A program to reverse a string.

[thatos]

I'm trying to write a program that when given a string it prints it back whats.
here is my program.Note I'm a biginner in C.

#include <stdio.h>
#include <string.h>
char input[];
main(){ 
  //gets(input);
  scanf("%s",input);
  reverse(input);
}
 
void reverse(){  
  int length,i;
  length = strlen(input);
  char reverse[length];
  for(i=0;i < length;i++){
    reverse[i] = input[length - i - 1];
  }
  printf("%s\n",reverse);
}
 

When I run the above program on input:

thato

I get as my result:

otaht&

I do not want that

&

[arnaudk]

There are many errors with that code.

• reverse is not declared before it is used and is wrongly defined: it does not take 0 arguments. The correct declaration is
  void reverse(char * input); and this should appear before main().
• Return type for main() is missing: "int main()" not "main()"
• line 13: you can not declare an array with a size that is not known at compile time. Use malloc() instead.
• line 3: char input[]; you cannot declare an array like this unless you initialize it when you declare it in which case the the compiler can fill in the size required, like char str[] = "hello";. Use char input[80]; or something like this instead if you don't expect the user to input strings longer than 80 characters.

I am amazed that your code could possibly compile with all those fundamental errors, I suggest you get a better compiler and enable all warnings, especially if you're learning because it's very important you don't develop bad habits. To clear up any confusion regarding arrays, I suggest you read this.

[newb16]

You should place terminating zero in the reversed array.

[thatos]

Thank you very much, now I know where I made errors.

There are many errors with that code.

• reverse is not declared before it is used and is wrongly defined: it does not take 0 arguments. The correct declaration is
  void reverse(char * input); and this should appear before main().
• Return type for main() is missing: "int main()" not "main()"
• line 13: you can not declare an array with a size that is not known at compile time. Use malloc() instead.
• line 3: char input[]; you cannot declare an array like this unless you initialize it when you declare it in which case the the compiler can fill in the size required, like char str[] = "hello";. Use char input[80]; or something like this instead if you don't expect the user to input strings longer than 80 characters.

I am amazed that your code could possibly compile with all those fundamental errors, I suggest you get a better compiler and enable all warnings, especially if you're learning because it's very important you don't develop bad habits. To clear up any confusion regarding arrays, I suggest you read this.

[JosAH]

Thank you very much, now I know where I made errors.

There also is a logic error in your code because lines 14-15 don't reverse
a string, e g. the string "abcd" is turned into "dccd" and not what you wrote.

kind regards,

Jos

[newb16]

There also is a logic error in your code because lines 14-15 don't reverse
a string, e g. the string "abcd" is turned into "dccd" and not what you wrote.

Why? He is not reversing it 'in place', there is a separate destination string.

[JosAH]

Why? He is not reversing it 'in place', there is a separate destination string.

My bad; I missed that one completely; I apologize for that remark.

kind regards,

Jos

[curiously enough]

The reason it gave you a random charachter in the end was because the last element of the array reverse that was given a value(in the for loop) was reverse[lengh-1].
And I don't see why you have to use another array called reverse to store the reverse of the string if all you want the function to do is print.

[thatos]

How do I place a terminating zero in the end of my string?

[boxfish]

How about

myCString[stringLength] = '\0'

[thatos]

Is this solution corrent?It works

#include <stdio.h>
#include <string.h>
#define max 100
main(){
  char string[max];
  scanf("%s",&string);
  int length = strlen(string);
  char reverse[100];
  int i;
  for(i=0;i<length;i++){
    reverse[i] = string[length - 1 - i];
  }
 
  reverse[i] = '\0';
  printf("%s\n",reverse);
}
 

[boxfish]

That looks good, but here

main(){

main should return an int. Make it

int main() {
    //...
    return 0;
}
 

And here

char reverse[100];

use max.

char reverse[max];

Looks good though.

[thatos]

Why should I make it return an integer, I often see this in other peoples programs? Whats the importance of returning an integer?

That looks good, but here

main(){

main should return an int. Make it

int main() {
    //...
    return 0;
}
 

And here

char reverse[100];

use max.

char reverse[max];

Looks good though.