C scanf - unknown array size - c

Question

I want to read values (float) into an array but I don't know number of values.
My input is this
Enter values: 1.24 4.25 1.87 3.45 .... etc
How can I load this input to an array? I know that input ends when enterring 0 or EOF.
while(0 or EOF){
scanf("%f", &variable[i])
i++;
}
Thank you.

Answer 1

You can dynamically allocate the array and then reallocate the memory for it when the previously allocated buffer is full. Note that the conversion specifier %f in the format string of scanf reads and discards the leading whitespace characters. From the man page of scanf -
scanf returns the number of items successfully matched and assigned
which can be fewer than provided for, or even zero in the event of an
early matching failure. The value EOF is returned if the end of input
is reached before either the first successful conversion or a matching
failure occurs.
This means that scanf will return EOF only when it encounters EOF as the first input when it is called because EOF must be preceded with a newline '\n' else it won't work (depending on the OS). Here's a small program to demonstrate how you can do it.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
size_t len = 4;
float *buf = malloc(len * sizeof *buf);
if(buf == NULL) { // check for NULL
printf("Not enough memory to allocate.\n");
return 1;
}
size_t i = 0;
float *temp; // to save buf in case realloc fails
// read until EOF or matching failure occurs
// signal the end of input(EOF) by pressing Ctrl+D on *nix
// and Ctrl+Z on Windows systems
while(scanf("%f", buf+i) == 1) {
i++;
if(i == len) { // buf is full
temp = buf;
len *= 2;
buf = realloc(buf, len * sizeof *buf); // reallocate buf
if(buf == NULL) {
printf("Not enough memory to reallocate.\n");
buf = temp;
break;
}
}
}
if(i == 0) {
printf("No input read\n");
return 1;
}
// process buf
for(size_t j = 0; j < i; j++) {
printf("%.2f ", buf[j]);
// do stuff with buff[j]
}
free(buf);
buf = NULL;
return 0;
}

Answer 2

I guess your actual concern is the unknown number of floats that user is going to input. You can use pointer to float, do malloc for some predefined size, if your limit has reached while taking an input then do a realloc to increase the memory. You need to take care of previously accepted data while doing a reaccloc.

Answer 3

You need a dynamic allocation of your array since you don't know its size at compile time.
// INITIAL_SIZE can be the average expected size of your array
#define INITIAL_SIZE 4
// size will track the current maximum size of youe array.
size_t size = INITIAL_SIZE;
// dynamic allocation for variable
float* variable = malloc(sizeof(float)*size);
// check that the allocation happened correctly
assert(variable != NULL);
// i contains the current actual size of your array
int i = 0;
while (0 or EOF) {
if (i >= size) {
// if the array is getting bigger than its max size, resize it.
size *= 2;
// This will reallocate enough memory for variable.
variable = realloc(variable, sizeof(float)*size);
// check that the allocation happened correctly;
assert(variable != NULL);
// (NB: It IS important to affect variable to the result of
// realloc, you can't simply realloc as in some cases the
// original pointer will become invalid!)
}
scanf("%f", &variable[i])
i++;
}
As an aside, please note that variable is not a wonderful variable name. Use a name describing what your variable is used for.
EDIT: corrected the size in realloc to alloc size*2 floats and avoid it break horribly, as unwind pointed out.

Answer 4

i = 0;
while(scanf("%f", &variable[i])!=-1)
{
i++;
}
scanf returns -1 when it tries to read after EOF.