Public Group

# How to read in a file in C.

This topic is 3889 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

I have an assignment that requires me to read in a .txt file and then perform calculations on the data. My professor has posted code that technically does it, however he's filled in a lot of other stuff that's good habit, but is complicating me being able to figure out what exactly is going on. Here it is.
/* files1.c */

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

int main()
{
char text[256] = "";
int test = 0 ;

/********* STEP 1 - define the files *****/
FILE* in;
FILE* out;

/********* STEP 2 - open the files for processing *****/
in   = fopen("input.txt", "r");  /* input file - read mode */
if (in == NULL)
{
printf("input file not opened");
exit(1);
}

out  = fopen("output.txt", "w");  /* output file - output mode */
if (out == NULL)
{
printf("output file not opened");
exit(1);
}

/********* STEP 3 - process the files *****/
fgets(text, sizeof(text) - 1 , in);
while (! feof(in))
{
printf("I just read %d bytes : %s", strlen(text), text);
fprintf(out, "%s\n", text);
fgets(text, sizeof(text) - 1 , in);
}

/********* STEP 4 - close the files *****/
fclose(out);
fclose(in);

return 0;
}

My issue with this is when I run it, the app window pops up and goes away (even if I add a scanf at the bottom) then the program creates a text document called output in which it writes it output to instead of on the screen. It also makes and .exe file which I can click and run and it will keep the application window up for me to look at. This is very confusing as usually I can run it straight from blood shed. My question for you lovely people is; would it be possible for you to post some sample code for the quickest/simplest way to read in a file and print it out on screen in C? Perhaps with that I could figure out how it works exactly and do the rest of the assignment. Thanks for your time. PS - yes I did make an input.txt for the program to use and it uses it.

##### Share on other sites
change 'fprintf(out, ...)' to 'fprintf(stdout, ...)'. If this is a console application, the output will go the console instead.

So 'fprintf(stdout, ...)' will basically behave the same as a regular 'printf'.

(Or am I missing your point? :)

system("pause");

...at the end of your program if the window disappears after execution. You can also just run the program from a console window ([windows key]+[R], cmd, [enter]) or put a breakpoint somewhere and run it through the debugger.

##### Share on other sites
int main()
{
FILE* f = fopen("file.txt", "rt");
fseek(f, 0, SEEK_END);

long fileLength = ftell(f);
fseek(f, 0, SEEK_SET);

char* bytes = (char*)malloc(sizeof(char) * fileLength);

printf("%s\n", bytes);
free(bytes);
return 0;
}

##### Share on other sites
hmm still getting the same thing, heres what I got

/* files1.c */#include <stdio.h>#include <stdlib.h>#include <string.h>int main(){  char text[256] = "";  int test = 0 ;  /********* STEP 1 - define the files *****/  FILE* in;  FILE* out;    /********* STEP 2 - open the files for processing *****/  in   = fopen("input.txt", "r");  /* input file - read mode */  if (in == NULL)  {    printf("input file not opened");    exit(1);  }  out  = fopen("output.txt", "w");  /* output file - output mode */  if (out == NULL)  {    printf("output file not opened");    exit(1);  }    /********* STEP 3 - process the files *****/  fgets(text, sizeof(text) - 1 , in);  while (! feof(in))  {    printf("I just read %d bytes : %s", strlen(text), text);    fprintf(stdout, "%s\n", text);    fgets(text, sizeof(text) - 1 , in);  }  /********* STEP 4 - close the files *****/  fclose(out);  fclose(in);  scanf ("%d", &test);  return 0;}

##### Share on other sites
Quote:
 Original post by longjumperint main(){ FILE* f = fopen("file.txt", "rt"); fseek(f, 0, SEEK_END); long fileLength = ftell(f); fseek(f, 0, SEEK_SET); char* bytes = (char*)malloc(sizeof(char) * fileLength); fread(bytes, sizeof(char), fileLength, f); printf("%s\n", bytes); free(bytes); return 0;}

If all you (OP) really want is code that reads an ASCII file and outputs it to the console, then longjumper's code is what you're looking for. Just keep in mind that it's not written in C ;)

Hint: in C variables can only be declared at the beginning of a scope.

##### Share on other sites
Quote:
 Original post by Jan-Lieuwechange 'fprintf(out, ...)' to 'fprintf(stdout, ...)'. If this is a console application, the output will go the console instead.So 'fprintf(stdout, ...)' will basically behave the same as a regular 'printf'.(Or am I missing your point? :)You can add something like:system("pause");...at the end of your program if the window disappears after execution. You can also just run the program from a console window ([windows key]+[R], cmd, [enter]) or put a breakpoint somewhere and run it through the debugger.

Yes the window disappears after execution, even with your solutions above. I can run it through the cmd prompt it also creates a .exe of the program in the folder the project it's saved in that I can run it from. My issue is, why is it disappearing as my other program never have. Why is it making an .exe as my other programs never have.

I think theres code in there I don't quite understand thats doing this and thats what makes me uneasy.

Quote:
Original post by Jan-Lieuwe
Quote:
 Original post by longjumperint main(){ FILE* f = fopen("file.txt", "rt"); fseek(f, 0, SEEK_END); long fileLength = ftell(f); fseek(f, 0, SEEK_SET); char* bytes = (char*)malloc(sizeof(char) * fileLength); fread(bytes, sizeof(char), fileLength, f); printf("%s\n", bytes); free(bytes); return 0;}

If all you (OP) really want is code that reads an ASCII file and outputs it to the console, then longjumper's code is what you're looking for. Just keep in mind that it's not written in C ;)

Hint: in C variables can only be declared at the beginning of a scope.

Ya, I'm still a beginner programmer so the fact that this code had half a dozen errors when all I did was copy and paste... it scared me away form it.

##### Share on other sites
Quote:
 Original post by JavaMavahmm still getting the same thing, heres what I got*** Source Snippet Removed ***

What do you mean with 'same thing'? The window disappears?

##### Share on other sites
Quote:
Original post by JavaMava
Quote:
 Original post by Jan-Lieuwechange 'fprintf(out, ...)' to 'fprintf(stdout, ...)'. If this is a console application, the output will go the console instead.So 'fprintf(stdout, ...)' will basically behave the same as a regular 'printf'.(Or am I missing your point? :)You can add something like:system("pause");...at the end of your program if the window disappears after execution. You can also just run the program from a console window ([windows key]+[R], cmd, [enter]) or put a breakpoint somewhere and run it through the debugger.

Yes the window disappears after execution, even with your solutions above. I can run it through the cmd prompt it also creates a .exe of the program in the folder the project it's saved in that I can run it from. My issue is, why is it disappearing as my other program never have. Why is it making an .exe as my other programs never have.

I think theres code in there I don't quite understand thats doing this and thats what makes me uneasy.

Quote:
Original post by Jan-Lieuwe
Quote:
 Original post by longjumperint main(){ FILE* f = fopen("file.txt", "rt"); fseek(f, 0, SEEK_END); long fileLength = ftell(f); fseek(f, 0, SEEK_SET); char* bytes = (char*)malloc(sizeof(char) * fileLength); fread(bytes, sizeof(char), fileLength, f); printf("%s\n", bytes); free(bytes); return 0;}

If all you (OP) really want is code that reads an ASCII file and outputs it to the console, then longjumper's code is what you're looking for. Just keep in mind that it's not written in C ;)

Hint: in C variables can only be declared at the beginning of a scope.

Ya, I'm still a beginner programmer so the fact that this code had half a dozen errors when all I did was copy and paste... it scared me away form it.

Your windows would only disappear if the CPU executes one of those 'exit(1)' statements. Those are executed when your program doesn't find 'input.txt' or when it cannot generate 'output.txt' (the latter being unlikely). Is 'input.txt' in the same folder as your executable?

All C / C++ projects would generate an .exe file, btw. So I don't understand what you're referring to when you say: "Why is it making an .exe as my other programs never have." - unless your previous projects were written using a different language.

##### Share on other sites
Quote:
Original post by Jan-Lieuwe
Quote:
 Original post by JavaMavahmm still getting the same thing, heres what I got*** Source Snippet Removed ***

What do you mean with 'same thing'? The window disappears?

yes

##### Share on other sites
Quote:
 Original post by Jan-LieuweYour windows would only disappear if the CPU executes one of those 'exit(1)' statements. Those are executed when your program doesn't find 'input.txt' or when it cannot generate 'output.txt' (the latter being unlikely). Is 'input.txt' in the same folder as your executable?All C / C++ projects would generate an .exe file, btw. So I don't understand what you're referring to when you say: "Why is it making an .exe as my other programs never have." - unless your previous projects were written using a different language.

I guess it muse be one of the exit commands, I'll go try that. It does find input.txt though as when I run the .exe it takes the text inside the input.txt and displays it on screen. Yes input.txt is in the same folder.

As for the confusion around generating .exe files, that must be my own mistake. We use a text based network, phobos, in my college so I never really see all my files unless I'm specifically looking for them. I guess when I'm at my home PC I just never noticed the .exe.

EDIT - just removed the exits, and the windows stays up. It displays the error message that it can't find input.txt so I guess I was some how mistaken there...

I just tried the .exe again and it finds the input.txt

1. 1
2. 2
Rutin
24
3. 3
4. 4
JoeJ
19
5. 5

• 14
• 26
• 11
• 11
• 9
• ### Forum Statistics

• Total Topics
631771
• Total Posts
3002252
×