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Program to check if a number belongs to a particular base or not

  • Difficulty Level : Medium
  • Last Updated : 27 Sep, 2021
Geek Week

Given a number N and its radix base R, find whether its valid or not according to its radix base. 
Find valid/invalid number for any base ranging from binary to base32. 


Input : 1000101010001
Output : Valid
1000101010001 is valid binary number.

Input : 0xFFFAG
Output : invalid
0xFFFAG is not a valid hexa-decimal number because of character 'G' .

Method used : strspn

  • strspn :
strspn(numStr, validNumber)
returns number of matching digits from 
character-set provided, so here it will return 1 - N,
where N is length of validNumber String.
numStr[]  : 
And that number is provided to numStr[] array.
  • numStr : 
numStr[strspn(numStr, validNumber)]
Using this number as index for numStr[] array,
we access the digit there and it will be NULL 
in case string is matched and it will be non-zero 
if string is not matched
  • !numStr : And Finally we invert the result using invert operator to match for true case ! 
!numStr[strspn(numStr, validNumber)]


// Program to check if a number belongs
// to particular base or not#include
#include <cstdio>
#include <cstdlib>
#include <cstring>
#define BINARY_BASE 2 // Defining binary base
#define OCTAL_BASE 8 // Defining octal base
#define DECIMAL_BASE 10 // Defining decimal base
#define HEXA_BASE 16 // Defining hexa-decimal base
#define BASE32_BASE 32 // Defining base32 base
// Function prototypes
bool isValid(const char* numStr, int base);
const char* print(bool status);
// Main method for base check
int main(void)
    // Defining valid/invalid numbers in radix bases
    char* binaryStr = "1000101010001";
    char* decimalStr = "45221";
    char* base32Str = "AD22F";
    // invalid
    char* octalStr = "7778A";
    char* hexStr = "FAG463";
    // Printing status of radix bases
    printf("Binary string %s is %s\n", binaryStr,
           print(isValid(binaryStr, BINARY_BASE)));
    printf("Octal string %s is %s\n", octalStr,
           print(isValid(hexStr, OCTAL_BASE)));
    printf("Decimal string %s is %s\n", decimalStr,
           print(isValid(decimalStr, DECIMAL_BASE)));
    printf("Hex string %s is %s\n", hexStr,
           print(isValid(hexStr, HEXA_BASE)));
    printf("Base32 string %s is %s\n", base32Str,
           print(isValid(base32Str, BASE32_BASE)));
    return 0;
bool isValid(const char* numStr, int base)
    // Defining valid base strings
    const char* validBinary = "01";
    const char* validOctal = "01234567";
    const char* validDecimal = "0123456789";
    const char* validHex = "0123456789abcdefABCDEF";
    const char* validBase32 = "0123456789abcdefghijklmnopqrstuvABCDEFGHIJKLMNOPQRSTUV";
    const char* validNumber = NULL;
    // Checking for valid base
    validNumber = (base == BINARY_BASE)
                      ? validBinary
                      : ((base == OCTAL_BASE)
                             ? validOctal
                             : (base == DECIMAL_BASE)
                                   ? validDecimal
                                   : (base == HEXA_BASE)
                                         ? validHex
                                         : (base == BASE32_BASE)
                                               ? validBase32
                                               : NULL);
    // Error checking for invalid base
    if (validNumber == NULL) {
        fputs("Invalid base encountered", stderr);
    // Check for valid base string using strspn
    return (!numStr[strspn(numStr, validNumber)]) ? true : false;
const char* print(bool status)
    return (status) ? "Valid" : "Invalid";


Binary string 1000101010001 is Valid
Octal string 7778A is Invalid
Decimal string 45221 is Valid
Hex string FAG463 is Invalid
Base32 string AD22F is Valid                                                                                         


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