how to read input string until a blank line in C? - c

Question

first of all i'm new to coding in C.
I tried to read a string of unknowns size from the user until a blank line is given and then save it to a file, and after that to read the file.
I've only managed to do it until a new line is given and I don't know how to look for a blank line.
#include <stdio.h>
#include <stdlib.h>
char *input(FILE* fp, size_t size) {
char *str;
int ch;
size_t len = 0;
str = realloc(NULL, sizeof(char)*size);
if (!str)return str;
while (EOF != (ch = fgetc(fp)) && ch != '\n') {
str[len++] = ch;
if (len == size) {
str = realloc(str, sizeof(char)*(size += 16));
if (!str)return str;
}
}
str[len++] = '\0';
return realloc(str, sizeof(char)*len);
}
int main(int argc, const char * argv[]) {
char *istr;
printf("input string : ");
istr = input(stdin, 10);
//write to file
FILE *fp;
fp = fopen("1.txt", "w+");
fprintf(fp, istr);
fclose(fp);
//read file
char c;
fp = fopen("1.txt", "r");
while ((c = fgetc(fp)) != EOF) {
printf("%c", c);
}
printf("\n");
fclose(fp);
free(istr);
return 0;
}
Thanks!

Answer 1

I would restructure your code a little. I would change your input() function to be a function (readline()?) that reads a single line. In main() I would loop reading line by line via readline().
If the line is empty (only has a newline -- use strcmp(istr, "\n")), then free the pointer, and exit the loop. Otherwise write the line to the file and free the pointer.
If your concept of an empty line includes " \n" (prefixed spaces), then write a function is_only_spaces() that returns a true value for a string that looks like that.
While you could handle the empty line in input(), there is value in abstracting the line reading from the input termination conditions.

Answer 2

Why not use a flag or a counter. For a counter you could simply increase the counter each character found. If a new line is found and the counter is 0 it must be a blank line. If a new line character is found and the counter is not 0, it must be the end of the line so reset the counter to 0 and continue.
Something like this:
int count = 0;
while ((ch = fgetc(fp)) != EOF)
{
if(ch == '\n')
{
if(count == 0)
{
break;
}
count = 0;
str[len++] = ch;
}
else
{
str[len++] = ch;
ch++;
}
}
Another way would be to simply check if the last character in the string was a new line.
while ((ch = fgetc(fp)) != EOF)
{
if(ch == '\n' && str[len - 1] == '\n')
{
break;
}
}

Answer 3

A blank line is a line which contains only a newline, right ? So you can simply keep the last 2 characters you read. If they are '\n', then you have detected a blank line : the first '\n' is the end of the previous line, the second one is the end of the current line (which is a blank line).
char *input(FILE* fp, size_t size) {
char *str;
int ch, prev_ch;
size_t len = 0;
str = realloc(NULL, sizeof(char)*size);
if (!str)return str;
while (EOF != (ch = fgetc(fp)) && (ch != '\n' && prev_ch != '\n')) {
str[len++] = ch;
if (len == size) {
str = realloc(str, sizeof(char)*(size += 16));
if (!str)return str;
}
prev_ch = ch;
}
str[len++] = '\0';
return realloc(str, sizeof(char)*len);
}
Note that parenthesis around ch != '\n' && prev_ch != '\n' are here to make the condition more understandable.
To improve this, you can keep your function that reads only a line and test if the line returned is empty (it contains only a '\n').