Exporting print statements to external file - c

Question

So I'm trying to teach myself , and Ive been doing online lab exercises to learn it. I wrote a program that goes into pretty good detail of arrays and structures. It uses random numbers to monitor for a spike of 100 psi, and then prints that as a 0 point, and prints the previous 10 seconds of the array and the next 10, as if I was collecting data. The next part of this exercise is to take the program's print statement, and have it write to an external file and have it print there. My thought process is to populate an array in the printout function that holds the values read from the file and then print from the array to screen. But I'm not sure how this would look, or really how to accomplish it. If anyone could point me in the right direction or give a good explanation it would be greatly appreciated!
My code so far:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h>
#define MAX_CHANGE 100
#define ARRAY_SIZE 21
typedef struct data_slice
{
int t; // -> Time
float tp; // -> Valve pressure
float tf; // -> Sodium flow
float tt; // -> Sodium temp in Celsius
} data_slice;
// Function Declarations
void get_values(float * pressure, float * flow, float * temp);
void printIt(data_slice * data);
void initializeArray(data_slice * data);
bool spikeValueRecorded(data_slice * data, int outputIndex);
int main()
{
srand((unsigned int)time(NULL));
data_slice data[ARRAY_SIZE];
int index = -1;
while (1)
{
// Initialize the entire array
initializeArray(data);
// If there's a spike.....
if (spikeValueRecorded(data, index))
{
// Set the previous "time" in array to negatives
int temp = index;
for (int i = 0; i >= -10; --i)
{
data[temp].t = i;
temp = temp - 1;
if (temp < 0)
temp = temp + ARRAY_SIZE;
}
// Record for 10 more seconds
for (int i = 0; i <= 10; ++i)
{
data[index].t = i;
index = (index + 1) % ARRAY_SIZE; // Increment the index of the circular array
get_values(&data[index].tp, &data[index].tf, &data[index].tt); // "Record" the values
}
break;
}
}
// Print the finished recording
printIt(data);
}
// Return: void
// in - Values of the data_slice struct
//
// Description: The three values of the struct (data_slice) to be filled in
void get_values(float * pressure, float * flow, float * temp)
{
*pressure = (float)(rand() % (700 - 500 + 1) + 500); // Range: 500 - 700
*flow = (float)(rand() % (20 - 10 + 1) + 10); // Range: 10 - 20
*temp = (float)(rand() % (200 - 100 + 1) + 100); // Range: 100 - 200
}
// Return: void
// in - The array of data_slice
//
// Description: Prints the entire array being passed in
void printIt(data_slice * data)
{
// Find the indice holding the time value of -10
int indice = 0;
for (int i = 0; i < ARRAY_SIZE; ++i)
{
if (data[i].t == -10)
{
indice = i;
break;
}
}
for (int i = 0; i < ARRAY_SIZE; ++i)
{
printf("%i\t %f\t %f\t %f\n", data[indice].t, data[indice].tp, data[indice].tf, data[indice].tt);
indice = (indice + 1) % ARRAY_SIZE;
}
}
// Return: void
// in - The array of data_slice
//
// Description: Initializes the entire array to random values and their times to 0
void initializeArray(data_slice * data)
{
for (int i = 0; i < ARRAY_SIZE; ++i)
{
data[i].t = 0;
get_values(&data[i].tp, &data[i].tf, &data[i].tt);
}
}
// Return: boolean
// in - The array of data_slice
// out - Indice of the pressure spike
//
// Description: Returns true if a positive spike in pressure has been recorded.
// outputIndex will hold the 0-indice of the pressure spike, else -1
bool spikeValueRecorded(data_slice * data, int outputIndex)
{
float oldValue = data[0].tp;
for (int i = 0; i < ARRAY_SIZE; ++i)
{
if (data[i].tp - oldValue < MAX_CHANGE)
{
outputIndex = i;
return true;
}
}
outputIndex = -1;
return false;
}

Answer 1

You can use the fprintf call exactly like the printf call, with one difference. The first argument will be a pointer to a file handle (FILE*) which you create using a call to fopen("full or relative path","w").
The string format is now the second argument and the variable args list starts at arg 3

Answer 2

This is way you can write to file by using libc API
void printIt(data_slice * data)
{
// Find the indice holding the time value of -10
int indice = 0;
for (int i = 0; i < ARRAY_SIZE; ++i)
{
if (data[i].t == -10)
{
indice = i;
break;
}
}
//write to file
FILE *fp = fopen("output.txt", "wb"); //Binary mode since the size of array is fixed.
fwrite(data, sizeof(char), sizeof(data), fp);
fclose(fp);
for (int i = 0; i < ARRAY_SIZE; ++i)
{
printf("%i\t %f\t %f\t %f\n", data[indice].t, data[indice].tp, data[indice].tf, data[indice].tt);
indice = (indice + 1) % ARRAY_SIZE;
}
}