What does the: = '-'; or ='/' means in strchr(I know it does locate work and strrchr of last occurence)?

Question

What does the = '-'; or = '/' mean in strchr (I know it does locate work and strrchr of last occurrence)?
This code does create two files.

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

int main()
{
    FILE *fpA = fopen("output_A.txt", "w"); 
    FILE *fpB = fopen("output_B.txt", "w"); 

    char *strA = ",-/"; 
    char temp[100]; 
    char str[5][60] = { { "summer is coming!" },
                        { "vacation will let you chill out" },
                        { "and, have a nice time" },
                        { "and, stay fit" },
                        { "and, wish you the best" }, };
    
    fprintf(fpA, "%s\n", str[0]); 
    fprintf(fpB, "%s\n", str[1]);

    fclose(fpA); 
    fclose(fpB); 

    fpA = fopen("output_A.txt", "r"); 
    fpB = fopen("output_B.txt", "w"); 

    *(strchr(str[2], ' ')) = '-'; 
    *(strrchr(str[2], ' ') + 1) = '/'; 

    strtok(str[2], strA); 
    strcpy(temp, strtok(NULL, strA)); 

    str[1][8] = '\n'; 
    str[1][9] = '\0'; 
    strncat(temp, str[1], strlen(str[1])); 

    if (strcmp(str[3], str[4]))
        strcat(temp, str[3]);
    else
        strcat(temp, str[4]); 

    fprintf(fpA, "%s", temp);
    fprintf(fpB, "%s", temp);

    fclose(fpA);
    fclose(fpB);

    return 0; 
}

Is the printing to file really relevant for the question? The code would be a lot shorter if you just print to stdout (using `printf()`) instead of to two files. See [mre]. — wovano, Jun 10 '22 at 06:25

Roberto Caboni · Answer 1 · 2022-06-10T06:34:23.653

The statements

*(strchr(str[2], ' ')) = '-'; 
*(strrchr(str[2], ' ') + 1) = '/';

basically substitute '-' to the first space occurrence, and '/' to the character next to the last space occurrence.

The first substitution:

strchr returns the pointer to the first occurrence of the searched character (' ')
* operator access that pointer
the assignment substitutes the character '-'

The second substitution:

strrchr returns the pointer to the last occurrence of the searched character (' ')
The pointer is incremented, and after that it points to the next character (dangerous action: what if the space whas the last character of the string?)
* operator access the pointer
the assignment substitutes the character '/'

The string str[2], originally "and, have a nice time" will be changed into "and,-have a nice /ime".

Warning: you need to check strchr and strrchr return values, as they can return NULL if the searched character is not found (check this out).
If that's the case, dereferencing the pointer will lead to Undefined Behavior (probably a segmentation fault on modern computers), making your program crash.

@Eronsee I missed that '+1'. A'll adapt the answer accordingly. It's not a bog deal if you know how pointers work. — Roberto Caboni, Jun 10 '22 at 06:25

score 1 · Answer 2 · edited Jun 10 '22 at 06:49

It searches for ' ', then it assigns '-' to the position it found the first/last occurrence of ' ' (depending on strchr, strrchr). I don't really know why would it do that, but that's what the code is doing.
Sidenote: strchrand strrchr return NULL when no match for delimiter is found in the string. If the string were to have no spaces, your code would be segfaulting.
An improvement could be:

char* aux = strchr(str[2], ' ');
if (aux != NULL)
    *aux = '-';

What does the: = '-'; or ='/' means in strchr(I know it does locate work and strrchr of last occurence)?

2 Answers2