How does this code print the last word the user inputs?

Question

This small piece of code takes a user input and returns the last word from the user input.

I don't exactly understand how those 2 lines of code work

can someone explain why or how printf( &last_word[i]); when I is at position 0 prints the last word of the sentence entered by the user input.

the purpouse of the program is to print the last word of the user input, but I dont understand those lines of code, If I change something on those 2 lines, the code stops working and it doesnt return the last word entered by the user.

#include <stdio.h>

int main(){    
int i = 0;
char *last_word;
char str[800];
printf("enter:");
fgets(str,100,stdin);

while (str[i] != '\0')    {    
    if (str[i] == ' ' && str[i + 1] > 32) 
        last_word = &str[i + 1]; 
    i++;                         
}

 /*------------------------these next lines of code-----------------------*/
 /*----->    i = 0; 
 /*----->    printf( &last_word[i]);


       return 0;
    }

Make sure to always use a string literal as the first argument of `printf`. — S.S. Anne, Sep 14 '19 at 22:31
@JL2210 Oops, typp: Code fails if `str[]` lacks a `' '` as .`last_word` is later used, but never set. — chux - Reinstate Monica, Sep 15 '19 at 03:34

Joshua · Accepted Answer · 2019-09-14T22:53:11.517

Holy undefined behavior batman.

The while loop stuffs the address of the next character past a space character into last_word; thus if there are any space characters it points one past the last one.

i = 0 therefore &last_word[i] is the same as last_word. So it uses the last word as a printf format string.

But that's not the same as last word. Try this input: " %n%n%n%n%n%n%n%n". Goodbye.

The first argument to printf should almost always be a constant. You want printf("%s", last_word); In fact, let us put it this way; the only reason to ever not pass a constant format string to printf and friends is your standard library is too old to support %*d for dynamic widths.

How does this code print the last word the user inputs?

1 Answers1