I have this function which receives the number 894867 (nbr) and the length 16 (i). When I run this code, it writes =A793 instead of what it should write DA793.

void ft_transformers(int nbr, int i) {

    char num[100];
    int t;
    int temp;
    int j;

    temp = 0;
    t = 0;
    while (nbr != 0) {
        temp = nbr % i;
        if (num[t] < 10) {
            num[t] = temp   48; 
        } else {
            num[t] = temp   55; 
        }         
        nbr /= i;
        t  ;
    }

    j = t;
    while (j >= 0) {
        ft_putchar(num[j]);
        j--;
    }   
}

CodePudding user response：

There are multiple problems:

• Instead of if (num[t] < 10), you should be testing if (temp < 10)

• instead of hardcoding ASCII values such as 48 and 55, you should use character constants and expressions '0' and 'A' - 10

• the final loop should decrement j before accessing the character in the array.

• the behavior is undefined if i is 0, 1 or -1.

Here is a modified version:

void ft_transformers(int nbr, int base) {

    char num[100];
    int t;
    int j;

    if (base < 2 || base > 36)
        base = 10;

    t = 0;
    while (nbr != 0) {
        int temp = nbr % base;
        if (temp < 10) {
            num[t] = '0'   temp; 
        } else {
            num[t] = 'A'   (temp - 10); 
        }         
        nbr /= base;
        t  ;
    }

    j = t;
    while (j > 0) {
        j--;
        ft_putchar(num[j]);
    }
}

Note also these shortcomings:

• the function will not output anything if the initial value of nbr is zero.

• the function will not output digits if nbr is negative.

Here is an improved version:

void ft_transformers(int nbr, int base) {

    char num[100];
    int t;
    int j;
    int isneg;

    if (base < 2 || base > 36)
        base = 10;

    isneg = 0;
    t = 0;
    for (;;) {
        int temp = nbr % base;
        if (temp < 0) {
            isneg = 1;
            temp = -temp;
        }
        if (temp < 10) {
            num[t] = '0'   temp; 
        } else {
            num[t] = 'A'   (temp - 10); 
        }         
        t  ;
        nbr /= base;
        if (nbr == 0)
            break;
    }

    if (isneg) {
        ft_putchar('-');
    }
    j = t;
    while (j > 0) {
        j--;
        ft_putchar(num[j]);
    }
}

CodePudding user response：

Your code has two errors:

• The condition num[t] < 10 is wrong. num[t] is uninitialized at that point. It should be temp < 10.
• The initialization of the second loop j = t; is wrong. t ; is executed at the end of the first loop and nothing is written to num[t] after that before the second loop. It should be j = t - 1;.

Also using magic numbers like 48 and 55 should be avoided. They should be '0' and 'A' - 10 to make it more clear. (unless your code has chance to be used in environments where character sets that are not compatible to ASCII are used)

CodePudding user response：

There are at least three drawbacks in the function.

For starters instead of the while loop you have to use do-while loop. Otherwise if nbr initially is equal to 0 the function will have undefined behavior.

Secondly, in this if statement

if (num[t] < 10) {

you have to use the variable temp instead of num[t]

if ( temp < 10) {

After the while loop the variable t points to after the last digit of the number. So in the first iteration of this loop

j = t;
while (j >= 0) {
    ft_putchar(num[j]);
    j--;
} 

there will be outputted an indeterminate value.

T least you should write

j = t;
while (j != 0) {
    ft_putchar(num[--j]);
} 

Pay also attention to that your function ignores the sign of the original number.

And it is a bad idea to use magic numbers like for example 48 or 55.

The function can be declared the following way as it is shown in the demonstration program be.low.

#include <stdio.h>

void ft_transformers( int nbr, int base ) 
{
    if ( base < 2 || base >  36 )
    {
        printf( "%d\n", nbr );
    }
    else
    {
        char num[8 * sizeof( int )];

        int sign = nbr < 0;

        size_t i = 0;

        do
        {
            int digit = nbr % base;

            if ( sign ) digit = -digit;
            
            if ( digit <  10 )
            {
                digit  = '0';
            }
            else
            {
                digit = digit - 10   'A'; 
            }

            num[i  ] = digit;
        } while ( nbr /= base );

        if ( sign ) putchar( '-' );

        while ( i != 0 ) putchar( num[--i] );
        putchar( '\n' );
    }
}

int main( void )
{
    int nbr = 0;

    ft_transformers( nbr, 16 );

    nbr = 894867;

    ft_transformers( nbr, 16 );

    nbr = -894867;

    ft_transformers( nbr, 16 );
}

The program output is

0
DA793
-DA793

In this declaration of the character array

char num[8 * sizeof( int )];

instead of the number 8 (the number of bits in a character) you can use the constant CHAR_BIT like

char num[CHAR_BIT * sizeof( int )];

To do so you need to include the header <limits.h>.