Text files: write then read

Files keep data after the program ends

• A file stores data on disk, so it is still there after your program stops.
• You work with a file through a FILE * — a pointer you get from fopen.
• In these tasks you write a file, then read it back in the same program, so everything is self-contained.

Opening and closing

• fopen("data.txt", "w") opens a file for writing (it creates or empties the file).
• fopen("data.txt", "r") opens it for reading.
• Always fclose(f) when you are done, so the data is saved and the file is released.

Writing with fprintf

• fprintf works like printf, but its first argument is the FILE *.
• fprintf(f, "%d\n", x); writes the number x and a newline into the file f.
• Write each value on its own line so it is easy to read back.

Reading with fscanf and fgets

• fscanf(f, "%d", &x) reads one number; it returns 1 when it succeeds.
• fgets(buf, size, f) reads one line of text into buf; it returns NULL at the end.
• Loop until the read fails to process the whole file.

#include <stdio.h>

int main(void) {
    FILE *f = fopen("nums.txt", "w");
    fprintf(f, "3\n4\n");
    fclose(f);

    f = fopen("nums.txt", "r");
    if (f == NULL) {          // the file could not be opened
        printf("cannot open nums.txt\n");
        return 1;
    }
    int x, total = 0;
    while (fscanf(f, "%d", &x) == 1) {
        total += x;
    }
    fclose(f);

    printf("%d\n", total);   // 7
    return 0;
}

Common mistakes

• Check that fopen did not return NULL before reading or writing.
• Always fclose the file when you are done.

Now you try

• These tasks are full programs: write inside main, and Check answer tests the output.
• Write the file first, fclose it, then open it again for reading and loop until the read fails.