Reverse an array in C Programming

You put <= instead of >=

for(i = line_len; i <= 0; --i) {

A simple silly mistake:

for(i=line_len; i<=0; --i){

What does this loop do? Well, first it sets i to line_len. Then it loops while i is less than or equal to zero, and at the end of each loop it decreases i by one.

This will loop for a long time (until i overflows) if line_len is zero, and otherwise it won't loop at all (because i will be greater than 0).

You probably meant >= instead of <=.

You also might've meant i=line_len-1 instead of i=line_len, but it's hard to tell.

EDITS: Changed the i<=0 in the for loop to i>=0.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define SIZE 80

void get_input(char *line){
 fgets(line, SIZE, stdin);
 char *ptr = strchr(line, '\n');
 if (ptr){
     *ptr = ' '; }
}

void reverse(char *line){
 char copy[SIZE];
 char word[SIZE];
 memset(copy, 0, SIZE);
 int line_len = strlen(line);
 int word_len = 0;
 int i;
 for(i=line_len; i>=0; --i){
     if(line[i] == ' ' && word_len > 0){
        memset(word, 0, SIZE);
        strncpy(word, line + i + 1, word_len);
        strcat(copy, word);
        strcat(copy, " ");
        word_len = 0;
     }else if(isalnum(line[i]) || line[i] == '\'')
          {word_len++;}
 }
  if(word_len>0){
     memset(word, 0, SIZE);
     strncpy(word, line, word_len);
     strcat(copy, word);}
     strcpy(line, copy);
}

int main(int argc, char *argv[]){
 int N = (int)strtol(argv[1], NULL, 10);
 if(N<0){
         printf("ERROR: Please provide an integer greater than or equal to 0\n");
         return 0;
        }
 if(N>SIZE){ printf("ERROR: Please provide an integer less than or equal to 80\n");
             return 0;
           }
     char line[SIZE];
     int i;
     for(i=0;i<N;i++){
                      get_input(line);
                      reverse(line);
                      printf("%s\n", line);
                     }
 return 0;
}

Very good attempt at the reversal. However, you may be making it a bit more difficult than it need be. If you are simply reversing the order of the arguments, you can either print them out in reverse, or if you want to build a string out of the reversed arguments, simply concatenate them in reverse order with strcat. There are a lot of ways to do it, but a fairly straight forward approach is:

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

#define MAXC 1024

int main (int argc, char **argv) {

    char string[MAXC] = "";
    int i, len = 0;

    for (i = argc - 1; i && len + strlen (argv[i]) + 2 < MAXC; i--) {
        strcat (string, argv[i]);
        strcat (string, " ");
        len = strlen (string);
    }
    printf ("%s\n", string);

    return 0;
}

Which simply starts with the last argument index argc - 1 and then so long as the argument index is greater than 0 (i) and you are within your allowed number of characters len + strlen (argv[i]) + 2 < MAXC (+2 to allow for the nul-terminator and the ' '), concatenate the argument to your string strcat (string, argv[i]); and add a space strcat (string, " ");

(you can add if (i > 1) before strcat (string, " "); to eliminate the trailing space).

Example Use/Output

$ ./bin/str_revargs The fox jumped over a log
log a over jumped fox The