Thread: I/O from .txt files.

Hi, I was wondering if it would be possible to get some help. I'm making a program for my chemistry course that requires a number of values to be read from a text file, a calculation done with those values, and the new numbers then printed onto the screen and saved into a file.

So far I have the following code,

Code:

//A program that calculates the reduced mass of diatomic molecules.
//The following libraries will be used.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
using namespace std;

int main()
{
int selection,b;//Defines the selection variable to determine the input.

printf("A program that works out the reduced mass of diatomic molecules.\n\n");//Prints the introductory message.
printf("Would you like to input the data from the keyboard or a .txt file?\n");//Allows the user to choose the input.
printf("1. From a .txt file ");
printf("2. From the keyboard\n");
printf(":");
scanf("%d",&selection);//Assigns the value of the selection int variable.

FILE *FP;
char a[100];
printf("Please input a filename including the .txt extension:"); scanf("%s",a);
FP = fopen(a,"r");

if (FP==NULL)
{
    printf("Error Reading File");
}
while (!feof(FP))
    {
        fgets(a,100,FP);
        printf("%s",a);
    }
    fclose(FP);

return 0;
}

I tried making an integer variable b which was equal to a*2 however this didn't seem to work, and instead I got an error message.

How can I perform a calculation on the numbers as they are read from the text file?

Let's start here:

Code:

while (!feof(FP))

Don't do this. FAQ > Why it's bad to use feof() to control a loop - Cprogramming.com

Code:

fgets(a,100,FP);

That gets one line, including the newline character. You need to parse what you get here with sscanf; how you do this will depend upon the format of the file.

You also keep going blindly on in the case of failure here:

Code:

if (FP==NULL)
{
    printf("Error Reading File");
}
while (!feof(FP))

which will result in a crash if FP is actually NULL.

Thankyou very much both of you for the replies, I'll have a read through the linked tutorial and how parsing works. Sorry to ask such a simple question, but I'd literally spent a whole day on it and was getting no where and didn't know where to look.

Exactly - the value read from the file is a sequence of characters, not a number. You can't multiply it by two any more than you can multiply a sentence by 2.

sscanf is a good way to do the conversion. All it's doing is converting the string to something else, in the same way that scanf reads user input into variables.

There's also an fscanf, which does the same thing from files. You could use fscanf directly, but I wouldn't recommend it -- , it can be pretty fiddly if unexpected input comes in (fscanf will not move the file pointer on unless it gets what it is trying to parse, so a simple while loop can easily get stuck in a tight infinite loop doing nothing). sscanf is similar in that it can fail to parse anything, but you have a lot more control with a string that you've already read than you do with a file pointer.

Another option is to use the conversion functions in stdlib.h (<cstdlib> (stdlib.h) - C++ Reference) that convert from strings to various number types. The at* functions (e.g. atoi) are flawed - they return 0 on failure, indistinguishably from returning 0 having successfully converted the string "0". The strto* functions are fine though -- they adjust a pointer to show how much of the string they parsed, so you can tell if an error occured.

If you're convinced there'll be no errors at all in the input data, you could use atoi and friends, but it's probably not a good habit to make.

Originally Posted by c99tutorial

It may also improve understanding by renaming the following variables:

a
b

What are the meanings of these? You are using `a' for two different purposes, the filename as well as a data buffer. Use two names for this.

a --> char filename[100], buffer[100];
b --> double result;

If `result' is a double value (or int, or whatever you want), then just by considering the type difference, it is obvious that `buffer' cannot be multiplied by two nor can it be stored into `result' so you must convert it first. This is what rags_to_riches means by parsing.

Hi, I'm going to sounds really stupid but I've just had a read of the C File I/O tutorial on here, and I'm still not really understanding. What is the difference between the filename and the data buffer?

Originally Posted by Alex Saunders

Hi, I'm going to sounds really stupid but I've just had a read of the C File I/O tutorial on here, and I'm still not really understanding. What is the difference between the filename and the data buffer?

Never mind, I've figured that out.
Thanks

This is what I've got so far, after having a mess around with it. No loop as of yet, but I will try to implement that later. I'm now having the problem that the string doesn't seem to print after obtaining it from the text file. The number int number does work, it's just char element_name[14] is displayed as an @ symbol on the screen rather than the text in the document. Any ideas?

This is essentially what the text file says.

100,Helium
200,Hydrogen
300,Nitrogen
400,Oxygen
500,Fluroine
abc,Sodium
def,Sulphur
ghi,potassium
jkl,nadonado

Code:

//A program that calculates the reduced mass of diatomic molecules.
//The following libraries will be used.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
using namespace std;

int main()
{
int selection;//Defines the selection variable to determine the input.

printf("A program that works out the reduced mass of diatomic molecules.\n\n");//Prints the introductory message.
printf("Would you like to input the data from the keyboard or a .txt file?\n");//Allows the user to choose the input.
printf("1. From a .txt file ");
printf("2. Fromc the keyboard\n");
printf(":");
scanf("%d",&selection);//Assigns the value of the selection int variable.

FILE *FP;
int number;
char element_name[14];
char filename[100];

do{
    printf("Please input a filename including the .txt extension:"); scanf("%s",filename);
    FP = fopen(filename,"r");
    if (FP==NULL){printf("\nError Reading File\n");}
}while (FP==NULL);



fscanf(FP, "%d[^,],%13\n", &number,&element_name);
printf("%d %s",number,element_name);

fclose(FP);

return 0;
}

Isn't the program going to be wrecked when the number first lines end, and the letter first lines begin, "abc,sodium", etc?

I'd prefer you used fgets() to bring the entire line into a char array of 100 elements or so, and THEN pick out the right way to sscanf() the parts from there - it makes setting it up a lot easier than working with a file, directly.

If the first char is a letter, instead of a number, you use a different sscanf() to get it, if it IS a digit (0-9), then you get it the same way you are getting it now. Either way, I'll show you how to get the last part of that line.

Do you have restrictions on what C functions you can use for this assignment?

Originally Posted by Adak

Isn't the program going to be wrecked when the number first lines end, and the letter first lines begin, "abc,sodium", etc?

I'd prefer you used fgets() to bring the entire line into a char array of 100 elements or so, and THEN pick out the right way to sscanf() the parts from there - it makes setting it up a lot easier than working with a file, directly.

If the first char is a letter, instead of a number, you use a different sscanf() to get it, if it IS a digit (0-9), then you get it the same way you are getting it now. Either way, I'll show you how to get the last part of that line.

Do you have restrictions on what C functions you can use for this assignment?

We haven't been given any no.

Originally Posted by c99tutorial

What is the purpose of this line?

For some reason our teacher was using a C++ compiler, but teaching us C commands from stdio.h.
The teacher didn't mind if we used C++ or C as long as it ran in the compiler, and I think I used some C++ code in a previous program and forgot to delete it when I was being lazy and just copied the libraries to the coding for this program.

It gets a bit difficult, because it doesn't have a consistent
number,letter

format.

This assumes you set up the char buffer[100], s1[100], and s2[100],
as well as int num, and include <string.h> as well.

Code:

   while(fgets(buff, sizeof(buff), fp)) {
      len=strlen(buff);
      if(buff[len-1]=='\n') //fgets adds a newline to the end of buff
         buff[len-1]='\0';   //which is removed, here
      printf("%s  ",buff);
      if(buff[0] >'0' && buff[0]<'9') {   //it is a digit first line of the file
         sscanf(buff, "%d,%s",&num,s1);
         printf("num: %d,  s1: %s\n",num,s1);
      }else {
         sscanf(buff, "%[^,],%s",s1,s2);   //the line has two strings
         printf("s1: %s, s2: %s\n",s1,s2); //and no numbers
      }
      printf("\n");
   }

Thankyou very much for the help everyone. Problem is now solved.