Hex calculator , parsing string using scanf [duplicate] - c

Question

This question already has answers here:
scanning a string to hex char array
(3 answers)
Closed 8 years ago.
Here is my Code :
char a[18], b[18];
char oper, clear;
char *test;
init_8051();
test="0x1234567890123456 + 0x1234567890123456\0";
printf("Please enter an equation: %s \n",test );
sscanf(test,"0x%s %c 0x%s",a,&oper,b);
printf(" a= %s \n" ,a);
printf(" oper= %s \n" ,oper);
printf(" b= %s \n" ,b);
I want to accept to hex numbers with an operation as a string and to be able to seperate those 2 numbers into 2 separate char arrays but it doesnt wanna work, here is the output of the following code :
Please enter an equation: 0x1234567890123456 + 0x1234567890123456
a= 1234567890123456
oper= Ò
b= 1234567890123456
As you can see the operation is not recognized and also i have to use spaces which i wish i didnt have to use i wish it to be in the format of 0x1234567890123456+0x1234567890123456
with no spaces between the plus and the number.
Thanks

Answer 1

From the sscanf manual
s Matches a sequence of non-white-space characters; the next pointer must be a pointer to character array that is long enough to hold the input sequence and the
terminating null byte ('\0'), which is added automatically. The input string stops at white space or at the maximum field width, whichever occurs first.
It means that %s consumes the + and the rest of the characters, leaving b and oper uninitalized, abd overflowing a since it only has space for 18 characters.
So when the input string is lacking the space after the first operand, sscanf will continue reading until it finds a whitespace character. Hence when the string does not contain the separating space between the operands and the operator, sscanf consumes all the input.
I'll append here a different approach to your problems solution
We copy the string, this is required by strtok you can't pass an inmutable string, there are plenty of methods to copy this string, you just have to pick the appropriate one for your case
input = strdup("0x1234567890123456 + 0x1234567890123456\0");
Now, we use strpbrk to find the operator
pointer = strpbrk(input, "+-*/" /* here go the operators */);
if (pointer != NULL)
oper = *pointer; /* this will contain the operator ascii value */
Create a string containing the operator as a delimiter
operstr[0] = oper;
operstr[1] = '\0'; /* strings must be null terminated */
Now, we use strtok to tokenize the string, and find the operands
pointer = strtok(input, operstr);
if (pointer != NULL)
fprintf(stderr, "first operand: %s\n", pointer); /* you can copy this string if you need to */
printf("Operator: %s \n", operstr);
Second call to strtok needs NULL first argument
pointer = strtok(NULL, operstr);
if (pointer != NULL)
fprintf(stderr, "second operand: %s\n", pointer); /* you can copy this string if you need to */
And finally free our copy of the input string.
free(input);
It is better to use strtok_r the reentrant version. But for now you could test my suggestions and may be, it is what you need.
Even though this will work for this particular situation it is not the preferred way of doing this kind of thing, you can try writing a parser and use Reverse Polish Notation, or you can try with a lexical analyzer and a parser generator like flex and bison.

Answer 2

My previous answer was downvoted, and didn't address all of OP's requirements, so I have rewritten this answer.
OP wants flexible input, either spaces or no spaces. I suggest not using sscanf() but the methods below. First the program finds a valid operator by using strcspn(), then breaks the string using strtok() on operators and whitespace. But using strtok() on a string literal is UB so I copy the "equation" to another string first.
I also corrected the printf() field spec for the operator, and made a and b different - it's always a bad idea using the same values for different variables in an example.
#include <stdio.h>
#include <string.h>
#define OPERATORS "+-*/"
#define DELIMS " \t\n" OPERATORS
int parse (char *test)
// return 1 if parsed successfully
{
char a[50], b[50];
char oper;
char *ptr;
int opind;
opind = strcspn (test, OPERATORS); // find operator
if (opind < 1) return 0; // fail
oper = test[opind]; // collect operator
ptr = strtok (test, DELIMS); // find a
if (ptr == NULL) return 0; // fail
strcpy (a, ptr); // collect 1st arg
ptr = strtok (NULL, DELIMS); // find b
if (ptr == NULL) return 0; // fail
strcpy (b, ptr); // collect 2nd arg
printf(" a %s \n" ,a);
printf(" oper %c \n" ,oper); // corrected format
printf(" b %s \n" ,b);
return 1;
}
int main (void)
{
char test[100];
strcpy (test, "0x123456789ABCDEF0+0xFEDCBA9876543210");
if (!parse (test))
printf("Failed\n");
printf("\n");
strcpy (test, "0x123456789ABCDEF0 + 0xFEDCBA9876543210");
if (!parse (test))
printf("Failed\n");
return 0;
}
Program output
a 0x123456789ABCDEF0
oper +
b 0xFEDCBA9876543210
a 0x123456789ABCDEF0
oper +
b 0xFEDCBA9876543210