Convert long integer(decimal) to base 36 string (strtol inverted function in C)

Question

I can use the strtol function for turning a base36 based value (saved as a string) into a long int:

long int val = strtol(\"ABCZX12         


        

Answer 1

Here is another option with no need for source array of charaters, but less portable since not all character encodings have contiguous alphabetic characters, for example EBCDIC. Test HERE

#include  
#include  
#include 
#include 
#include 

char get_chars(long long value) 
{ 
    if (value >= 0 && value <= 9) 
        return value + '0'; 
    else
        return value - 10 + 'A'; 
} 

void reverse_string(char *str) 
{ 
    int len = strlen(str); 

    for (int i = 0; i < len/2; i++) 
    { 
        char temp = str[i]; 
        str[i] = str[len - i - 1]; 
        str[len - i - 1] = temp; 
    } 
} 

char* convert_to_base(char *res, int base, long long input) 
{ 
    bool flag = 0;
    int index = 0;   
    if(input < 0){  
       input = llabs(input);
       flag = 1;
    }
    else if(input == 0){
       res[index++] = '0';
       res[index] = '\0';
       return res;
    }      
       while(input > 0)
       {          
          res[index++] = get_chars(input % base); 
          input /= base; 
    } 
    if(flag){
        res[index++] = '-';
    }       
    res[index] = '\0';   
    reverse_string(res); 
    return res; 
} 

int main() {  
    long long input = 0;
    printf("** Integer to Base-36 **\n ");
    printf("Enter a valid number: ");
    scanf("%lld", &input); 
    if(input >= LLONG_MAX && input <= LLONG_MIN){
      printf("Invalid number");  
      return 0; 
    }

    int base = 36; 
    char res[100]; 
    printf("%lld -> %s\n", input, convert_to_base(res, base, input));

    return 0; 
}