Can't break a while loop. Reading undefined array size - c

Question

I have this sequence of letters and numbers, in which the letters are always these four: s, S, m, M. The numbers can have any value. Since the size of the sequence is not given, I just can't use a for loop, so I decided to use a while loop, but I'm having issues on breaking the loop.
Some input examples are:
12 s 80 s 3 m 12 M 240 S 8 m 30 s 240 s 1440 S 8 m 18 s 60 M
5 m 120 s 30 s 360 S 6 M 5 s 42 S 36 M 8 m 66 M 3240 S 14 m
Here is my code so far:
#include <stdio.h>
int main () {
int n[100], i = 0;
char x[100];
while(x[i] != '\n')
{
scanf(" %d %c", &n[i], &x[i]);
printf("%d %c ", n[i], x[i]);
i++;
}
return 0;
}
Any toughts on how to break the loop, and have all this values saved correctly on the array?

Answer 1

like this
#include <stdio.h>
int main(void){
int n[100], i, j;
char x[100];
do {
for(i = 0; i < 100; ++i){
int ch;
while((ch = getchar()) != EOF && ch != '\n'){//check newline
if(ch == '-' || '0' <= ch && ch <= '9'){
ungetc(ch, stdin);//back to stream a character
break;
}
}
if(ch == EOF || ch == '\n')
break;
if(2 != scanf("%d %c", &n[i], &x[i])){
fprintf(stderr, "invalid format.\n");
i = 0;//End the outer do-while loop
break;
}
}
//print
for(j = 0; j < i; ++j){
printf("(%d, %c)", n[j], x[j]);
}
printf("\n");
} while(i != 0);//End with empty line
}
#include <stdio.h>
#define DATA_MAX_LEN 100
int main(void){
int n[DATA_MAX_LEN], i, len, loop_end;
char x[DATA_MAX_LEN], newline[2], ch;
while(scanf("%1[\n]", newline) != 1){//End with empty line(only newline), Need EOF check
for(loop_end = len = i = 0; i < DATA_MAX_LEN && !loop_end; ++i){
//format: integer character[space|newline]
if(scanf("%d %c%c", &n[i], &x[i], &ch) != 3)
loop_end = printf("invalid format.\n");
else if(ch == '\n')
loop_end = len = ++i;
else if(ch != ' ')
loop_end = printf("invalid format.\n");
}
for(i = 0; i < len; ++i){
printf("%d %c ", n[i], x[i]);
}
printf("\n");
}
}

Answer 2

scanf and fscanf have many problems so it's best to avoid them.
The general pattern for dealing with input is to create a large input buffer, and then process that into smaller chunks.
char line[4096];
fgets( line, sizeof(line), stdin );
Since line is reused it's ok to make it large enough to hold any reasonable input.
Now that you've read a line into memory, it's a string of known size to process as you like. sscanf (scanf on a string) doesn't have most of the problems of scanf, but it's also not suited to moving through a string. One approach is to split the string into tokens on whitespace with strtok, and process them alternately as letters and numbers.
const char sep[] = " \t\n";
bool expect_letter = false;
for(
char *token = strtok( line, sep );
token != NULL;
token = strtok( NULL, sep )
) {
if( expect_letter ) {
printf("Letter %s\n", token);
expect_letter = false;
}
else {
printf("Number %s\n", token);
expect_letter = true;
}
}
If you want to store them in an array, it's bad practice to allocate what you hope is enough memory. You'll have to use an array that grows as needed. C does not have these built in. You can write your own, and it's a good exercise, but it's easy to get wrong. For production use one from a library such as Glib.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <glib.h>
int main() {
// Read a single line of input into a buffer.
char line[4096];
fgets( line, sizeof(line), stdin );
// Create arrays to hold letters and numbers, these will grow as needed.
GArray *numbers = g_array_new( FALSE, TRUE, sizeof(int) );
GArray *letters = g_array_new( FALSE, TRUE, sizeof(char) );
// Split the string on whitespace into tokens.
const char sep[] = " \t\n";
gboolean expect_letter = FALSE;
for(
char *token = strtok( line, sep );
token != NULL;
token = strtok( NULL, sep )
) {
if( expect_letter ) {
// Need an error check to ensure that `token` is a single character.
g_array_append_val( letters, token[0] );
expect_letter = FALSE;
}
else {
// strtol is a better choice, it has error checking
int num = atoi(token);
g_array_append_val( numbers, num );
expect_letter = TRUE;
}
}
// Print the numbers and letters.
for( guint i = 0; i < letters->len; i++ ) {
printf(
"%d%c\n",
g_array_index( numbers, int, i ),
g_array_index( letters, char, i )
);
}
}
Note that GLib provides its own boolean, so I switched to that instead of stdbool to keep things consistent.
As noted in the comments, this does not include checks that the token is what you expect. It's also possible to have a number with no letter, so checking that letters and numbers are the same size would be good. Or you can make a struct to hold the letter/number pairs and have a single list of those structs.