File Handling in C Print out the reverse String in the output from the input

Question

Okay, so i have a lot of problems with file handling and strings in C. The point of this task is to reverse the string from the output and to convert lower letters to upper and vice-versa. For example i have to get an output of ENALPOREa from an Aeroplane. I have to have 2 files. An input file where i have written Aeroplane and an output file where i have got ENALPOREa. Any help is appreciated !

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

void inputOfAString(const char*nameOfTheFile){
FILE *fin = fopen(nameOfTheFile,"w");

if(fin == NULL){
   printf("Error");
   exit(1);
}

char s[20];
fgets(s,20,stdin);
fprintf(fin,"%s",s);

fclose(fin);
}


void readTheString(const char *inputOfTheFile,const char*outputOfTheFile){
   FILE *fout = fopen(outputOfTheFile,"w");
   FILE *fin = fopen(inputOfTheFile,"r");
   if(fout == NULL || fin == NULL){
       printf("Error");
       exit(1);
   }
   char s[20];
   while(!feof(fin)){
       fgets(s,20,inputOfTheFile);
       for(int i = 0;i<20;i++){
           if(s[i] == isupper(s[i])){
               s[i] = tolower(s[i]);
           }else{
               s[i] = tolower(s[i]);
           }
       }
       fputs(s,fout);
   }

   fclose(fout);
   fclose(fin);
}

int main(){
   inputOfAnString("input1.txt");
   readTheString("input1.txt","output1.txt");
   return 0;
}

You haven't actually asked a question or described any errors/incorrect behaviours. Please review - [How to ask a good question](https://stackoverflow.com/help/how-to-ask). — kaylum, Dec 21 '20 at 21:38
[Why `while(!feof(file))` is always wrong](https://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong) — Barmar, Dec 21 '20 at 21:46
And `if (s[i] == isupper(s[i]))` should be `if (isupper(s[i]))` — Barmar, Dec 21 '20 at 21:47
Shouldn't the `else if` be the opposite condition? And if it's the opposite, you should just use `else`. — Barmar, Dec 21 '20 at 21:48
The loop where you lower-case `s` could be made much simpler, and without possibly going out beyond the null-terminator: `for (size_t i = 0; i < strlen(s); ++i) { s[i] = tolower(s[i]); }` — Some programmer dude, Dec 21 '20 at 21:53
@Someprogrammerdude Better to use `for(int i = 0; s[i]; i++){` rather than call `strlen()` repeatedly. — chux - Reinstate Monica, Dec 21 '20 at 21:56
Worst Vayne Antartic, `tolower()` in both side of the `if()`? — chux - Reinstate Monica, Dec 21 '20 at 21:59
Instead of `printf("Error")`, write an informative message to stderr with `perror(nameOfTheFile)`. — William Pursell, Dec 21 '20 at 22:04
@WilliamPursell Agree that the error message could/should be made more informative, yet failed values can have _bad_ attributes to them - like the file name containing control characters (e.g. `'\n'`) , etc. Care is warranted printing _bad_ data. — chux - Reinstate Monica, Dec 21 '20 at 22:09

chux - Reinstate Monica · Accepted Answer · 2020-12-21T22:53:01.243

Test the result of fgets(), not feof().
(Whatever text or instructor suggested feof() - treat with suspicion).

Use the result of isupper() as T/F test.

One branch should use toupper()

Iterate to the end of the string, which may be less than 20.

   char s[20];
   //while(!feof(fin)){
   //    fgets(s,20,inputOfTheFile);
   while(fgets(s, sizeof s, inputOfTheFile)) {
       // for(int i = 0; i < 20;i++){
       for(int i = 0; s[i];i++){
           // if(s[i] == isupper(s[i])){
           if(isupper(s[i])){
               s[i] = tolower(s[i]);
           }else{
               //s[i] = tolower(s[i]);
               s[i] = toupper(s[i]);
           }
       }
       fputs(s,fout);
   }

Advanced: use unsigned char for avoid calling is...(), to...() with negative values.

Tight alternative

   #define S_SIZE 20
   char s[S_SIZE];
   while (fgets(s, sizeof s, inputOfTheFile)) {
       const unsigned char *p = (const unsigned char *) s;
       while (*p) {
           if(isupper(*p)) {
               *p = tolower(*p);
           } else {
               *p = toupper(*p);
           }
           p++;
       }
       fputs(s, fout);
   }

File Handling in C Print out the reverse String in the output from the input

1 Answers1