Reverse command-line arguments (C) - c

Question

The overall gist of the program is to accept a command-line argument and for each string to print out backwards with variable length.
For example:
$ ./reversecommand hello 102
dnammocesrever/. olleh 201
I am having difficulty in implementing the thought process into code (e.g. with hello below). Any thoughts?
argc[0] ./reversecommand
argc[1] hello
argc[1][0] h -> argc[1][4] o
argc[1][1] e -> argc[1][3] l
argc[1][2] l -> argc[1][2] l
argc[1][3] l -> argc[1][1] e
argc[1][4] o -> argc[1][0] h
argc[2] 102
argc[3] [null]

Answer 1

#include <stdio.h>
#include <string.h>
static void print_reversed(const char *str, size_t len)
{
const char *ptr = str + len;
while (ptr > str)
putchar(*--ptr);
}
int main(int argc, char **argv)
{
for (int i = 0; i < argc; i++)
{
if (i != 0)
putchar(' ');
print_reversed(argv[i], strlen(argv[i]));
}
putchar('\n');
return 0;
}
There's no need to modify the strings; simply print the characters out one at a time in reverse order.

Answer 2

The logic is fairly straightforward: For each string in argv, reverse it and then print it.
#include <stdio.h>
#include <string.h>
void swap_characters(char *s, char *t) {
char tmp = *s;
*s = *t;
*t = tmp;
}
void reverse_string(char *s, int length) {
for (int i = 0; i < length / 2; ++i)
swap_characters(s + i, s + length - 1 - i);
}
int main(int argc, char **argv) {
for (int i = 0; i < argc; ++i) {
reverse_string(argv[i], strlen(argv[i]));
printf("%s ", argv[i]);
}
printf("\n");
}

Answer 3

Try this code give File name as reverse and put command line argument:-

Answer 4

There are lots of ways to do string reversal, and it is worth spending some time looking into because it is a standard interview question.
a simple implementation for null-terminated strings in C looks like
char * reverse(char * string){
int i;
int len = strlen(string);
for (i = 0; i < len/2; i++){
char c = string[i];
string[i] = string[len - i - 1];
string[len - i - 1] = c;
}
return string;
}
This version does manipulate the string, so we'll make a copy of the string just in case.
Once you have string reversal, you will need to run reverse on each argument, and print it out.
NOTE: arguments in a standard console application like this start with the name of the program as it was typed by the user. So if the program is invoked as ./my-program arg1 arg2, argv[0] will be "./my-program", so we will skip argv[0]
Using this, all you have to do is call reverse on each argument. like so
int main(int argc, char ** argv){
int i;
char * copy;
for ( i = 1; i < argc; i++){ // skip argv[0]
copy = strdup(argv[i]); // copy the string;
copy = reverse(copy);
printf("argv[%d] = \"%s\"\n", i, copy);
free(copy); // clean up the copy
}
}
All together, you get
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char * reverse(char * string){
int i;
int len = strlen(string);
for (i = 0; i < len/2; i++){
char c = string[i];
string[i] = string[len - i - 1];
string[len - i - 1] = c;
}
return string;
}
int main(int argc, char ** argv){
int i;
char * copy;
for ( i = 1; i < argc; i++){ // skip argv[0]
copy = strdup(argv[i]); // copy the string;
copy = reverse(copy);
printf("argv[%d] = \"%s\"\n", i, copy);
free(copy); // clean up the copy
}
}