Odtnhj

I need to write a function in C that converts a string (data pointed to by a char *) to an int. I've already done this successfully with this code:

int charTOint(char * c) 
 char p = *c;
 int ergebnis = 0;

 while (p) 
 ergebnis = ergebnis * 10 + (p - '0');
 c++;
 p = *c;
 

 return ergebnis;

My problem now is, that I need it to also work with negative numbers / char arrays starting with a '-'.

I know that I have to check, whether the first char is a '-', but I'm stuck after that.

I know the '-' is the 45th character of the ASCII table, but I somehow can't think of a way to make it work.

I've already done this successfully (with positive) with this code:

Good. An important insight is to build on success.

As int charTOint(char * c) { works well with positive values, perhaps re-write it using unsigned. We get more range as UINT_MAX is typically greater than INT_MAX.

unsigned charTOunsigned(const char * c) 
 char p = *c;
 unsigned ergebnis = 0;
 while (p) 
 ergebnis = ergebnis * 10 + (p - '0');
 c++;
 p = *c;
 
 return ergebnis;

I need it to also work with negative numbers / char arrays starting with a '-'.

Now armed with charTOunsigned(), use that variation of good existing code to re-make a int charTOint() that meets the additional goal. With the usual extra positive range of charTOunsigned(), int charTOint() will readily handle a string that converts to INT_MIN.

int charTOint(const char * c) 
 return (*c == '-') ? -charTOunsigned(c+ 1) : charTOunsigned(c);

Certainly one could code a stand-alone charTOint(), yet I wanted to emphasize code re-use. That makes for a productive coder.

Another variable could be added to recognize the sign. Then multiply by the sign variable.

A check should be added to confirm that only digits are processed. If the input was 123abc, this will stop after 123

int charTOint(char * c) '+' == *c) 
 if ( '-' == *c) 
 sign = -1;
 
 c++;
 
 while (*c) 
 p = *c - '0';
 if ( 0 <= p && 9 >= p) // digit 0 to 9
 ergebnis = ergebnis * 10 + p;
 c++;
 
 else 
 break;//not a digit
 
 

 return ergebnis * sign;

EDIT 3:

#include <stdio.h>
#include <string.h>
#include <limits.h>

int charTOint(char *c, int *number) 

int main ( void) 
 char text[100] = "";
 int value = 0;

 do 
 printf ( "enter an integer or enter donen");
 if ( fgets ( text, sizeof text, stdin)) 
 text[strcspn ( text, "n")] = 0;
 if ( charTOint ( text, &value)) 
 printf ( "value = %dn", value);
 
 else 
 printf ( "input [%s]n", text);
 
 
 else 
 fprintf ( stderr, "fgets EOFn");
 return 0;
 
 while ( strcmp ( text, "done"));
 return 0;