be rewound), and (2) the file might
> change between counting and reading.  How much either matters will
> depend on the context.  I like piping to programs so (1) would bother
> me.
> 
>> Any 'better' way?
> 
> I'd allocate the array on the fly.  It's one of those things that, once
> you've done it, becomes a stock bit of coding.  In fact, you can write a
> simple dynamic array module, and use it again and again.
>> ----------------------------------------------------------
>> #include <stdio.h>
>> #include <stdlib.h>
>>
>> int main(int argc, char *argv[]) {
>>
>> 	int N=0, i=0, j=0;
>> 	int *nums;
>> 	
>> 	FILE* datafile = fopen(argv[1], "r");
>> 	while(fscanf(datafile, "%d", &j) != EOF){
> 
> It's always better to loop while fscanf succeeds rather than trying to
> handle all the errors.  You might not care about case where this loop
> fails, but it's just better to get into the right habit:
> 
>    while (fscanf(datafile, "%d", &j) == 1) ...
> 
>> 	nums = calloc(N, sizeof(int));
> 
> The cost is low, but there's no need to use calloc here as you are going
> to assign exactly N locations.
> 
>> 	rewind(datafile);
>> 	while(fscanf(datafile, "%d", &j) != EOF){
>> 		nums[i++] = j;
>> 	}
> 
> As above, though I'd read into &nums[i] directly.
> 
>> 	fclose (datafile);
>> 	printf("\n");
>> 	
>> 	for(i=0;i<N;i++) {
>> 		printf("%d. %d\n", i+1, nums[i]);
>> 	}
>> 	printf("\n");
>> 	free(nums);
>> 	return(0);
> 
> Because I have acquired the habit, I'd also check for errors,
> particularly on argc, fopen and malloc.
> 
>> 				
>> }
>> ----------------------------------------------------------

Thanks for the tips.

I'm not into error checking on my personal code.  But I am into brief 
and efficient.

New effort
* dropped 2 variables
* allocate 'on the fly'
* one fscanf thru the file
* 4 less lines of code

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

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

	int N=0;
	int *nums = malloc(2 * sizeof(int));
	
	FILE* datafile = fopen(argv[1], "r");
	while(fscanf(datafile, "%d", &nums[N++]) == 1){
		nums = realloc(nums, (N+1) * sizeof(int));
	}
	fclose (datafile);
	
	N--;
	for(int i=0;i<N;i++) {
		printf("%d.%d  ", i+1, nums[i]);
	}	
	free(nums);
	
	printf("\n");
	return 0;
				
}
----------------------------------------------------------




original 19 lines not incl close brackets
----------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

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

     int N=0, i=0, j=0;
     int *nums;

     FILE* datafile = fopen(argv[1], "r");
     while(fscanf(datafile, "%d", &j) != EOF){
         N++;
     }

     nums = calloc(N, sizeof(int));
     rewind(datafile);
     while(fscanf(datafile, "%d", &j) != EOF){
         nums[i++] = j;
     }
     fclose (datafile);
     printf("\n");

     for(i=0;i<N;i++) {
         printf("%d. %d\n", i+1, nums[i]);
     }
     printf("\n");
     free(nums);
     return(0);

}
----------------------------------------------------------