C Exercises: Check if a given string can be interpreted as a decimal number

Last update on August 07 2025 12:49:22 (UTC/GMT +8 hours)

6. Decimal Number String Check Variants

Write a C program to check if a given string can be interpreted as a decimal number.

Example:
Input:
str_num1[ ] ="1234"
str_num2[ ]=" 0.1 "
str_num3[ ]=" -90e3 "
str_num4[ ]=" 99e2.5 "
Output:
Is the above string is a number? 1
Is the above string is a number? 1
Is the above string is a number? 1
Is the above string is a number? 0

Sample Solution:

C Code:

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

// Function to determine if a given string is a valid number
bool is_Number(char* str1) {
    int n, m;

    // Skip leading spaces
    while (*str1 == ' ') str1 ++;

    n = m = 0;

    // Skip the sign of the number
    if (*str1 == '+' || *str1 == '-') str1 ++;

    // Loop to check for digits before a possible decimal point
    while (*str1 >= '0' && *str1 <= '9') {
        n ++;
        str1 ++;
    }

    // Check if there is a decimal point
    if (*str1 == '.') {
        str1 ++;
        // Loop to check for digits after the decimal point
        while (*str1 >= '0' && *str1 <= '9') {
            m ++;
            str1 ++;
        }
        // If no digits found before or after the decimal point, return false
        if (!n && !m) return false;
    } else if (!n) {
        // If no digits found before a decimal point, return false
        return false;
    }

    // Check for exponent notation (e.g., 1.23e45)
    if (*str1 == 'e' || *str1 == 'E') {
        str1 ++;
        // Check for sign after 'e' or 'E'
        if (*str1 == '+' || *str1 == '-') str1 ++;
        n = 0;
        // Loop to check for digits in the exponent
        while (*str1 >= '0' && *str1 <= '9') {
            n ++;
            str1 ++;
        }
        // If no digits found in the exponent, return false
        if (!n) return false;
    }

    // Skip trailing spaces
    while (*str1 == ' ') str1 ++;

    // Return true if the entire string has been checked and no invalid characters found
    return *str1 == 0 ? true : false;
}

// Main function to test the is_Number function with different strings
int main(void) {	
    char str_num1[] ="1234";
    printf("\nstr_num = %s", str_num1);
    printf("\nIs the above string a number? %d ", is_Number(str_num1));

    char str_num2[]=" 0.1 ";
    printf("\n\nstr_num = %s", str_num2);
    printf("\nIs the above string a number? %d ", is_Number(str_num2));

    char str_num3[]=" -90e3   ";
    printf("\n\nstr_num = %s", str_num3);
    printf("\nIs the above string a number? %d ", is_Number(str_num3));

    char str_num4[]=" 99e2.5 ";
    printf("\n\nstr_num = %s", str_num4);
    printf("\nIs the above string a number? %d ", is_Number(str_num4));

    return 0;
}

Sample Output:

str_num = 1234
Is the above string is a number? 1 

str_num =  0.1 
Is the above string is a number? 1 

str_num =  -90e3   
Is the above string is a number? 1 

str_num =  99e2.5 
Is the above string is a number? 0

Flowchart:

$Flowchart: Check if a given string can be interpreted as a decimal number$

For more Practice: Solve these Related Problems:

Write a C program to validate if a string represents a valid decimal number using state machine logic.
Write a C program to check decimal validity by manually parsing the string character by character.
Write a C program to determine if a string is a number, handling optional spaces, signs, and decimal points.
Write a C program to validate a numeric string and flag errors if multiple decimal points or invalid characters are found.

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