C函数，反向字符串

Question

I want help regarding a function which you call from main that reverse text. However, the program works, well "more or less" but it crashes. Here is how code looks

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

void reverse(char * array, int numberOfChars) {
    int begin = 0;
    int end = 0;
    char temp;

    end = strlen(&array) - 1;
    printf("%s", &array);

    while (begin < end) {
        temp = array[begin];
        array[begin] = array[end];
        array[end] = temp;
        begin++;
        end--;
    }
}

int main() {
    reverse('supm', 4);
    return(0);
    getchar();
}

The string gets reversed to mpus, but then crashes, apparently it seems also like the arrays can only take in 4 characters, if i change it to 5 and the integer value to 5, it won't work at all. Any help would be appreciated.

Answer 1

strlen(&array) and printf("%s", &array); are wrong. They will pass where the pointer is instead of where the string is. use strlen(array) and printf("%s", array); .

reverse('supm', 4); is also wrong because the first argument of this call is not a string but an implementation-defined integer.

Be careful that just changing it to reverse("supm", 4); won't work because modifying string literals isn't allowed.

Also, the last getchar(); won't be executed because it is after return 0; .

Try this:

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

void reverse(char * array, int numberOfChars) {
    int begin = 0;
    int end = 0;
    char temp;

    end = strlen(array) - 1;
    printf("%s", array);

    while (begin < end) {
        temp = array[begin];
        array[begin] = array[end];
        array[end] = temp;
        begin++;
        end--;
    }
}

int main() {
    char supm[] = "supm";
    reverse(supm, 4);
    puts(supm); /* added this to see if this code is working well */
    return(0);
}

Maybe you should use the parameter numberOfChars instead of strlen(array) because you pass the parameter, but it is not used at all.

Answer 2

A few things immediately pop out at me:

1. strlen should take a char* , where you are passing a char** . Instead of strlen(&array) , use strlen(array) .
2. Similar story with printf . Instead of printf("%s", &array) , use printf("%s", array) .
3. Your string literal in main incorrectly uses single quotes, when it should be using double quotes. Your 'supm' should be "supm" . You also cannot modify a string literal, so you'll need to set a variable containing your string and then pass the variable instead.
4. You return(0) before calling getchar . That means getchar will never be called.
5. numberOfChars is unused. Either remove that parameter or use it instead of strlen .

Answer 3

In your function reverse -

end = strlen(&array) - 1;
printf("%s", &array);

You pass char ** to strlen which expects a const char * and try to print a char ** with %s . Jusr pass array to both these functions. And in main -

reverse('supm', 4);

If you even do "spum" which would be string literal , thus constant (modifying it would cause problem).

Do this instead -

char s[]="spum"; 
reverse(s, 4);

Compiler must have issued warnings for these statements and if not then enable compiler warnings.

Answer 4

1. array is already a pointer-- &array gives you a pointer to the pointer. so change the strlen call to this:

    strlen(array); 

2. you need to define the string before you pass to the function-- otherwise the string only exists inside your function, you won't be able to access it outside.

    char mystr[] = "supm"; reverse(mystr, 4); printf("%s\\n", mystr); return 0; 

Answer 5

Try this code I have fixed all your code problems.

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

void reverse(char * array, int numberOfChars) {
    int begin = 0;
    int end = 0;
    char temp;

    end = strlen(array) - 1;
    printf("%s\n", array);

    while (begin < end) {
        temp = array[begin];
        array[begin] = array[end];
        array[end] = temp;
        begin++;
        end--;
    }
}

int main() {
    char str[] ="sump";
    reverse(str, 4);
    puts(str);    
    return(0);

}

What you can learn from problems of this code ?

void reverse(char * array, int numberOfChars) /* definition */

reverse('supm', 4); /*calling function */

Compare this both two in definition first argument is to pass character array char *array But in calling you are passing 'sump' .Now onwards please remember for more than character you can use "" so here you should use "sump"

end = strlen(&array) - 1;

Refer strlen

Its argument is constant character pointer . But here you are passing &array .I guess you think &array is address of array but that is wrong for array array represent address and array[i] represent value

getchar()

This function is for reading input not for printing strings. refer getchar

For printing reversed output you should use puts at last not getchar

Answer 6

In your main function, the line reverse('supm', 4); has a couple of things wrong with it. First of all, it is a character constant, not a string, so you will end up passing an invalid pointer value to your reverse function. You probably meant to use "supm" which is a string and does match the char * expected by your reverse function. However, "supm" is a string literal constant, so although you can point to it, you are not allowed to modify it. So you should do something like this:

    char rev[] = "supm";
    reverse(rev, 4);

Here, rev is an array of 5 char , initialized as {'s', 'u', 'p', 'm', '\\0'} .

You are not currently using the numberOfChars parameter of reverse , so you could remove it.