how to change a word in a string by its number? - c

Question

I have this assignment:
Replace the numbered word in the sentence with three characters: ???
The problem is I can't replace the whole word by its number, my program changes only 3 first characters.
Here is my code:
#include <stdio.h>
#include <string.h>
int main() {
char sentence[100];
int word_number;
int pos;
int i;
printf("Enter sentence: ");
fgets(sentence, 100, stdin);
printf("Enter word number: ");
scanf("%d", &word_number);
pos = 0;
for (i = 0; i < strlen(sentence); i++) {
if (sentence[i] == ' ') {
word_number--;
}
if (word_number == 1) {
pos = i;
break;
}
}
// Update the sentence
sentence[pos] = '?';
sentence[pos + 1] = '?';
sentence[pos + 2] = '?';
printf("Result sentence: %s\n", sentence);
return 0;
}

Answer 1

There are multiple issues in the code:
Your approach only works if the word to be replaced has exactly 3 letters. For longer or shorter words, you need to move the contents of the remainder of the string.
Another restriction in your code is you assume that words a separated by a single space, which may of may not be explicitly stated in the assignment.
Similarly, you assume the n-th word is followed by a space, which may not be the case if the word is the last one in the sentence.
you should check the return values of fgets() and scanf() to detect invalid or missing input.
Here is a modified version.
#include <stdio.h>
#include <string.h>
int is_sep(char cc) {
// separators are space, newline and the null byte
return (cc == ' ' || cc == '\n' || cc == '\0');
// you could also accept punctuation as word separators:
//return strchr(" \t\n.,:;?!'`"##$%^&*()[]{}<>/`~+=|", cc);
}
int main() {
char sentence[200];
int word_number;
printf("Enter sentence: ");
// read a sentence, allow for 2 extra bytes to insert replacement
if (!fgets(sentence, sizeof(sentence) - 2, stdin))
return 1;
printf("Enter word number: ");
if (scanf("%d", &word_number) != 1)
return 1;
int len = strlen(sentence);
int start = 0;
int wn = 1;
char last = ' ';
for (int i = 0;; i++) {
char cc = sentence[i];
if (is_sep(cc)) {
if (!is_sep(last)) {
/* end of word */
if (wn == word_number) {
// replace this word with `???`
memmove(sentence + start + 3, sentence + i, len + 1 - i);
memcpy(sentence + start, "???", 3);
break;
}
wn++;
}
} else {
if (is_sep(last)) {
/* start of a word */
start = i;
}
}
last = cc;
if (cc == '\0')
break;
}
printf("Result sentence: %s\n", sentence);
return 0;
}

Answer 2

I’m not sure I’d try to do this in-place. For small data, it doesn’t hurt to just create a new buffer, even if it is just a temporary.
But, since we are posting solutions, here’s a single-pass version:
#include <iso646.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void replace_nth_word_with( char * dest, int n, const char * src, const char * word )
{
char last = ' ';
while (*src) // For each character in src:
if ((*src != ' ') and (last == ' ') and !n--) // If we found a word-begin AND it is the Nth word:
{ //
for (size_t k = 0; word[k]; k++) // Copy the new word to dest
*dest++ = word[k]; //
while (*src and (*src != ' ')) ++src; // Skip the old word in src
} //
else // Otherwise just copy the character over
last = *dest++ = *src++; //
*dest = '\0'; // End of string
}
void ask_string( const char * prompt, char * s, size_t n )
{
printf( "%s", prompt );
fgets( s, (int)n, stdin );
char * p = strpbrk( s, "\n\r" );
if (p) *p = '\0';
}
void ask_int( const char * prompt, int * n )
{
char s[20];
printf( "%s", prompt );
fgets( s, sizeof s, stdin );
*n = strtol( s, NULL, 10 );
// It hurts to write this without error checking, LOL
}
int main(void)
{
char sentence[1000];
ask_string( "Sentence? ", sentence, sizeof sentence );
int word_number;
ask_int( "Word number (counting from 1)? ", &word_number );
char new_word[100];
ask_string( "New word? ", new_word, sizeof new_word );
char new_sentence[sizeof sentence + sizeof new_word];
replace_nth_word_with( new_sentence, word_number-1, sentence, new_word );
printf( "\"%s\"\n", new_sentence );
return 0;
}