Read lines from a file into character arrays

Question

I'm completely new to C and I'm working on a program which has to read in 3 lines from a text file(two numbers and a mathematical symbol) and write out the result. So for example:

The text file looks like:

1

4

*

and my program should be able to read the 3 lines and write out something like "1*4 = 4" or something.

I managed to get to a point where i can read the 3 lines in and show them on screen, so I thought I should put the two numbers in one array and the symbol in another one. The problem is, that I tried to see if the arrays contain the numbers I put in them and my output has some huge numbers in it and I'm not sure why.

Here's the code i wrote:

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

    int main(void)
    {
        int res = 1;                                /*Creates an integer to hold the result of the check for the file*/
        const char *file = "input.txt";             /*String holding the name of the file with the input data*/

        res = access(file,R_OK);                    /*Checks if the file "input.txt" exists*/

        if(res == -1)
        {                                           /*IF the file doesn't exist:*/
            FILE *input = fopen("input.txt","w");   /*This creates a file called "input.txt" in the directory of the program*/
            char write[] = "1\n1\n+";               /*This variable holds the string that's to be written to the file*/
            fprintf(input,"%s",write);              /*This writes the variable "write" to the file*/
            printf("input.txt file created!");      /*Tells you the file is created*/
            fclose(input);                          /*Closes the file after it's done*/
        }
        else
        {                                           /*IF the file exists:*/
            FILE *f = fopen("input.txt","r");

            //char line[ 5000 ];
            //while ( fgets ( line, sizeof line, f ) != NULL )
            //{
            //    fputs ( line, stdout );
            //}

            char line[5000];
            char nums[2];
            char symbol[1];

            int i = 0;
            while(fgets(line,sizeof line,f)!=NULL)
            {
                i++;
                if(i < 3)
                {
                    fputs(nums,f);
                }
                else
                {
                    fputs(symbol,f);
                }

                printf("%d,%d",nums,symbol);
            }

            printf("\n\n\n");
            scanf("\n");
        }

        return 0;
}

Any help would be greatly appreciated! Thank you in advance If you require any more information i will provide it.

Answer 1

What are you reading from the files are simply characters codes: the program has no way of figuring by itself that the character "4" corresponds to the integer number 4. The %d placeholder of printf expects int variables, or it won't work.

If you want just to print the characters you have to save them in char variables (or a char array) and use the placeholder %c in printf. If you want to actually use the numbers and symbols in your program you have more work to do.

Not only in C, but I think in most languages you have to "parse" the characters to numbers.

In C you can use the functions atoi or atol (you have to #include <stdlib.h>) in order to do this conversion.

In order to parse the symbol I'm afraid you will have to use an if or a switch to read the character and perform the operation accordingly.

For example your loop could look like:

  while(fgets(line,sizeof line,f)!=NULL)
    {
      int op1;
      int op2;
      int res;
      char symbol;
      i++;
      switch (i) {
        case 1:
          //First line is first operand
          op1 = atoi(line);
          printf("op1 %d\n",op1);
          break;
        case 3:
          //Second line is second operand
          op2 = atoi(line);
          printf("op2 %d\n",op2);
          break;
          //Fifth line is the operator, could be +,-,%./ or anything
        case 5:
          symbol = line[0];
          printf("operand %c\n",symbol);
          switch(symbol) {
            case '+':
              res = op1+op2;
              break;
            case '-':
              res = op1-op2;
              break;
            default:
              //operation not defined, return
              return;

          }
          printf("%d%c%d = %d",op1,symbol,op2,res);
      }
    }