【415】C语言文件读写

A program can open and close, and read from, and write to, a file that is defined by the user

This is generally done when you have

• large volumes of stored data, or
• complex data (such as structs) or
• non-printable data

These don't happen often. Nevertheless, for the sake of completeness, here is a program that

• reads a number from a file input.txt

• writes the count from 1 to that number to the file output.txt

  • it is user-friendly : it tells the user that an output file has been created

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// files.c // read a number 'num' from a file input.txt // write a count from 1 to 'num' to the file OUT   #define IN  "input.txt" #define OUT "output.txt"   #include <stdio.h> #include <stdlib.h>   #define NUMDIG 6 // size of numerical strings that are output   int main(void) {    FILE *fpi, *fpo; // these are file pointers    char s[NUMDIG];      fpi = fopen(IN, "r");    if (fpi == NULL) { // an important check        fprintf(stderr, "Can't open %s\n", IN);        return EXIT_FAILURE;    }    else {        int num;        if (fscanf(fpi, "%d", &num) != 1) { // an important check            fprintf(stderr, "No number found in %s\n", IN);            return EXIT_FAILURE;        }        else {            fclose(fpi); // don't need the input file anymore            fpo = fopen(OUT, "w");            if (fpo == NULL) { // an important check                fprintf(stderr, "Can't create %s!\n", OUT);                return EXIT_FAILURE;            }            else { // got input and got an output file                fprintf(fpo, "%s", "Counts\n");                for (int i=1; i<=num; i++) {                    sprintf(s, "%d", i);                    fprintf(fpo, "%s\n", s);                }                fclose(fpo);                printf("file %s created\n", OUT);                return EXIT_SUCCESS;            }        }    } }

input.txt

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1

output.txt

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prompt$ dcc files.c prompt$ ./a.out file output.txt created prompt$ more output.txt Counts 1 2 3 4 5 6 7 8 9 10 11 12 13