Thread: Reverse String

The reverse-problem was not forgotten, and from time
to time I have tried to find a solution. Now I found
an answer but for Windows only. - This program run correctly:

Code:

//Reverse a sentence by "mh2x", and "strrev" reverse it 
//than word by word. For Windows only. - Oktober 2018


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


char *ReverseChars(char * pStart, char * pEnd);
char *ReverseSentence(char * pSentence);


int main(void)
{
  char sentence[] = "They never come back!";


  ReverseSentence(sentence);
		
	//Bad! ;)
  printf("%s\n", strrev(sentence));


	return 0;
}


char *ReverseChars(char *pStart, char *pEnd)
{
	if (NULL == pStart || NULL == pEnd)
		return NULL;


	char *pString = pStart;
	
	while (pEnd > pStart)
	{
		char temp = *pStart;
		*pStart = *pEnd;
		*pEnd = temp;


		pStart++;
		pEnd--;
	}
	return pString;
}


char *ReverseSentence(char *pSentence)
{
	if (NULL == pSentence)
		return NULL;


	char *pStart = pSentence;
	char *pEnd = pSentence;


	while (*pEnd)
	{
		if (*pEnd == ' ')
		{
			//Reverse a word
			ReverseChars(pStart, pEnd - 1);
			pStart = pEnd + 1;
		}
		pEnd++;
	}
	//Revese last word
	ReverseChars(pStart, pEnd-1);
	
	//Reverse the whole sentence
	ReverseChars(pSentence, pEnd - 1);


	return pSentence;
}

This is still a problem:

Code:

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


char *ReverseChars(char * pStart, char * pEnd);
char *ReverseSentence(char * pSentence);


int main(int argc, char *argv[])
{
	if (argc < 2) return 1;
	char *dateiname = argv[1];
	FILE *dz = fopen(dateiname, "r");
	if (dz == NULL) return 1;
	
	char sentence[100];
	char c;
	while((sentence[] = fgetc(dz)) != EOF)
	{
		//sentence[c]; Error!
	}
	fclose(dz);	
 
  ReverseSentence(sentence);
		
	//Bad! ;)
  printf("%s\n", strrev(sentence));


	return 0;
}

. . . and so on.

Where is the thinking error?

So please describe the problem.

If you're getting compile errors post the complete error messages, all of them, exactly as they appear in your development environment.

By the way do you realize that strrev() is a non-standard function?

Originally Posted by jimblumberg

So please describe the problem.

If you're getting compile errors post the complete error messages, all of them, exactly as they appear in your development environment.

By the way do you realize that strrev() is a non-standard function?

Code:

int main(int argc, char *argv[])
{
    if (argc < 2) return 1;
    char *dateiname = argv[1];
    FILE *dz = fopen(dateiname, "r");
    if (dz == NULL) return 1;
    
    char sentence[100];
    char c;
    //Mit "[]" oder ohne - Fehler
    while((sentence[] = fgetc(dz)) != EOF)
    {
        sentence[c];
    }
    fclose(dz);    
 
  ReverseSentence(sentence);
        
    //Bad! ;)
  printf("%s\n", strrev(sentence));


    return 0;
}

The compile errors:


If the comment code out than are no errors, but also no output. Look first screenshot above. I understand the message from the compiler but I don't find an answer. - Thanks for Your effort.

The original code is so (this run correctly):

Code:

int main(int argc, char *argv[])
{
    //This program reads a file and prints it word by word.
    if (argc < 2) return 1;
    char *dateiname = argv[1];
    FILE *dz = fopen(dateiname, "r");
    if (dz == NULL) return 1;
    
    char c;
                 
    while((c = fgetc(dz)) != EOF)
    {
         printf("%c", c);
    }
    fclose(dz);    
    
    return 0;
}

. . . and I thought I could doing the input in the array "sentence[]". Wrong thought!

Thanks for Your tip @laserlight! So run it correctly - almost. Where does the droll signs come from?

Code:

//Reverse a sentence by "mh2x", and "strrev" reverse it 
//than word by word. For Windows only. - Oktober 2018

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


char *ReverseChars(char *pStart, char *pEnd);
char *ReverseSentence(char *pSentence);


int main(int argc, char *argv[])
{
    if (argc < 2) return 1;
    char *dateiname = argv[1];
    FILE *dz = fopen(dateiname, "r");
    if (dz == NULL) return 1;
    
    char *sentence;
    char c;
    int z = 0;
    
    while((c = fgetc(dz)) != EOF)
    {
        sentence[z++] = c;        
    }
    fclose(dz);    
 
  for(int i = 0; i < z; i++)
        putchar(sentence[i]);
    
    printf("\n\n");
    ReverseSentence(sentence);

    printf("%s", strrev(sentence));
    printf("\n\n");

    return 0;
}


char *ReverseChars(char *pStart, char *pEnd)
{
    if (NULL == pStart || NULL == pEnd)
        return NULL;


    char *pString = pStart;
    
    while (pEnd > pStart)
    {
        char temp = *pStart;
        *pStart = *pEnd;
        *pEnd = temp;


        pStart++;
        pEnd--;
    }
    return pString;
}


char *ReverseSentence(char *pSentence)
{
    if (NULL == pSentence)
        return NULL;


    char *pStart = pSentence;
    char *pEnd = pSentence;


    while (*pEnd)
    {
        if (*pEnd == ' ')
        {
            //Reverse a word
            ReverseChars(pStart, pEnd - 1);
            pStart = pEnd + 1;
        }
        pEnd++;
    }
    //Revese last word
    ReverseChars(pStart, pEnd-1);
    
    //Reverse the whole sentence
    ReverseChars(pSentence, pEnd - 1);


    return pSentence;
}

When you build a string character-by-character, you should take care to make sure it ends with '\0' since any of the native string things are going to expect it. Your putchar loop gets away with it, since you're looping based on the number of characters read; however, your ReverseSentence uses the null character to find the end of the string, so since you didn't put one in it keeps going.