Program that prints specific characters from a line in a file - c

Question

This is a programming problem i stumbled upon. It requires some specific stuff to be done and that is the reason that i copy files etc. The problem is that it requires that i print out the last character from onoma, the first character from epithetoand the product of the four digits of afm. The thing is that it must be done by reading the file. My code is the following:
#include <stdio.h>
main()
{
FILE *fp, *fp2;
char ch;
char onoma[12];
char epitheto[25];
char afm[4];
char adt[8];
fp = fopen("C:\\Users\\kostikas\\Desktop\\New folder\\myfile.txt", "w");
printf("Dose onoma: \n");
scanf("%s", &onoma);
printf("Dose epitheto: \n");
scanf("%s", &epitheto);
printf("Dose afm: \n");
scanf("%s", &afm);
printf("Dose adt: \n");
scanf("%s", &adt);
fprintf(fp, "%s \n%s \n%s \n%s \n", onoma, epitheto, afm, adt);
fclose(fp);
fp = fopen("C:\\Users\\kostikas\\Desktop\\New folder\\myfile.txt", "r");
fp2 = fopen("C:\\Users\\kostikas\\Desktop\\New folder\\myfile2.txt", "w");
while((ch = fgetc(fp)) != EOF)
{
fputc(ch, fp2);
}
fclose(fp);
fclose(fp2);
fp2 = fopen("C:\\Users\\kostikas\\Desktop\\New folder\\myfile2.txt", "r");
}
I have absolutely no idea on how to continue from here. I'm utterly baffled. If my post is against community guidelines please let me know and i will delete it. Your help would be greatly appreciated though. Thank you.

Answer 1

This is a bunch of tasks smashed together, any one of which is a challenge to a beginner. Let's break this into parts.
The problem is that it requires that i
print the last character from onoma
print the first character from eponimo
print the product of the four digits of afm.
We'll focus on those one at a time.
The thing is that it must be done by reading the file.
And leave this for later, it just complicates everything. First get the logic working with hard coded values, then worry about where the input comes from.
print the first character from eponimo
Each character can be accessed with eponimo[n] starting at 0. First character is eponimo[0], then eponimo[1], and so on. Printing the first character is just...
char eponimo[] = "Basset hounds got long ears";
printf("%c\n", eponimo[0]);
We use %c, not %s, because it is a single character, not a string which is a pointer to an array of characters.
print the last character from onoma
The last character would be the length of the string, minus 1 because it starts from 0. You get the length of a string with strlen.
char onoma[] = "Oodles of poodles jump der strudel";
size_t onoma_length = strlen(onoma);
printf("%c\n", onoma[onoma_length-1]);
print the product of the four digits of afm.
This is a little trickier. It requires math, loops, and converting characters to integers.
First, check that afm is long enough, again using strlen.
if( strlen(afm) < 4 ) {
fprintf(stderr, "afm must have at least four digits\n");
exit(1);
}
You can also check if the characters are digits with isdigit.
Then we can loop through the first four characters.
char afm[] = "123456";
for( int i = 0; i < 4; i++ ) {
printf("%c\n", afm[i]);
}
Now we need to turn them into integers. Whole strings are done with atoi (ASCII to Integer), but we want single characters. The thing about characters in C is they're just integers and we can do math on them. 0 is 48, 1 is 49, and so on. We take advantage that all the integers are in a sequence.
char afm[] = "123456";
for( int i = 0; i < 4; i++ ) {
// '0' - '0' is 0.
int digit = afm[i] - '0';
printf("%d\n", digit);
}
Note that I've switched to %d for printing an integer (a "digit").
Now we multiply them.
char afm[] = "123456";
int product = 1; // start with 1, not 0, else it will always be 0
for( int i = 0; i < 4; i++ ) {
// '0' - '0' is 0.
int digit = afm[i] - '0';
product *= digit; // same as product = (product * digit)
}
printf("%d\n", product);
Now that you know how to do it with fixed strings, you can substitute what you've read from the file. First, open it for reading and check that it worked. This check will save you a lot of misery.
char filename[] = "C:\\Users\\kostikas\\Desktop\\New folder\\myfile.txt";
fp = fopen(filename, "r"); // open for reading
if( fp == NULL ) {
perror("Could not open input file");
}
Now allocate memory and read lines. scanf reads from "standard input", roughly what you tyope into the program. fscanf reads from a filehandle.
// Allocate space for 10 characters.
// Read only 9 because strings need an extra null character to indicate the end.
char onoma[10];
fscanf(fp, "%9s", onoma);
char eponimo[10];
fscanf(fp, "%9s", eponimo);
char afm[10];
fscanf(fp, "%9s", afm);
// We're done reading, close the file.
fclose(fp);
However, scanf and fscanf have a lot of surprising behaviors. And we need to preallocate our best guess of how much we're going to read and be careful not to read in more than we've allocated.
If at all possible, use getline instead. Most compilers support it. It will allocate memory for you. It doesn't have the caveats of fscanf, but it also doesn't strip the newline off the end like fscanf does.
size_t size = 0;
char *onoma = NULL;
getline(&onoma, &size, fp);
Writing is easier. Open the file for writing, you know how to do that, and use fprintf instead of printf, like fscanf instead of scanf.
fprintf(fp, "%c\n", eponimo[0]);
That should get you going. This exercise is throwing multiple problems at a beginner. Break them up into parts and tackle them one at a time.