Why strlen() returning 2 or 1 everytime?

Question

Why doesn't strlen() return an exact value?
For example, when I input 123 it returns 2. For 12345 it returns 1. Why?

#include<stdio.h>
#include<string.h>
int main()
{
    long int ui,remainder,i=0,len;
    char binary[20];
    scanf("%ld",&ui);
    while(ui!=0) {
        remainder=ui%2;
        binary[i]=(char)remainder;
        printf("%d ",remainder);
        ui=ui/2;
        i++;
    }
    binary[i]='\0';
    printf("len is %ld\n",strlen(binary));
    for(i=len-1;i>=0;i--) printf("%d",binary[i]);
    return 0;
}

Answer 1

strlen returns the length of an array of characters until it finds the character '\\0' (with numeric value 0 ) in it. Your binary array is an array of characters, but you are treating them as integers as you're storing the numeric values 0 and 1 . When calculating its length, strlen stops when it finds the first 0 you wrote.

To get the answer you need, change the binary definition to char binary[20] = { 0 }; , save the remainder values as ASCII characters ( binary[i] = '0' + (char)remainder; ), and print them as characters printf("%c ",binary[i]);

Answer 2

The reason it isn't working is because of how you're using binary .

The strlen function is meant to operate on NULL terminated strings. But binary isn't treaded as a string, but as an array of bytes.

The strlen function searches until it find a null bytes, ie a byte with a value of 0. In the case of "123", the value 0 appears after two bytes. In the case of "12345", the value 0 appears after one byte. This explains the output you're getting.

Also, you're using len without initializing it. This leads to undefined behavior. You want to set it to i after you exit the while loop. You also need to change the format specifier for your first printf from %d to %ld since remainder is declared as a long int .

Answer 3

There are a few problems here binary is a char so it's '0' + value print it out that way as well;

#include<stdio.h>
#include<string.h>
int main()
{
    long int ui,remainder,i=0,len;
    char binary[20] = {0};
    scanf("%ld",&ui);
    if (ui==0) { binary[i] = '0'; i++; } // Special case for zero
    while(ui!=0) {
        remainder=ui%2;
        binary[i]=(char)remainder + '0';
        printf("%d ",remainder);
        ui=ui/2;
        i++;
    }
    binary[i]='\0';
    printf("len is %ld %s\n",len = strlen(binary), binary);
    for(i=len-1;i>=0;i--) 
        printf("%c",binary[i]);
    return 0;
}

For my run below I got

32

0 0 0 0 0 1 len is 6 000001

100000