Strcmp not comparing strings from argv

Question

** Updated 26/10 -> First of all thank you all for you help, I am getting closer now, I need more work and studying but I really appreciate you are helping me a lot :-)

Still don't know why the first "rain" word in the input.txt file is not getting the positive output from strcmp, and from the cmd I can see the "<" bracket don't appear except for the LAST line which is the line that works.

Also checked the highlighted response from Removing trailing newline character from fgets() input

and even if I change the code to the following:

while( fgets (line, sizeof line, fp)!=NULL ) {

  /* remove \n from at the end of the str buffer*/   
  char * pos;
  /*
  if ((pos = strchr(line, '\n')) != NULL)
     *pos = '\0';
  */
  line[strcspn(line, "\n")] = 0;

I get the same result as if I use the if block instead. Maybe I'm getting extra \0 which might be the case. Anyone has a link where I can read about the delimiters I just used, or a nice reference of a debugger, etc. ... which I will have a look as soon as I come here? Thank you so much in advance!

read5.c version: Now from that input.txt file, it had an extra space on the last "rain" word, I removed the space, and it was able to find and get that last word compare as a true result, running in the strcmp if block. but that was the only string that was a true positive result from that if block.

on the cmd I can see:

$./read5 input.txt rain output.txt sun
>Maria
>rain
>manel
>Bla bla
<rain>
Found it! rain

On the output.txt it becomes:

Maria
rain
manel
Bla bla
sun

read5.c

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

/**
* Compile program:
*    gcc read3.c -o read3
*
*/
int main (int argc, char **argv) {
   FILE *fp, *fo;
   char *compare, *replace;
   char line[246];

   if (argc <= 4){
      printf(">Missing arguments on the command line.\n");
      printf(">Be sure you run the program as\n\"./read3 input.txt compare outout.txt replace\"\n\n");
   }

   /* opening file for reading */
   fp = fopen(argv[1] , "r");
   if(fp == NULL){
      perror("Error opening input file");
      return 1;
   }
   compare = argv[2];

   fo = fopen(argv[3], "w");
   if(fo == NULL){
      perror("Error opening output file");
      return 1;  //TODO check if: return 1 because it was expected, right?
   }
   replace = argv[4];

   /*
   printf(); made to test version 2
   //printf("We are going to compare %s\n", compare);
   //printf("We are going to replace it with %s\n", replace);
   */


   while( fgets (line, sizeof line, fp)!=NULL ) {

      /* remove \n from at the end of the str buffer*/   
      char * pos;
      if ((pos = strchr(line, '\n')) != NULL)
         *pos = '\0';

      /* print str enclosed in <> so we can see what str actually contains */
      //printf("Inside the loop, got the string: %s\n", line);

      //printing the strings with defined delimiters
      printf("<%s>\n", line);

      if(strcmp(compare, line) == 0){
         printf("Found it! %s \n", line);
         fprintf(fo, "%s\n", replace);
      }
      else{
         fprintf(fo, "%s\n", line);
      }

   }
   fclose(fp);
   fclose(fo);

   return 0;
}

---

First question with no edits: 25/10

I need to make a program that is run like this:

./read2 input.txt rain output.txt sun 

It reads the input.txt, searches for rain string and if finds it, replaces it with sun string and outputs all the text from input.txt with the replacements to the output.txt.

But with the code that I have so far, the strcmp is not comparing the strings I want, maybe it has the extra space that I get on the command line, I don't know... for now what is doing is copying everything from input.txt to output.txt... It's running the else block always...

Read2.c:

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

int main(int argc, char **argv) {
    FILE *fp, *fo;
    char str[60];

    //char* token;
    /* opening file for reading */
    fp = fopen(argv[1], "r");
    char *compare = argv[2];

    fo = fopen(argv[3], "w+");
    char *replace = argv[4];

    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }

    //printf("We are going to compare %s\n", compare);

    //printf("We are going to replace it with %s\n", replace);

    while (fgets(str, 60, fp) != NULL) {
        /* writing content to stdout */
        //Take the \n out 
        //token = strtok(str, "\n");

        printf("Inside the loop, got the string: %s\n", str);
        if (strcmp(compare, str) == 0) {
            //puts(str);
            printf("Found it! %s \n", str);

            fprintf(fo, "%s", replace);
        } else {
            fprintf(fo, "%s", str);
        }
    }
    fclose(fp);

    return(0);
}

input.txt:

Maria
rain
manel
Bla bla
rain 

Ouput.txt becomes exactly as input.txt and before it was empty, so the code is working, except the if block that tests with strcmp.

Answer 1

The problem is the \n at the end of the str buffer. fgets adds the \n at end end of the line it reads, you need to get rid of it before comparing.

This is what you need:

  while (fgets(str, 60, fp) != NULL) {

    /* remove \n from at the end of the str buffer*/    
    char *pos;
    if ((pos = strchr(str, '\n')) != NULL)
      *pos = '\0';

    /* print str enclosed in <> so we can see what str actually contains */
    printf("Inside the loop, got the string: <%s>\n", str);

    if (strcmp(compare, str) == 0) {
      printf("Found it! %s\n", str);
      fprintf(fo, "%s\n", replace);
    }
    else {
      fprintf(fo, "%s\n", str);
    }
  }

Look at the comments in the code for explanations.

Answer 2

read.c

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

/**
* How to compile program:
*    gcc read.c -o read
*
* How to run the program: 
*      .> ./read input.txt rainy output.txt sunny
* (On Windows MinGW compiler, simply: 
*      .> read input.txt rainy output.txt sunny - without ./)
*
*/
int main (int argc, char **argv) {
   FILE *fp, *fo;
   char *compare, *replace;
   char line[246];

   if (argc <= 4){
      printf(">Missing arguments on the command line.\n");
      printf(">Be sure you run the program as\n\"./read input.txt compare outout.txt replace\"\n\n");
   }

   /* Opening files for reading */
   fp = fopen(argv[1] , "r");
   if(fp == NULL){
      perror("Error opening input file");
      return 1;
   }
   compare = argv[2];

   fo = fopen(argv[3], "w");
   if(fo == NULL){
      perror("Error opening output file");
      return 1; 
   }
   replace = argv[4];

   while( fgets (line, (sizeof line), fp)!=NULL ) {
      line[strcspn(line, "\n")] = 0;
       if(strcmp(compare, line) == 0){
         printf("Found it! %s \n", line);
         fprintf(fo, "%s\n", replace);
      }
      else{
         fprintf(fo, "%s\n", line);
      } 
   }
   fclose(fp);
   fclose(fo);
   return 0;
}

/* 
Important info

strcspn :: 
Locate first occurrence of character in string, 
after locating the first occurrence of \n, replaces it by 0.


Sources::
https://stackoverflow.com/questions/2693776/removing-trailing-newline-character-from-fgets-input/28462221#28462221

Used to debug:
.>printf("1st: Reads input.txt, removes '\\n' from fgets, and prints it \n");
.>printf("2nd: Compares each line with 'rainy' \n");


.>printf("<%s>\n", line);

*/

input.txt

cloudy
rainy
chilly
rainy
rainy

Answer 3

Your approach fails because the lines read from the input file contain a trailing newline '\n' that makes the comparison return non zero.

You can strip the newline before comparing with the search string.

Note that there are other problems:

• you should verify that enough command line arguments have been passed by testing argc > 4.
• there is no need to open the output file in update mode "w+", "w" is simpler and better.
• 60 bytes is a bit small for the line array, limiting the longest line handled correctly to 58 bytes.

Here is an improved version:

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

int main(int argc, char **argv) {
    FILE *fp, *fo;
    char *compare, *replace;
    char line[256];

    if (argc <= 4) {
        printf("missing command line arguments\n");
        return 1;
    }
    fp = fopen(argv[1], "r");
    if (fp == NULL) {
        perror("Error opening input file");
        return 1;
    }
    compare = argv[2];
    fo = fopen(argv[3], "w");
    if (fo == NULL) {
        perror("Error opening output file");
        return 1;
    }
    replace = argv[4];

    while (fgets(line, sizeof line, fp) != NULL) {
        line[strcspn(line, "\n")] = '\0';
        if (strcmp(line, compare) == 0) {
            printf("fount it!);
            fprintf(fo, "%s\n", replace);
        } else {
            fprintf(fo, "%s\n", line);
        }
    }
    fclose(fp);
    fclose(fo);

    return 0;
}

Note that long lines will be broken into chunks that fit in the line array, so there may be false positives with the above naive approach.

You can remove this limitation completely with this inner loop:

int c;
int pos = 0;
int cmplen = strlen(compare);
for (;;) {
    c = getc(fp);
    if (c == '\n' || c == EOF) {
        if (pos == cmplen) {
            fprintf(fo, "%s", replace);
        } else
        if (pos > 0) {
            fprintf(fo, "%*s", pos, compare);
        }
        pos = 0;
        if (c == EOF)
            break;
    } else {
        if (pos >= 0) {
            if (compare[pos] == (char)c) {
                pos++;
                continue;
            }
            if (pos > 0) {
                fprintf(fo, "%*s", pos, compare);
            }
            pos = -1;
        }
    }
    putc(c, fo);
}