通过动态内存分配计算字符串中的字符

Question

I am writing a C program where I write a function to count the number of character in a function. I have to use a dynamic memory allocation to allocate memory for an array take the array and copy it to another with fixed size. I have wrote the following code for it:

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
int count_insensitive(char *str, char ch){
    int n = 0;
    int i;
    for (i=0;i<strlen(str);i++){
        if (tolower(*(str+i))== tolower(ch)){
            n++;
        }
    }
    return n;
}
int main(){
    char *a,ch;
    int i,n;
    n=50;
    a = (char*) malloc(sizeof(char) * n);
    if (a==NULL){
        exit(1);
    }
    fgets(a,sizeof(a),stdin);
    char str[strlen(a)];
    strcpy(str,a);
    free(a);
    char c[] = {'b','H','8','u','$'};
    for (i=0;i<5;i++){
        ch = c[i];
        printf("The character '%c' occurs %d times.\n",c[i],count_insensitive(str,ch));
    }
    return 0;
}

The program runs but it take only the first 3 characters of a string and prints the position. Can you help me what I am doing wrong.

Answer 1

From fgets() :

Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or the end-of-file is reached, whichever happens first.

where num is the second argument passed to fgets() .

In your code, you are passing sizeof(a) as second argument to fgets() :

fgets(a,sizeof(a),stdin);

and a is char pointer.

The size of pointer will be 4 byte on a 32-bit system and 8 on a 64-bit system.

Seems that yours is 32-bit system and you must be giving input of more that 4 characters, that's why fgets() is reading only first 3 characters of the given input.

Since, you are allocating memory of n characters to pointer a , you should pass n as second argument to fgets() :

fgets(a, n, stdin);

Also, strlen() returns length of a null terminated string without including the terminating null character itself. So, you should add 1 to strlen(a) to ensure that str should be long enough to accommodate null terminating character:

char str[strlen(a)+1];
                  ^^