在 C 中使用 isdigits 、 isalpha 和 ispunct

Question

I'm trying to identify the number of alphabets, number and punctuation by the user input , I got the No. of number but the alphabets and punctuation is incorrect!! .

i'm not sure why.. this is my code

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main()
{
    char str[50];
    int alphabet = 0 , number = 0, punct = 0;
    printf("Enter your sentence: ");
    fgets(str, sizeof(str),stdin);
    for(int i=0 ; i<50; i++)
   {
       if(isalpha(str[i]) != 0)
       {
           alphabet++;
       }
       else if (isdigit(str[i]) != 0 ){
           number++;
       }
       else if (ispunct(str[i]) != 0){
           punct++;
       }
   }
    printf("Your sentence contains:\n");
    printf("Alphabets: %d",alphabet);
    printf("\nDigits: %d",number);
    printf("\nPunctuation: %d",punct);

    return 0;
}

Answer 1

This loop

for(int i=0 ; i<50; i++)

is incorrect. The entered string can be less than the size of the character array str .

So instead use

for( size_t i = 0 ; str[i] != '\0'; i++ )

Take into account that the function fgets can append the new line character \\n' to the entered string. If you want to remove it before the loop then write

#include <string.h>

//…

str[strcspn( str, "\n" )] = '\0';

Also in if statements you should convert the given character to the type unsigned char . For example

   if( isalpha( ( unsigned char )str[i] ) != 0)

or

   if( isalpha( ( unsigned char )str[i] ) )

Otherwise in general without casting such a call can invoke undefined behavior if the code of a character is negative.

Answer 2

You have hardcoded the upper boundary of the loop: for(int i=0 ; i<50; i++) , however can you guarantee your input is 49 characters long? If you cannot, then you will be reading garbage values past '\\0' inserted by fgets. Either change the loop (for example): for(int i=0; i<50 && str[i]; i++) so that you only loop through to the end of the input.

Alternatively, zero-initialize the input buffer, ie char str[50] = {0}; , I believe isalpha(), isdigit() and ispunct() all return false with a zero input, therefore not affecting your results.