C Program to print string from commandline in reverse [duplicate] - c

Question

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Reversing a string in C
(27 answers)
Closed 2 years ago.
/* Pgm to print string from commandline and reverse it */
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>
int main(int argc, char *argv[]) {
if(argc<1){
perror("Not enough arguments");
}
// Printing the string
for(int i=1; i<argc ; i++){
printf("%s\n",argv[i]);
}
//Part to print the string in reverse
char *arr = (char*) malloc(sizeof(argv[1])+1); // +1 for the NULL terminator
strcpy(arr,argv[1]);
char *str = (char*) malloc(sizeof(argv[1])+1); //buffer array
//Reverse part begins
int j=0;
for(int i= sizeof(argv[1]); i>=0 ; i--){
str[j] = arr[i];
j++;
}
for(int j=0;j<sizeof(argv[1]);j++){ // Printing the reverse string
printf("R=%s\n",&str[j]);
}
free(arr);
free(str);
return 0;
}
This program is expected to print the text from argv[1] on commandline in reverse order. But the output I get is weird.
output
user#DESKTOP-KI53T6C:/mnt/c/Users/user/Documents/C programs$ gcc linex.c -o linex -Wall -pedantic -std=c99
user#DESKTOP-KI53T6C:/mnt/c/Users/user/Documents/C programs$ ./linex hello
hello
R=
R=
R=
R=
R=olleh
R=lleh
R=leh
R=eh
Also when the input is above a certain number of characters, it automatically truncates it:
user#DESKTOP-KI53T6C:/mnt/c/Users/user/Documents/C programs$ ./linex strcmpppssdsdssdsd
strcmpppssdsdssdsd
R=spppmcrts
R=pppmcrts
R=ppmcrts
R=pmcrts
R=mcrts
R=crts
R=rts
R=ts
All I want is the output to be : 'olleh' when I type 'hello'

Answer 1

The expression argv[1] has the type char *. That is it is a pointer. sizeof( char * ) that is equivalent to the expression sizeof( argv[1] ) is equal to 4 or 8 depending on the used system.
So for example in this line
char *arr = (char*) malloc(sizeof(argv[1])+1);
there are allocated not enough memory to store a string pointed to by the pointer argv[1]. Instead of the expression sizeof( argv[1] ) you need to use expression strlen( argv[1] ).
Also to output a string in the reverse order there is no any need to allocate dynamically a character array. This is just a bad idea.
Here is a demonstrative program that shows how to output a string in the reverse order.
#include <stdio.h>
#include <string.h>
int main(void)
{
const char *s = "Hello";
for ( size_t i = strlen( s ); i != 0; )
{
putchar( s[--i] );
}
putchar( '\n' );
return 0;
}
The program output is
olleH
If you want not just to output a string in the reverse order but to create a new string in the reverse order then what you need is to write a function that performs copying of a string stored in one character array into another character array.
Here is a demonstrative program.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char * copy_reverse( char *s1, const char *s2 )
{
size_t n = strlen( s2 );
s1[n] = '\0';
while ( *s2 ) s1[--n] = *s2++;
return s1;
}
int main(void)
{
const char *s1 = "Hello";
char *s2 = malloc( strlen( s1 ) + 1 );
if ( s2 ) puts( copy_reverse( s2, s1 ) );
free( s2 );
return 0;
}
The program output is the same as shown above
olleH