C++ program to check if a string is a number

Using strings or characters is very helpful when solving logic programming difficulties. Characters in a string are 1-byte data types that can store symbols in ASCII values. A string is a collection of characters. These symbols can be special characters, numbers from the number system, or letters from the English alphabet. This article will teach you how to use C to determine whether a character is a numeric character.

Check if the string is a number

To check if a given string is a number, we need to check if each character in it is a number. If any of the characters is a non-numeric character, the string is non-numeric, otherwise it is numeric. The algorithm is as follows -

algorithm

• Read string s as input
• For each character c in string s, perform the following operations
• If c is a non-number, then
  • Return error
• End if
• Finish
• return true

Example

#include <iostream>
#include <ctype.h>

using namespace std;
string solve( string s ) {
   for( int i = 0; i < s.length(); i++ ) {
      if( !isdigit( s[i] )) {
         return "False";
      }
   }
   return "True";
}

int main()
{
   cout << "Is "589" a numeric string? : " << solve( "589" ) << endl;
   cout << "Is "69a" a numeric string? : " << solve( "69a" ) << endl;
   cout << "Is "2979624" a numeric string? : " << solve( "2979624" ) << endl;
   cout << "Is "25\%4A" a numeric string? : " << solve( "25\%4A" ) << endl;
   cout << "Is "889" a numeric string? : " << solve( "889" ) << endl;
}

Output

Is "589" a numeric string? : True
Is "69a" a numeric string? : False
Is "2979624" a numeric string? : True
Is "25%4A" a numeric string? : False
Is "889" a numeric string? : True

This solution can check if the given string is a number, but will not return true when the input is a negative number. For negative numbers, special checks are required.

Check whether the string is negative or positive

To check if the given string is a number, we just need to check if each character is a number. But for negative numbers, the first character must be a "-" sign. So first check if the first character is negative, then check if the next character is a number, and if so, check if the remaining characters are numbers. The algorithm is as follows -

algorithm

• Read string s as input
• If the first character of s is '-' and the next character is a number, then
  • st = 1
• otherwise
  • st=0
• If it ends
• For each character c in string s starting from index st, perform the following operations
  • If c is a non-number, then
    • Return error
  • If it ends
• Finish
• return true

Example

#include <iostream>
#include <ctype.h>

using namespace std;
string solve( string s ) {
   int start;
   if( s[0] == '-' && isdigit( s[1] ) ) {
      start = 1;
   }
   else {
      start = 0;
   }

   for( int i = start; i < s.length(); i++ ) {
      if( !isdigit( s[i] )) {
         return "False";
      }
   }
   return "True";
}

int main()
{
   cout << "Is "687" a numeric string? : " << solve( "687" ) << endl;
   cout << "Is "256l" a numeric string? : " << solve( "256l" ) << endl;
   cout << "Is "-5845" a numeric string? : " << solve( "-5845" ) << endl;
   cout << "Is "-89A2" a numeric string? : " << solve( "-89A2" ) << endl;
   cout << "Is "-256" a numeric string? : " << solve( "-256" ) << endl;
}

Output

Is "687" a numeric string? : True
Is "256l" a numeric string? : False
Is "-5845" a numeric string? : True
Is "-89A2" a numeric string? : False
Is "-256" a numeric string? : True

in conclusion

To check if the given string is a number, we need to check every character of it. A string is a number when all characters are numbers. In this article, we also use logic to check negative numbers. When the first character is a negative sign, then check if the next character is a number. If yes, then check the rest. This program can be extended to check floating point numbers. Now it only works for positive and negative integers.

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