Thread: open file with numbers and multiply them

But how would you even see if it worked ? :|

If you are writing C (rather than C++), you should declare FILE *in, *out; before any of the code, (so before the first printf).

You don't need to use global variables, all the variables could be moved inside of main (before the first printf).

To be strictly correct, you should check that the character at strlen(ficheiro)-1 is really a newline before you replace it - it would fail if someone enters an EXACTLY 199 char long filename, as the last char of the filename would be overwritten by a zero.

You should check the return value from sscanf() to ensure that the data is correctly read.

To be strictly correct, you also should use PATH_MAX from limits.h instead of a constant of 200 for your filename strings - that way, any valid filename would be acceptable, not just ones that are shorter than 200 chars [not really a BIG problem, but if someone wants to use a file that is some very deep directory structure, then I suppose they should be allowed, yes?]

--
Mats

Well the final answer i suppose to your problem is the following:

Code:

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


int WriteMulToFile(char *inFile, char *outFile, const int factor)
{
	if(inFile == NULL || outFile == NULL)
	{
		fprintf(stderr, "File Error, check input or output files.\n");
		return -1;
	}
	else
	{
		FILE *in = fopen(inFile, "r");
		FILE *out = fopen(outFile, "w");
		//Index.
		int i;
		//Variables.
		float x,y,z;
		if(in == NULL)
		{
			fprintf(stderr,"Error can not open file &#37;s in read mode.\n", inFile);
			return -2;
		}
		else if(out == NULL)
		{
			fprintf(stderr,"Error can not open file %s in read mode.\n", outFile);
			return -3;
		}
		else
		{
			//File opening OK!!
			char *Buf = malloc(1024);
			if(Buf == NULL)
			{
				fprintf(stderr, "Error Memory.\n");
				return -4;
			}
			else
			{
				//Init memory block.
				memset(Buf, 0, 1024);
				//Init variables.
				x = 0.0;y = 0.0;z = 0.0;
				//Read records from file.
				while(fgets(Buf, 1024, in) != NULL)
				{
					if(Buf[strlen(Buf) - 1] == '\n')
						Buf[strlen(Buf) - 1] = '\0';
					//Now we have in Buf the whole line lets parse it.
					if(sscanf(Buf, "%f %f %f", &x, &y, &z) != 3)
					{
						printf("Error reading from buffer.\n");
						return -5;
					}
					else
						fprintf(out, "%.3f %.3f %.3f\n", factor * x, factor * y, factor * z);
					memset(Buf, 0, strlen(Buf));
				}
				//Close Files.
				fclose(in);
				fclose(out);
				//Free memory.
				free(Buf);

				//Return 
				return 0;
			}
		}
	}

}

int main(int argc, char *argv[])
{
	int factor = 0;
	printf("Welcome...............\n");
	printf("Give me the factor to multiply the values that will be read from file:");
	scanf("%d", &factor);
	WriteMulToFile("input.txt", "output.txt", factor);
	return 0;
}

@Bokarinho, your use of memset doesn't achieve anything useful.
fgets() will always append a single \0, you don't have to pre-fill the buffer to begin with.

Yes, fgets() will overwrite what was there previously, from the beginning of the buffer (it's not like say strcat).

If the first line of the file was "Hello world\n", then the buffer would contain "Hello world\n\0"
If the second line of the file was "Hi\n", then the buffer would contain "Hi\n\0o world\n\0"

Yes, the end of the longer lines will remain stored in memory, but they will not be accessible through normal use of the standard string API calls (strcpy etc). Thus in a correct program, you'll never see past the first \0 stored in the buffer, which marks the end of the most recently read line.

One possible good reason for using memset() would be to ensure that no traces of sensitive information could leak into other parts of the program, or perhaps be written out to a file. But this requires a great amount of skill to do properly, if you're also going to contend with page files, hibernation files and the like.

I cleaned up as sugested(thanks matsp), but i still have one last question: how can i change the code to accept: an argument such as "./fescala infile outfile" supposing that i would specify the factor before compiling?

Code:

/*
/* Programa para recalcular coordenadas XYZ      
/* Dado um ficheiro de entrada xyz e um factor   
/* de conversão recalcula as coordenadas originais 
/* e escreve um ficheiro de saída
/* 2007-09-10
*/

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
 
int main() {
    double x; 
    double y;
    double z;
    double factor;
    char line[PATH_MAX];
    char factorin[PATH_MAX];
    char ficheiro[PATH_MAX];
    char ficheiro_mod[PATH_MAX];
    
    FILE *in, *out; 
    
    /* inicializa o nome do ficheiro de entrada */
    printf("Introduzir nome do ficheiro: ");
    fgets(ficheiro, sizeof(ficheiro), stdin);
    ficheiro[strlen(ficheiro)-1] = '\0'; /* elimina o ultimo caracter */
    return (0); /* verifica se o numero foi lido correctamente */
    
   	/* Qual o factor de multiplicacao?*/
    printf("Introduzir o factor de conversao [0,1]: ");
	fgets(factorin, sizeof(factorin), stdin);
	sscanf(factorin, "&#37;lf", &factor);
	return (0); /* verifica se o numero foi lido correctamente */
    	
    in = fopen( ficheiro, "r" ); /* abre o ficheiro de leitura */
    if( in == NULL ) {
    	printf("ERRO: nao e poss&#237;vel encontrar o ficheiro %s\n", ficheiro);
        exit(1);
        }
        
    /* alterar nome do ficheiro de saida */
    strcat(ficheiro_mod, "esc-");
    strcpy(ficheiro_mod, ficheiro);
    
    out = fopen( ficheiro_mod, "w" ); /* abre o ficheiro de saida */
	if( out == NULL ) {
		printf("ERRO: nao e possivel abrir o ficheiro de saida\n");
		exit(1);
		}
	
	/* leitura e escrita */ 
	while (fgets(line, sizeof(line), in) != NULL) { 
	sscanf(line, "%lf %lf %lf", &x, &y, &z); /* mudar de espa&#231;o para virgula se necessario */
	fprintf(out, "%.3lf,%.3lf,%.3lf\n", x * factor, y * factor, z * factor); /* o output e escrito com 3 casas decimais */
	}
   
    /* fecha os ficheiros */
    fclose( in );
    fclose( out );
    return (0);
    }

You need to add argc, argv to main.

argc will contain the number of arguments (first one is normaly the applications name). These ones are in argv, and the first one is the application name, so in your example:
argv[0] = "fescala"
argv[1] = "input"
argv[2] = "output"

Make sure you check that you have the right number of arguments before you try to use them, as a common cause for errors is to "use arguments that aren't there". This is undefined behaviour, so "it may do almost anything" - but it's almost certain that it won't do what you want, that's 99.99999% certain!

--
Mats

Originally Posted by matsp

You need to add argc, argv to main.

argc will contain the number of arguments (first one is normaly the applications name). These ones are in argv, and the first one is the application name, so in your example:
argv[0] = "fescala"
argv[1] = "input"
argv[2] = "output"
Mats

I tried this out but i am doing something wrong. It reads the file, deletes the input and produces an empty output file.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
 
int main(int argc, char **argv) {
    double x; 
    double y;
    double z;
    float factor;
    char line[PATH_MAX];
    char output[PATH_MAX];
    
    FILE *in, *out; 
    
    if ( argc != 3 ) {
    fputs ( "usage: mfescala.exe input output", stderr );
    exit ( EXIT_FAILURE );
  }       
    factor = 0.97;
    	
    in = fopen(argv[1], "r"); /* abre o ficheiro de leitura */
    if( in == NULL ) {
    	printf("ERRO: nao e possivel encontrar o ficheiro &#37;s\n", argv[1]);
        exit(1);
        }
    
    out = fopen(argv[2], "w" ); /* abre o ficheiro de saida */
	if( out == NULL ) {
		printf("ERRO: nao e possivel abrir o ficheiro de saida\n");
		exit(1);
		}
	
	/* leitura e escrita */ 
	while (fgets(line, sizeof(line), in) != NULL) { 
	sscanf(line, "%lf %lf %lf", &x, &y, &z); /* mudar de espa&#231;o para virgula se necessario */
	fprintf(argv[2], "%.3lf,%.3lf,%.3lf\n", x * factor, y * factor, z * factor); /* o output e escrito com 3 casas decimais */
	}
   
    /* fecha os ficheiros */
    fclose( in );
    fclose( out );
    return (0);
    }

It looks about right to me, but fprintf(argv[2]) is definitely not the right thing to do... You should have compiler warnings enabled so that you get a big fat warning message that argv[2] isn't a "FILE *".

Also, if you use the same name for both input and output files, then you would get the result you describe. There's little you can do about that - best approach may be to compare input and output file names and see if they are the same, and exit if that's the case.

--
Mats

Looking at it briefly, it looks good.

Some minor comments:
- Not sure what PATH_MAX has to do with your line. Line is the data you read from the file, which, in theory could be "any lenght", but realistically should be within 100 chars (33 chars per number) - nothing wrong with making your buffer 1000 long (about 333 chars per number), or some such, tho'. But PATH_MAX is really used for filepaths and filenames, not as a "generic constant". Ideally, I'd like to see a #define constant, such as this:

Code:

#define LINE_MAX (1000)
...
int main(...)
...
  char line[LINE_MAX];
...

Is three decimal points what you should be doing? There's no real right or wrong answer, I'm just asking, because if you have quite small numbers, chopping at 3 decimal places is perhaps a bit low. The larger the number is, the less important the decimal places become, of course.

--
Mats

For extra points, go with

Code:

if ( sscanf(line, "%lf %lf %lf", &x, &y, &z) == 3 ) {
  fprintf(out, "%.3lf,%.3lf,%.3lf\n", x * factor, y * factor, z * factor);
} else {
  // complain about a badly formatted line
}

For maximum points, use strtod() in place of sscanf(), because the former will also detect overflow and underflow.