Question

Quick question, What have I done wrong here. The purpose of this code is to get the input into a string, the input being “12 34”, with a space in between the “12” and “32” and to convert and print the two separate numbers from an integer variable known as number. Why doesn’t the second call to the function copyTemp, not produce the value 34?. I have an index_counter variable which keeps track of the string index and its meant to skip the ‘space’ character?? what have i done wrong?

thanks.

#include <stdio.h>
#include <string.h>
int index_counter = 0;
int number;
void copyTemp(char *expr,char *temp);

int main(){
 char exprstn[80]; //as global?
 char tempstr[80];

 gets(exprstn);
 copyTemp(exprstn,tempstr);
 printf("Expression: %s\n",exprstn);
 printf("Temporary: %s\n",tempstr);
 printf("number is: %d\n",number);
 copyTemp(exprstn,tempstr);      //second call produces same output shouldnt it now produce 34 in the variable number?
 printf("Expression: %s\n",exprstn);
 printf("Temporary: %s\n",tempstr);
 printf("number is: %d\n",number);

 return 0;
}
void copyTemp(char *expr,char *temp){
 int i;
 for(i = index_counter; expr[i] != '\0'; i++){
  if (expr[i] == '0'){
   temp[i] = expr[i];
  }
  if (expr[i] == '1'){
   temp[i] = expr[i];
  }
  if (expr[i] == '2'){
   temp[i] = expr[i];
  }
  if (expr[i] == '3'){
   temp[i] = expr[i];
  }
  if (expr[i] == '4'){
   temp[i] = expr[i];
  }
  if (expr[i] == '5'){
   temp[i] = expr[i];
  }
  if (expr[i] == '6'){
   temp[i] = expr[i];
  }
  if (expr[i] == '7'){
   temp[i] = expr[i];
  }
  if (expr[i] == '8'){
   temp[i] = expr[i];
  }
  if (expr[i] == '9'){
   temp[i] = expr[i];
  }
  if (expr[i] == ' '){ 
   temp[i] = '\0';
   sscanf(temp,"%d",&number); 
   index_counter = i+1; //skips?
  }
 }
 // is this included here? temp[i] = '\0'; 
}

Answer 1

There are a few problems in your program:

• You are using the same index into
  expr and temp arrays. This works for
  the first time since both will be 0
  to start with but when you want to
  process the 2nd number, you need to
  reset the index into the temp array
  back to 0. Clearly this cannot be
  done using a single index. You’ll
  have to use two indices, i and j.
• By the time you complete the
  processing of the 2nd number ( 34 in
  "12 34") you’ll reach the end of the
  string and hence the sscanf never
  gets run on the second occasion ( in
  general for the last occasion). So
  after the for loop you need another
  sscanf to extract the last number. Also you should return from the function once you’ve extracted the number from the string and incremented i.
• You should avoid using gets() and use
  fgets() instead because of security
  reasons.
• You can combine the multiple test for
  the digits into a single test as
  shown:

Something like this.

void copyTemp(char *expr,char *temp){
    int i;
    int j = 0;
    for(i = index_counter; expr[i] != '\0'; i++){

        if (expr[i] >= '0' && expr[i]<='9'){
            temp[j++] = expr[i]; // copy the digit into temp..increment j.
        }    
        else if (expr[i] == ' '){ // space found..time to extract number.
            temp[j] = '\0'; // terminate the temp.
            sscanf(temp,"%d",&number); // extract.
            index_counter = i+1; // skip the space.
                    return; // done converting...return..must not continue.
        }
    }
    // have reached the end of the input string..and still need to extract a 
    // the last number from temp string.
    temp[j] = '\0';
    sscanf(temp,"%d",&number);
}

After these changes it works as expected:

$ gcc b.c 2> /dev/null && ./a.out
12 34
Expression: 12 34
Temporary: 12
number is: 12
Expression: 12 34
Temporary: 34
number is: 34

Your approach is very fragile…if a user gives multiple spaces between the input numbers..your program will fail.