C - Getting a string from an int

Question

I have a list of all strings of a given length in the format

e.g length 3

000
100
200
.
.
900
A00
B00
.
.
Z00
a00
b00
.
.
z00
010
.
.
.
zzz

I am trying to write valueof(int pos, int len) which accepts a position int as parameter and prints the String at that position (e.g valueof(1,3) prints 000 and valueof(109,3) prints k10

This is what I tried and it is not working:

void valueof(int pos,int len)
{   

    int i=0;
    char arry[62] = {'0','1','2','3','4','5','6','7','8','9',
        'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
        'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
    char * string = malloc(sizeof(char)*(len+1));
    for(i=0; i<len; i++)
    {
        string[i]='0';
    }
    string[len]='\0';
    i=0;
    while(pos >= 62)
    {
        string[i]=arry[(pos%62)-1];
        i++;
        pos/=62;
    }
    string[i]=arry[pos];

    printf("%s\n",string);
}

How is it not working? For example, if we call valueOf(109, 3) what currently is reported? — Bill Lynch
– Bill Lynch, Commented Oct 1, 2014 at 17:02
Read about snprintf(3) & strdup(3) and perhaps (on Linux) asprintf(3) — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 1, 2014 at 17:11

Bill Lynch · Accepted Answer · 2014-10-01 17:05:23Z

5

So, pos % 62 returns a number between [0, 61]. So when you do (pos%62)-1, you get a number between [-1, 60]. Which you likely don't want.

You should probably rewrite this line:

string[i]=arry[(pos%62)-1];

as:

string[i] = arry[pos % 62];

answered Oct 1, 2014 at 17:05

Bill Lynch

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Johan Snowgoose · Accepted Answer · 2014-10-01 17:22:03Z

2

Here's a test harness. I would add some bounds checking to the input otherwise you can overrun the array.

    #include <stdio.h>
    #include <stdlib.h>

    void valueof(int pos,int len)
...

   int main (int argc, char **argv) {
     valueof(atoi(argv[1]), atoi(argv[2]));
   }

To compile gcc vo.c -o vo

./vo 1 3
000
./vo 109 3
k10

edited Oct 1, 2014 at 17:22

answered Oct 1, 2014 at 17:15

Johan Snowgoose

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