Reputation: 7694

Read number from a file and replace it with another number

12 23 34 45 56
34 23 56 21 43
12 57 98 34 12

The above is the content of a txt file.

With C, i can use fgetc(myFile) to get the first integer and store into an integer variable.

I will check whether it is 12.

if it is 12, i want to replace with 25. How do i exactly replace it a certain number.

How do i rewrite a certain part of it?

Or do i store every number into an array, replace all 12s with another numbers and overwrite the whole file??

Upvotes: 1

Answers (3)

Alex Voikov

Reputation: 121

Save result to another file, than renames it. This code opens homework.txt, replaces all 12 -> 25 and writes result to homework_new.txt

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

#define MAXBUF 42
#define HOMEWORKFILE "homework.txt"
#define HOMEWORKNEWFILE "homework_new.txt"

int main(int argc, char **argv)
{
    char buf[MAXBUF+1];
    char str[MAXBUF+1];
    FILE *hw;
    FILE *hw_new;
    int length;
    int i, j;
    int number;
    char is_first;
    int n_line = 0;

    hw = fopen(HOMEWORKFILE, "r");
    hw_new = fopen(HOMEWORKNEWFILE, "w");

    if (!hw)
    {
        fprintf(stderr, "File not found: %s\n", HOMEWORKFILE);
        return 5;
    }

    while(!feof(hw))
        if (fgets(buf, MAXBUF, hw) != NULL)
        {
            length = strlen(buf);
            j = 0;
            str[0] = 0;
            is_first = 1;
            n_line++;
            /* parse string */
            for(i = 0; i < strlen(buf); ++i)
            {
                if (isblank(buf[i]) || buf[i] == '\0' || buf[i] == '\n')
                {
                    str[j] = 0;
                    number = atoi(str);

                    if (is_first)
                        is_first = 0;
                    else
                        fprintf(hw_new, " ");

                    if (number == 12)
                        fprintf(hw_new, "%d", 25);
                    else
                        fprintf(hw_new, "%d", number);

                    j = 0;
                }
                else if (isdigit(buf[i]))
                {
                    str[j++] = buf[i];
                }
                else
                {
                    fprintf(stderr, "bad input on line %d '%s'\n", n_line, buf);
                    return 100;
                }
            }
            fprintf(hw_new, "\n");

        }

    fclose(hw_new);
    fclose(hw);
    return 0;
}

Upvotes: 1