如何使用 C append 是 a.txt 文件的第一行？

Question

I have a program where I am trying to add wind data to a.txt file, but I am facing the issue where I add data, I close the program, try to add data again, and it eliminates the rest of the data. The following is an example of the.txt file:

3
3.60000 N
4.30000 E
5.40000 S

The first line is the number of data entries and the following are each wind speed and then wind direction.

My code works fine if I just add the data once and be done with it, but not if I want to add more work to what I previously did. I know I can use append to add data to the end, but it will not update the counter at the top. I want to be able to add data and also update the counter at the top. I need the counter for other functions in the code. The following is my code for the problem:

void addWindData(FILE* outFile, int numNumbers, double windSpd[], char windDir[]){
    int numItems;
    
    printf("How many data items would you like to add? ");
    scanf("%d", &numItems);

    fprintf(outFile, "%d \n", numItems);

    for(int i = 0; i < numItems; i++){
        printf("Wind speed? ");
        scanf("%lf", &windSpeed[i]);

        printf("Wind direction? ");
        scanf(" %c", &windDir[i]);

        fprintf(outFile, "%lf %c \n", windSpd[i], windDir[i]);
    }
}

I call the function as so:

fp = fopen(FILE_NAME, "w");
if(fp == NULL){
    printf("File could not be found! \n");
}
else{
    addWindData(fp, numNumbers, windSpd, windDir);
    fclose(fp);
}

Any help I could get will be greatly appreciated. I understand that 'w' will always write, but when I use append, it starts a new count at the end and then adds the data. I just need help to figure out how to update the counter. I am very new to the C programming language and am still doing my best to learn, I am trying my best to use logic and reasoning, so it helps me to get inspiration from others! Thank you in advance!

Answer 1

File is open as stream, so you have 2 choices: Change chars or rewrite all file. You must know that you can change only existing chars! As fopen reference says to change chars you can use "r+" mode. This mode allow to read/write, starts from beginning and allows to rewind .

Answer 2

you can change fp = fopen(FILE_NAME, "w") to fp = fopen(FILE_NAME, "r+") because w mode will erase the previous text in.txt file but r+ do not because r is upgrade mode.