Argthtjtr

My program is supposed to read 4 numbers from a .txt file, add 10 to the numbers, then print it back. I have a function which loads the text file with four numbers, then another function which adds 10 and appends the file. The program currently works, but I am very confused within my addTen() function.

Why don't I need to fscanf the file again? How does my function already know the values that are held inside my .txt file? I found this accidentally when trying to get my program to work with EOF indicators.

#include <stdio.h>



// function prototypes

void loadTextFile(FILE *file);

void addTen(FILE *file);



// begin main function

int main(void){



    FILE *text = fopen("question6.txt","w");



    if(text == NULL)

    {

        printf("question6.dat cannot be opened!n");

        fprintf(stderr, "Error opening the fil!n");

    }



    else

        printf("nquestion6.dat was opened successfully for Writing.nn");



    loadTextFile(text);

    addTen(text);



    fclose(text);



    return 0;

} // end main function





void addTen(FILE *file){



    int i=0;

    int number[4];



    for (i=0;i<4;i++)

    {   

        fprintf(file, "%dn", number[i]+10);

        printf("nt%d was written to the filen", number[i]+10);

    }



} 



// function to load the .txt file with array values so we may execute main on any computer. 

void loadTextFile(FILE *file){





    int numberArray[4]={56, 23, 89, 30};



    int i=0;



    for(i=0;i<4;i++)

    {

        fprintf(file, "%dn", numberArray[i]);

        printf("nt%d was written to the filen", numberArray[i]);



    }



    printf("nThe data was successfully writtenn");



} // end function loadTextFile

The program currently works

No, it doesn't!

Your addTen function has undefined behavior. It is outputting values from an uninitialized array. If it "works" for you, that's probably because it just happens to be in the same part of the stack as the local variable numberArray that you populate in loadTextFile before discarding. You cannot rely on this at all.

I suggest that you pass an array into your functions:

void addTen(int array, int size) {

    for (int i=0; i < size; i++) {   

        array[i] += 10;

    }

}



void loadTextFile(FILE *file, int array, int size) {

    for (int i=0; i < size; i++) {   

        fscanf(file, "%d", &array[i]);

    }

}

And maybe a separate function to print the data:

void printArray(FILE* fp, int array, int size) {

    for (int i=0; i < size; i++) {   

        fprintf("%dn", array[i]);

    }

}

Now invoke it all:

int array[4];

loadTextFile(text, array, 4);

addTen(array, 4);

printArray(text, array, 4);

Note that you have some other errors in your program framework. One issue is that you are opening the input file with openmode "w", which will open the file for write-only, and truncate its current contents.

It may be better to open it for read ("r") first, load the data and close it. Then open it for write and output the modified data.

Turns out I do need a fscanf and the only reason mine worked was because I got lucky! With the help of @paddy and other commentators here, I was able to solve the program. I also solved the program using the idea of pre-loading the text file with specific numbers. The codes below are a refined version of the code posted in the question and will work as a solution to this problem.

//Program which reads four numbers from a text document, adds 10 to them and appends them back to the text file



#include <stdio.h>



// function prototypes

void loadTextFile(FILE *file, int array, int size);

void printArray(FILE* fp, int array, int size);

void addTen(int array, int size);



// begin main function

int main(void){



    FILE *text = fopen("question6.txt","r"); // open the file to read it



    int array[4]; 



    loadTextFile(text, array, 4); // scan the .txt file into an array

    addTen(array, 4);   // add ten to each number in the array



    fclose(text); // close the file for writing



    fopen("question6.txt","a"); // open the file to append it



    printArray(text, array, 4); // print the array to the file



    fclose(text); // close the file after appending. 



    return 0;

} // end main function





void loadTextFile(FILE *file, int array, int size) {

    int i=0;

    for (i=0; i < size; i++) {   

        fscanf(file, "%d", &array[i]);

    }

}



void addTen(int array, int size) {

    int i=0;

    for (i=0; i < size; i++) {   

        array[i] += 10;

    }

}



void printArray(FILE* fp, int array, int size) {

    int i=0;

    for (i=0; i < size; i++) {   

        fprintf(fp, "%dn", array[i]);

        printf( "%d was written to the array.n", array[i]);

    }

}

I have also gotten my original idea to work by pointing the pointer to the beginning of the file. By only opening the file using fopen once, the pointer was at the end of the file at all times.

#include <stdio.h>



// function prototypes



void loadTextFile(FILE *file);



void returnStats(FILE *file);



// begin main function



int main(void){



FILE *text = fopen("question6.txt","w");



if(text == NULL)



{



printf("question6.dat cannot be opened!n");



fprintf(stderr, "Error opening the fil!n");



}



else



printf("nquestion6.dat was opened successfully for Writing.nn");



loadTextFile(text);

fclose(text);

FILE *text1 = fopen("question6.txt","r");



returnStats(text1);





return 0;



} // end main function



void returnStats(FILE *file){



int i=0, count;



int number[4];





while(fscanf(file, "%d", &number[i++])!=EOF){



printf("n%d : Was Read from the Filen", number[i-1]);

count++;



}

fclose(file);

FILE *text1 = fopen("question6.txt","a");

for(i=0;i<count-1;i++){

    fprintf(text1, "%dn", number[i]+10);

    printf("n%d: was appended to the text filen", number[i]+10);

}

} // end returnStats



// function to load the .txt file with array values so we may execute main on any computer.



void loadTextFile(FILE *file){



int numberArray[4]={56, 23, 89, 30};



int i=0;



for(i=0;i<4;i++)



{



fprintf(file, "%dn", numberArray[i]);



printf("n%d : was written to the filen", numberArray[i]);



}



printf("nThe data was successfully writtenn");



} // end function loadTextFile

Your loadTextFile(FILE *file) has an array with 4 integers. It is not reading numbers from the file, it is writing numbers to the file. It has this array: int numberArray[4]={56, 23, 89, 30};

For reading from file you need to use fscanf()