Reversing array logic in C not working properly

Question

I am trying to read the input to my program (a string of chars) and invert the order of the words that are in it.

For example, if I were to pass my program ABC DEF GHI JKL it would output JKL GHI DEF ABC. I am using the whitespace as separators.

My code:

char toReverse[1000];
char outputArray[1000];
int charCount = //Size of the toReverse array. Varies on the input
                //It is the total number of chars stored in the array
...
int i;
int tempCharCount = charCount;
int wordSize = 0;
int outputIndex = 0;
int sentenceIndex = 0;
int charStep = 0;
for(i = charCount-1; i>=0; i--){
    if(toReverse[i] == ' '){
        int j;
        sentenceIndex = tempCharCount - wordSize;
        for(j = 0; j<charStep; j++){
            outputArray[outputIndex++] = toReverse[sentenceIndex++];
        }
        outputArray[outputIndex] = ' ';
        outputIndex++;
        charStep = 0;
    }
    wordSize++;
    charStep++;

}

There is a flaw in my code. I do know why this happens though. For example, if I were to pass as input ABC DEF GHI, it will only output GHI DEF. This is because whenever the outer for loop reaches index 0 of my toReverse array, since it is not a space ' ', it does not do the if(toReverse[i]) inner for(j = 0; j<charStep; j++) since the condition is not met.

Do you have any advice regarding to the logic? I have tried reversing my logic, such as if(toReverse[i] != ' ') but it brings more problems than it solves.

Thanks for your advice and comments!

Cheers

Edit 1

I am reading my input from a file By the way!

Update 1

Here I am basically trying to open various files and read chars from them

int main(int argc, char * argv[])
{
    int i = 1;
    FILE * fp = NULL;

    if(1==argc){
    do_read(stdin);
    }else{
    // cycle through all files in command line arguments and read them
        for (i=1; i < argc; i++) {
            if ((fp = fopen(argv[i], "r")) == NULL) {
                printf("Failed to open file.\n");
            }
            else {
                do_read(fp);
                fclose(fp);

            }
        }
    }

    //printf("\n");

    //printf("\n");
    printf("%i",charCount);
    return 0;
}

`char toReverse[1000] = " "; scanf("%[^\n]", &toReverse[1]);` — BLUEPIXY, May 04 '15 at 19:29
I don't think I can use this approach if I am reading the input from a file, or can I? — Cesar A, May 04 '15 at 19:32
Eventually you either have to read the whole file in memory or read file block by block from the end back to the beginning (if you have random access to file and can use fseek), By the way my answer works in O(n) and is optimal for large data — Mohammad Jafar Mashhadi, May 25 '15 at 12:13

BLUEPIXY · Answer 1 · 2015-05-05T01:31:00.310

2

sample

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

void proc_rev(char toReverse[], char outputArray[]){
    int charCount = strlen(toReverse);
    int i;
    int tempCharCount = charCount;
    int wordSize = 0;
    int outputIndex = 0;
    int sentenceIndex = 0;
    int charStep = 0;
    for(i = charCount-1; i>=0; i--){
        if(toReverse[i] == ' '){
            int j;
            sentenceIndex = tempCharCount - wordSize;
            for(j = 0; j<charStep; j++){
                outputArray[outputIndex++] = toReverse[sentenceIndex++];
            }
            outputArray[outputIndex] = ' ';
            outputIndex++;
            charStep = 0;
        }
        wordSize++;
        charStep++;
    }
    outputArray[outputIndex] = '\0';
}

int main(void){
    FILE *fp = stdin;

    char toReverse[1000] = " ";
    char outputArray[1000];

    while(1 == fscanf(fp, "%998[^\n]%*c", &toReverse[1])){
        proc_rev(toReverse, outputArray);
        puts(outputArray);
    }

    return 0;
}

void do_read(FILE *fp){
    char toReverse[1000] = " ";
    char outputArray[1000];

    while(1 == fscanf(fp, "%998[^\n]%*c", &toReverse[1])){
        proc_rev(toReverse, outputArray);
        puts(outputArray);
    }
}

edited May 05 '15 at 01:31

answered May 05 '15 at 00:34

BLUEPIXY

39,699
7
33
70

How would I print it? – Cesar A May 05 '15 at 01:16
Also, I am trying to read chars from a file... and I would like to use say, the feed from stdin if the command `myprog < file1` is given, and the `fopen` and `fclose` functions to read from files if the program is called such as `myprog file1 file2`. Could you help me achieve this? See my update for more code – Cesar A May 05 '15 at 01:20
By the way, my do_read(fp) function would be the equivalent to your proc_rev fun – Cesar A May 05 '15 at 01:23
I think that there is no problem. – BLUEPIXY May 05 '15 at 01:27
How would you set it up though? Could you explain the while() condition? how come you are using pointers? When can I use pointers? – Cesar A May 05 '15 at 01:30
I add sample of `do_read`. `1 == fscanf(fp, "%998[^\n]%*c", &toReverse[1])` Read one line from file. We can use pointers. (e.g FILE *fp) – BLUEPIXY May 05 '15 at 01:34
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/76943/discussion-between-cesar-a-and-bluepixy). – Cesar A May 05 '15 at 01:36

score 1 · Answer 2 · answered May 04 '15 at 19:37

This code is not tested but basic idea is reversing the whole string once and then reverse it word by word. idea is correct, implementation may have bugs

void swap(char* s, int i, int j) {
    char tmp = s[i];
    s[i] = s[j];
    s[j] = tmp;
}


void rev(char* stirng, int start, int len) {
    for (int i=0; i<len/2; ++i) {
       swap(string, i, len-i-1);
    }
}

int main() {
    char* string = read from file
    int len = strlen(string);
    rev(string, 0, len);

    for (int i=0; i<len;) {
        int l = 0;
        int j=i;
        while (j<len && string[j]!=' ') ++j;
        rev(string, i, j-i);
        i=j+1;
    }
}

score 0 · Answer 3 · answered May 04 '15 at 19:53

0

The current logic reverses individual words from last to second word. However to reverse the first word you will have to add a check apart from if(toReverse[i] == ' ') as the first character need not be a space. A separate check can be used when counter 'i' reaches zero, i.e. first character

answered May 04 '15 at 19:53

Akshay

36
6

is this an answer or a comment on BLUEPIXY's answer? – Mohammad Jafar Mashhadi May 25 '15 at 12:14

Reversing array logic in C not working properly

3 Answers3