input redirected

[AChrapko]

Hello, how can you tell if a program that expects the input to be redirected fails to do so? For example:

./gauss < gauss1.txt

is called like

./gauss

instead.

The gist of my program is to read from some given files which are formatted like

3
1 2 3 6
4 5 6 15
7 8 9 24

Where the 3 is the number of rows, and the rest is intended to be put in a 2d array.

I have been working with scanf() to read each number (as a double).
My problem is that if there is no input redirection to a file my program will just hang waiting for input, and I need to instead display some sort of error message. I have tried looking at the number of arguments using

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

but it does not store the redirection.

Thanks for any suggestions

[msh]

You don't need to "store the redirection". You need to call gauss with gauss1.txt as an argument, which will be stores as argv[1]. You then will use this argument to open and read the file.

[AChrapko]

Thanks for the reply, but for the specifications of my program, I do need to use input redirection.
It is for a class, and they do batch testing so I have to make sure it works according to the way they call the program.

[msh]

In that case, program as if you were dealing with user input. If you're using scanf() and it "hangs" it means that you're most likely forgetting skip the \n at the end of each line.

[AChrapko]

I'm doing just that. It hangs because it's as though the user called

./gauss

and then walked away from their computer.
It is treated as though the program is intended to take actual input from a physical user, rather than a file.

[msh]

You're doing it wrong, then. Show us the code.

[AChrapko]

That was a rather harsh accusation.

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

#define MAXROWS 100
#define MAXCOLS 101

void printMatrix(int rows, int columns, double matrix[][MAXCOLS]);

/* Add helper functions and global variables here */

int main()
{
	/* Add your implementation here */
	int i = 0;
	int rows;
	int r,c;
	
	scanf("%d",&rows);				//get the first number (num. of rows)
	double matrix[rows][rows+1];			//create 2d array based on value of rows
	
	//scanf("%lf",&matrix[0][0]);
	double num = 0;
	for (r = 0; r < rows; r++) {
		for (c = 0; c < rows+1; c++) {
			if((scanf("%lf",&num)) != EOF){		//scanf() until the end of file is reached
				//printf("scanned: %lf\n",num);
				matrix[r][c] = num;
				printf("m[%d][%d] = %lf\n",r,c,matrix[r][c]);
			}else{
				printf("Element a[%d][%d] is missing\n",r,c);
				exit(0);
			}
		}
	}
	//printf("%d, %d\n",r-1,c-1);
	printf("\t\ttest: %lf\n",matrix[1][0]);
	
	printf("initial matrix\n");
	printMatrix(rows,rows+1,matrix);
	
	return 0;
}

void printMatrix(int rows, int columns, double matrix[][MAXCOLS])
{
	/* Add your implementation here */
	int j, k;
	
	printf("MATRIX: \n");
	//printf("rows = %d\n",rows);
	//printf("columns = %d\n",columns);
	for (j = 0; j <= rows; j++) {
		for (k = 0; k < columns; k++){
			printf("m[%d][%d] : %lf\n",j,k,matrix[j][k]);
		}
	}
}

The section to focus is within the first imbedded for-loop.

There may be some mistakes that I have not figured out yet because when I attempt to print the array with the printMatrix function it does not work properly, and as to why I honestly have no idea at all.

Thanks in advance.

[msh]

That was a rather harsh accusation.

It's accurate, though. You're not skipping over newlines anywhere in that code.

Here is my implementation, it lacks your error checking, but it works with the sample input you provided.

// Read data...
for( i = 0; i < rows; ++i )
{
    for( t = 0; t < ROW_LEN; ++t )
    {
        scanf( "%lf", &data[i][t] );
    }
    clean_buff( );
}


void clean_buff( void )
{
    int ch;
    while( ( ch = getchar() ) != EOF && ch != '\n' );
}

You'll also need to skip the newline after you read the number of rows.

[AChrapko]

I think you have a misconception of what the program is intended to do. If the program is called as such:

./gauss

rather than

./gauss < gauss1.txt

then it should not wait for any input, but instead return

Expected N (number of equations)

and nothing further.

After having implemented your code my program looks like

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

#define MAXROWS 100
#define MAXCOLS 101

void printMatrix(int rows, int columns, double matrix[][MAXCOLS]);
void clean_buff( void);

/* Add helper functions and global variables here */

int main()
{
	/* Add your implementation here */
	int i = 0;
	int rows;
	int r,c;
	
	scanf("%d",&rows);
	double matrix[rows][rows+1];
	
	//scanf("%lf",&matrix[0][0]);
	double num = 0;
	/*for (r = 0; r < rows; r++) {
		for (c = 0; c < rows+1; c++) {
			if((scanf("%lf",&num)) != EOF){
				//printf("scanned: %lf\n",num);
				matrix[r][c] = num;
				printf("m[%d][%d] = %lf\n",r,c,matrix[r][c]);
			}else{
				printf("Element a[%d][%d] is missing\n",r,c);
				exit(0);
			}
		}
	}*/
	for(r=0; r < rows; ++r){
		for(c = 0; c < rows+1; ++c){
			scanf("%lf",&matrix[r][c]);
		}
		clean_buff();
	}
	
	
	
	//printf("%d, %d\n",r-1,c-1);
	printf("\t\ttest: %lf\n",matrix[1][0]);
	
	printf("initial matrix\n");
	printMatrix(rows,rows+1,matrix);
	
	return 0;
}

void clean_buff( void ){
	int ch;
	while (( ch = getchar() ) != EOF && ch != '\n');
}

void printMatrix(int rows, int columns, double matrix[][MAXCOLS])
{
	/* Add your implementation here */
	int j, k;
	
	printf("MATRIX: \n");
	//printf("rows = %d\n",rows);
	//printf("columns = %d\n",columns);
	for (j = 0; j <= rows; j++) {
		for (k = 0; k < columns; k++){
			printf("m[%d][%d] : %lf\n",j,k,matrix[j][k]);
		}
	}
	
}

After typing

gcc test.c -o test1
./test1

the program will sit and await user input.
If I type any amount of characters and press return it says "Illegal Instruction" and terminates

[Salem]

I'm doing just that. It hangs because it's as though the user called

./gauss

and then walked away from their computer.
It is treated as though the program is intended to take actual input from a physical user, rather than a file.

Well there isn't a lot you can do about that, especially in a "standard C" student work scenario.

There are a few ways out, but none of them are what you would call "easy 1-liners" that a new student would be expected to know.

If your program spec says "redirected input from a file", them you just have to take them at their word.

These are synonymous
./gauss (and you type in your input)
./gauss < file.txt
cat file.txt | ./gauss

[Salem]

> It's accurate, though. You're not skipping over newlines anywhere in that code.
Except %lf (and %d) SKIP white-space automatically, so it seems a moot point to me.

> If I type any amount of characters and press return it says "Illegal Instruction" and terminates
What exactly did you type?

Use script to capture your terminal session of what you typed in, and what it printed in response.
UNIX man pages : script ()

[Adak]

I think this is what he's looking for:

Compile this, and run it in a terminal window. It shows what you want.

/* compile this into an exe, first. then run it with some words after the name of the 
program: ie:


>argcargv hello world how are you <enter>

Output similar to:

Argc: 6

1. argv hello
2. argv world
3. argv how
4. argv are
5. argv you
*/

#include <stdio.h>

int main(int argc, char *argv[]) {
  int i, n; 
  printf("\n\n");
  printf("Argc: %d \n\n", argc);
  i=0;
  while(*argv[i]) {
    printf("Argv %d: %s\n", i, argv[i]);
    ++i;
  }
  printf("\n\n\t\t\t     press enter when ready");

  i = getchar(); ++i;
  return 0;
}

[Adak]

What you want is an usage message for the user, telling them to redirect input to your program.

That is not unusual, at all. Check the argc number. If that int is less than 2, then the user has NOT redirected any input to your program.

print an error message, and terminate the program with return, right from the top of main.

[AChrapko]

the specs do not directly say "redirected input from a file" however they are tested by machine and called with

./gauss < file.txt

and with my problem of not having any clue as to how to do the bulk of the program, I figured I would get at least the matrix to input properly and make sure the user input proper arguments. Though I don't think "arguments (flags,parameters whatever)" is the proper term for "< fileName.txt" redirecting the input. Anyways, the other problem I was having is if you refer to my first post of the full code, my array does not print off properly.

the first printf()

printf("m[%d][%d] = %lf\n",r,c,matrix[r][c]);

gets the correct number, and the loops control where it is placed. After the for-loops I even printed one of the problem elements, the first being "matrix[1][0]" and that worked properly, but when using the printMatrix() function my results look like

m[0][0] : 7.000000
m[0][1] : 8.000000
m[0][2] : 10.000000
m[0][3] : 25.000000
m[1][0] : 899156430874148739915381448845229089616319085089529831162663631101506972863985960828040243303953133619257521263629665420168260295724615073305771911999367898772537344.000000
m[1][1] : 0.000000
m[1][2] : 17676154007756128403861346874008211219880184843688977024706402509217698083584330072617614175375678388925763597874710561778820251748349828351438029372236052957308649472.000000
m[1][3] : 0.000000
m[2][0] : 0.000000
m[2][1] : 0.000000
m[2][2] : 0.000000
m[2][3] : 0.000000
m[3][0] : 0.000000
m[3][1] : 0.000000
m[3][2] : 258686043184358362686196289935145724172259836220883361143274666226839157675633155966362818606293835110643936222715950291052218218230256283650594491132653621739520.000000
m[3][3] : 0.000000

the file I input with looks like this:

3
7 8 10 25
1 2 3 6
4 5 6 15

[AChrapko]

Adak, that is exactly correct in that I want a Usage message, however I tried using argc and argv already, but when I printed the value of argc after calling the program

./gauss < file.txt

the value is still 1. It seems like the system does not consider "< ..." to be part of the arguments, I think because it is a platform specific operation(?) Like that is how you use a file for stdin on a Unix machine (what I'm working on) but not others? At least that's how I believed it to be