Extract integers from a string in C - c

Question

I'm trying to extract all numbers from a string add 56 89 29. There is something wrong with my code which I'm not able to figure out. The output looks like this:
Current no: 56
Current no: 89
Current no: 9
Current no: 29
Current no: 9
Current no: 9
Current no: 0
Current no: 0
where as, I would expect this:
Current no: 56
Current no: 89
Current no: 29
Current no: 29
Current no: 29
Current no: 29
There are better ways to do it but I want to know what is wrong with this implementation.
#include <string.h>
#include <stdio.h>
int main(int argc, char * argv[]){
char temp[100];
char op[3];
char *buf = "add 56 89 29";
int offset = 0;
int rcount = 0;
int lastno = -999;
int currno;
bzero(&temp, sizeof(temp));
sscanf(buf, "%s", op); // Get the operator (add)
if(!strcmp(op, "add")){
while(1){
sscanf(buf + offset + 4, "%s", temp); // strlen("add ") == 4
currno = atoi(temp);
offset += strlen(temp);
if(currno == lastno){
rcount++;
// Repeating numbers means string finished
if(rcount == 3){
break;
}
}
printf("Current no: %d\tOffset: %d\n", currno, offset + 4);
lastno = currno;
}
}
return 0;
}
Thanks in advance!

Answer 1

The "%n" specifier will record the number of characters used by a scan. The result can be added to the offset to advance through the string.
#include <string.h>
#include <stdio.h>
int main(int argc, char * argv[]){
char temp[100];
char op[4];
char *buf = "add 56 89 29";
int offset = 0;
int used = 0;
int rcount = 0;
int lastno = -999;
int currno;
bzero(&temp, sizeof(temp));
sscanf(buf, "%3s%n", op, &used); // Get the operator (add)
offset += used;
if(!strcmp(op, "add")){
while( ( sscanf(buf + offset, "%99s%n", temp, &used)) == 1) {
currno = atoi(temp);
offset += used;
printf("Current no: %d\tOffset: %d\n", currno, offset);
lastno = currno;
}
}
return 0;
}

Answer 2

There are a few things I can see that would cause your program to fail
It's unecessary to write a loop like while (1) and then check for the variable inside the loop to break, because there is do {} while ();, this makes the code hard to understand.
This initialization bzero(&temp, sizeof(temp)); is wrong, unecessary, and inefficient. And also, bzero() is deprecated use memset().
You have a very important mistake in scanf("%s", op); you allocate space for 3 characters and type add which requires 3 + 1 for the nul terminator. The correct way to do it would be
char op[4];
if (sscanf(buf, "%3s", op) != 1)
{
fprintf("unexpected problem while reading the operator.\n");
return -1;
}
sscanf(buf + offset + 4, "%s", temp) is very dangerous too, because you could easily overflow buf or temp, and besides it's much more readable like this
sscanf(&buf[offset + 4], "%s", temp)
This implementation would work better
#include <string.h>
#include <stdio.h>
int main(int argc, char * argv[])
{
char op[4];
const char *buffer;
int count;
int last;
int length;
int offset;
const char *pointer;
buffer = "add 56 89 29";
if (sscanf(buffer, "%3s%n", op, &length) != 1)
{
fprintf(stderr, "unexpected problem while reading the operator.\n");
return -1;
}
pointer = &buffer[length];
if (strcmp(op, "add") != 0)
{
fprintf(stderr, "sorry, I can't understand this operator `%s'.\n", op);
return -1;
}
count = 0;
last = -999;
offset = length;
do {
int number;
int length;
if (sscanf(pointer, "%d%n", &number, &length) != 1)
{
fprintf(stderr, "input format error, exiting.\n");
return -1;
}
pointer += length;
if (number == last)
++count;
printf("Current no: %d\tOffset: %d\n", number, offset);
last = number;
offset += length;
} while ((count < 3) && (*pointer != '\0'));
return 0;
}
read about the scanf() "%n" specifier.
You can also use strtol() like this
#include <string.h>
#include <stdio.h>
int main(int argc, char * argv[])
{
char op[4];
char buffer[] = "add 56 89 29";
int count;
int last;
int length;
int offset;
char *pointer;
if (sscanf(buffer, "%3s%n", op, &length) != 1)
{
fprintf(stderr, "unexpected problem while reading the operator.\n");
return -1;
}
pointer = &buffer[length];
if (strcmp(op, "add") != 0)
{
fprintf(stderr, "sorry, I can't understand this operator `%s'.\n", op);
return -1;
}
count = 0;
last = -999;
offset = length;
do {
int number;
char *previous;
previous = pointer;
number = strtol(pointer, &pointer, 10);
if (number == last)
++count;
printf("Current no: %d\tOffset: %d\n", number, offset);
last = number;
offset += pointer - previous;
} while ((count < 3) && (*pointer != '\0'));
return 0;
}

Answer 3

Hi I tried your code and the following worked for me. I am not a hardcore C programmer so please forgive me if I am used any non standard C functions here and but I love to work in C.
int main(int argc, const char * argv[])
{
char *buf = "add 56 89 29";
int sum = 0;
if(!strncmp(buf, "add", 3))
{
buf += 3;
while(*buf != '\0')
{
if(isdigit(*buf) == 0)
{
++buf;
continue;
}
int digit;
int num = 0;
while(isdigit(*buf))
{
digit = (int)*buf - 48;
num = (num * 10) + digit;
++buf;
}
sum += num;
printf("Current no: %d\t Sum: %d\n", num, sum);
}
}
return 0;
}

Answer 4

A much better and convenient way of doing this is with strtok which you can use to get tokens one after the other.
#include <stdio.h>
#include <string.h>
int main(int argc, char * argv[]){
char *buf = "add 56 89 29";
char *token;
int count = 0;
token = strtok(buf, " ");
while(token != NULL){
count++;
if(count == 1)
continue;
printf("%s\n", token)
token = strtok(NULL, " ");
}
}