Using sscanf() function to read a string

Question

I'm a begginer so I'm stuck in this part. I need to type a message, and a shift amount by which letters should be shifted to 'encrypt' a message. The problem is that It's not displaying any text, it just never exits the while loop.

Any help would be greatly appreciated.

#include <stdio.h>
#include <stdlib.h>

int main(void)
{

   char ch,message[50]={0};
   int shift;


   printf("Enter message to be encrypted: ");
   scanf("%s",message);
   printf("Enter shift amount (1-25): ");
   scanf("%d",&shift);

   printf("Encrypted message: ");

   while((sscanf(message," %c",&ch) == 1) && (ch != '\n'));
   {
       ch += shift;
       putchar(ch);
   }

    return 0;
}

Output:

Enter message to be encrypted: abcABC
Enter shift amount (1-25): 3
Encrypted message:

(Program is stucked there in an infite loop)

Also take a look at cplusplus.com/reference/cstdio/sscanf you are always reading only the first character form the message — Coldsteel48
– Coldsteel48, Commented Sep 25, 2016 at 4:29
But since your message has a defined size I would suggest to use a for loop for this -> it will be much safer — Coldsteel48
– Coldsteel48, Commented Sep 25, 2016 at 4:33
when I add message++ it's giving me a warning ' lvalue required as increment operand' — tadm123
– tadm123, Commented Sep 25, 2016 at 4:34

Coldsteel48 · Accepted Answer · 2016-09-25 05:01:40Z

1

There is couple of things with your code a) you have ";" at the end of your while b) you always reading only the first character inside the sscanf thats why you are in the infinite loop. I would suggest to replace while with the for loop like shown below.

scanf automagically appends'\0' to the end of the string.

int i;
for(i=0 ; i<50 && mesage[i] != '\0'; i++) 
{ 
    ch = message[i]; 
    ch += shift; 
    putchar(ch); 
}

edited Sep 25, 2016 at 5:01

answered Sep 25, 2016 at 4:41

Coldsteel48

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1 Comment

tadm123 Over a year ago

I'm getting the encrypted message but with lots of characters after it.

Ravi Shankar Bharti · Accepted Answer · 2016-09-25 04:37:30Z

1

You are wrongly using sscanf. It will always read only the first character.

Try this to achieve what you want.

int i=0;
while(i<strlen(message))
{
    ch=message[i++];
    ch += shift;
    putchar(ch);
}

answered Sep 25, 2016 at 4:37

Ravi Shankar Bharti

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6 Comments

tadm123 Over a year ago

I might be mistaken, but is there any reason why you can use 'fscanf' to go through each character on a file but not 'sscanf', which is suppose to do the same thing but for a string?

Coldsteel48 Over a year ago

@tadm123 fsacnf is sued to work with FILES see reference cplusplus.com/reference/cstdio/fscanf .

tadm123 Over a year ago

I know, that's what I'm saying @user3351949. I thought that using sscanf for a string was equivalent to using fscanf for a file.

Ravi Shankar Bharti Over a year ago

sscanf is basically used to retrieve information from a string in a specific format. See use of sscanf here : cplusplus.com/reference/cstdio/sscanf

Coldsteel48 Over a year ago

@tadm123 Well it is not, when reading from file, it has a pointer to the place you are reading/writing (don't remember it's name it) and it increments that pointer after you are reading. for string it doesn't have that pointer you should manage it your self.

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