Thread: urgent help needed with itoa

Below is the reverse function logic.

Code:

void reverse(char output[])
 {
     int i, j;
     char c;
 
     for (i = 0, j = strlen(output)-1; i<j; i++, j--) {
         c = output[i];
         output[i] = s[j];
         output[j] = c;
     }
 }

I changed code logic as below, but getting compile errors. what is wrong here?

Code:

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

void itoa (const int input, char* output, int rad){
int i = 0, n,sign;
n = sign;

while (n > 0)
{
   sign = n%rad;
   n = n/rad;
   output[i++] = '0' + sign;
}
output[i] = '\0';
strrev(output);
}


int main (){
char output[13];
itoa(726, output, 13);
printf("\n %s  \n", output);
return 0;

}
main.c:24:1: warning: implicit declaration of function ‘strrev’; did you mean ‘strsep’? [-Wimplicit-function-declaration]
   24 | strrev(output);
      | ^~~~~~
      | strsep
/usr/bin/ld: /tmp/ccFVXKei.o: in function `itoa':
main.c:(.text+0x79): undefined reference to `strrev'
collect2: error: ld returned 1 exit status

can anyone help me? i am stuck with this code.

Why "reverse" the buffer if you can fill it backwards?

Code:

#include <stdio.h>

static int digits10( unsigned int n )
{
  static const unsigned int vals[] =
    { 10U, 100U, 1000U, 10000U, 100000U, 1000000U,
      10000000U, 100000000U, 1000000000U };
  int i;

  if ( n >= 1000000000U ) return 10;

  i = 0;
  while ( i < sizeof vals / sizeof vals[0] )
    if ( n < vals[i++] ) 
      break;

  return i;
}

char *itoa_( int n, char *str )
{
  char *endp;
  unsigned int m;
  int signal;

  signal = n < 0;

  m = signal ? -n : n;

  // Points to the end of the string.
  endp = str + digits10( m ) + signal;

  *endp-- = '\0';
  do 
    *endp-- = '0' + m % 10;
  while ( m /= 10 );

  if ( signal ) *endp = '-';

  return str;
}

int main( void )
{
  char buff[12];

  printf( "%s\n", itoa_( -726, buff ) );
}

As per your logic, it output as -726. How can I used void function to implement itoa? I need to use below signature.

Code:

void itoa (const int input, char* output) {}

The only errors I saw on your original code were these:

Code:

$ gcc -Wall -Wextra xx.c
xx.c: In function ‘itoa’:
xx.c:5:7: error: assignment of read-only parameter ‘input’
 input = -input;
       ^
xx.c:9:16: error: assignment of read-only parameter ‘input’
 }while ((input /= 10) > 0);
                ^~
xx.c:13:1: warning: implicit declaration of function ‘reverse’ [-Wimplicit-function-declaration]
 reverse(output);
 ^~~~~~~

It seems like you need to not declare your input parameter as const, since you are modifying it. And you need to provide a definition or declaration of the reverse function.

Originally Posted by leo2008

As per your logic, it output as -726

As it should.

How can I used void function to implement itoa? I need to use below signature.

Code:

void itoa (const int input, char* output) {}

Simple. Change the code.

How to change code logic to handle negative to positive? I mean change -726 to +726. In your itoa function, something like this?

Code:

 
if((signal = n)<0)//record signal
n=-n;//make n a positive number

I have removed the const input and below is my code. please check it.

Code:

#include <stdio.h>
#include <stdbool.h>
#define true 1
#define false 0

 
void reverse(char output[], int len)
{
    int start, end;
    char temp;
    for(start=0, end=len-1; start < end; start++, end--) {
        temp = *(output+start);
        *(output+start) = *(output+end);
        *(output+end) = temp;
    }
}
 
void itoa (int input, char* output, int rad){
int i = 0;
bool isnegative = false;
 
if(input == 0) {
output[i] = '0';
output[i + 1] = '\0';
//return output;
}
 
if(input < 0 && rad == 10){
bool isnegative = true;
input = -input;
}
 
while (input != 0){
  int rem = input%rad;
  output[i++] = (rem > 9) ? (rem-10) + 'A' : rem + '0';
  input = input/rad;
}
 
if(isnegative){
output[i++] = '-';
}
 
output[i] = '\0';
reverse(output, i);
return output;
}
 
 
int main (){
 
int i, b;
char array[13];
printf("Enter a number and base\n");
scanf("%d %d", &i, &b);
      
//printf("String : %s", itoa(i, array, b));
//return 0;   
 
}

leo2008:

You also #include stdbool.h, but then redefine "true" and "false" which are already defined in stdbool.h!

Compile with all warnings turned on and see the results!

Have you actually tested the code yourself?

If I remove stdbool.h, I get the below errors.

Code:

main.c:28:1: error: unknown type name ‘bool’
   28 | bool isnegative = false;
      | ^~~~
main.c:37:1: error: unknown type name ‘bool’
   37 | bool isnegative = true;
      | ^~~~

I have modified the below code and tested output below. But it doesnt print any string values after providing number and base. Can someone help me understand what is wrong in this code?

Code:

#include <stdio.h>
#include <stdbool.h>
#define true 1
#define false 0

 
void reverse(char output[], int len)
{
    int start, end;
    char temp;
    for(start=0, end=len-1; start < end; start++, end--) {
        temp = *(output+start);
        *(output+start) = *(output+end);
        *(output+end) = temp;
    }
}
 
void itoa (int input, char* output, int rad){
int i = 0;
bool isnegative = false;
 
if(input == 0) {
output[i] = '0';
output[i + 1] = '\0';
//return output;
}
 
if(input < 0 && rad == 10){
bool isnegative = true;
input = -input;
}
 
while (input != 0){
  int rem = input%rad;
  output[i++] = (rem > 9) ? (rem-10) + 'A' : rem + '0';
  input = input/rad;
}
 
if(isnegative){
output[i++] = '-';
}
 
output[i] = '\0';
reverse(output, i);
//return output;
}
 
 
int main (){
 
int i, b;
char array[13];
printf("Enter a number and base\n");
scanf("%d %d", &i, &b);
      
//printf("String : %s", itoa(i, array, b));
//return 0;   
 
}


OUTPUT:

Enter a number and base
10 2