fgets() and scanf() not working properly in unison. Buffer problem encountered - c

Question

My assignment: -
Write a program that replaces the occurence of a given character (say
c) in a primary string (say PS) with another string (say s).
Input: The first line contains the primary string (PS) The next line
contains a character (c) The next line contains a string (s)
Output: Print the string PS with every occurence of c replaced by s.
Test case 1: -
Input: -
abcxy
b
mf
Expected output: -
amfcxy
Test case 2: -
Input: -
Al#bal#20owL
l
LL
Expected output: -
ALL#baLL#20owL
My code below: -
#include <stdio.h>
#include <stdlib.h>
int main() {
char PS[101];
char c;
char S[11];
fgets(PS, 101, stdin); //PS value input.
scanf("%c", &c);
if (c == '\n' || c == '\0') {
scanf("%c", &c); //Clearing the buffer. I want the real value of 'c' from STDIN not '\n'
}
fgets(S, 11, stdin); //S value input.
int i = 0;
while (PS[i] != '\0') { //Removing the '\n' from PS
if (PS[i] == '\n') {
PS[i] = '\0';
break;
}
i++;
}
i = i - 1; //i now holds the value of the size of the string PS (excluding '\0')
int j = 0;
while (S[j] != '\0') {
if (S[j] == '\n') {
S[j] = '\0';
break;
}
j++;
}
j = j - 1; //j now holds the value of the size of the string S (excluding '\0')
int k = 0; //work as an initializer
int move = 0; //work as an initializer.
while (PS[k] != '\0') { //This loops checks the whole array for the same character mentioned in char 'c'
if (PS[k] == c) {
for (move = i; move > k; move --) { //This loop advances the all the characters in PS by '(j - 1)' steps to make space for string S characters.
PS[move + (j - 1)] = PS[move];
}
for (move = 0; move < j; move++) { //This loop adds all the characters of string S into string PS at the relevant place.
PS[k + move] = S[move];
}
i = i + (j - 1); // 'i' now holds the new value of size of string PS after adding all the characters of string S.
}
k++;
}
puts(PS);
return 0;
}
Now the problem is that the code is not taking the input for string S.
After inputting first 2 inputs, it executes and gives a gibberish answer. I cannot figure out the bug, but what I do know is that there is some issue related to the buffer in C. Please help.
Edit: -
Thanks to #WeatherVane I have now edited the code with this: -
scanf("%c", &c);
if (c == '\n' || c == '\0') {
scanf("%c", &c); //Clearing the buffer. I want the real value of 'c' from STDIN not '\n'
}
char x;
x = getchar(); //New addition. It eats the '\n' after scanf().
fgets(S, 11, stdin); //S value input.
Now my code is working fine but the output is still not correct. It is sometimes failing to copy the last char from string S or giving me gibberish output.

Answer 1

The problem with the code was: -
i = i - 1; //i now holds the value of the size of the string PS (excluding '\0')
j = j - 1; //j now holds the value of the size of the string S (excluding '\0')
The value of i and j are the true values of the size of string PS and string S; not i = i - 1 and j = j - 1.
Lesson learnt from this assignment: -
scanf() does not treat '\n' in any way. It WILL be left in the
buffer.
If possible use fgets and then remove '\n' from your respective array/pointer.
Be extra careful of your C buffer when dealing with chars and strings.
The final correct code is: -
#include <stdio.h>
#include <stdlib.h>
int main()
{
char PS[101];
char c;
char S[11];
fgets(PS, 101, stdin); //PS value input.
scanf("%c", &c);
if(c == '\n' || c == '\0')
{
scanf("%c", &c); //Clearing the buffer. I want the real value of 'c' from STDIN not '\n'
}
char x;
x = getchar(); //New addition. It eats the '\n' after scanf().
fgets(S, 11, stdin); //S value input.
int i = 0;
while(PS[i] != '\0') //Removing the '\n' from PS
{
if(PS[i] == '\n')
{
PS[i] = '\0';
break;
}
i++;
}
i = i; //i now holds the value of the size of the string PS (excluding '\0')
int j = 0;
while(S[j] != '\0')
{
if(S[j] == '\n')
{
S[j] = '\0';
break;
}
j++;
}
j = j; //j now holds the value of the size of the string S (excluding '\0')
int k = 0; //work as an initializer
int move = 0; //work as an initializer.
while(PS[k] != '\0') //This loops checks the whole array for the same character mentioned in char 'c'
{
if(PS[k] == c)
{
for(move = i; move > k; move --) //This loop advances the all the characters in PS by '(j - 1)' steps to make space for string S characters.
{
PS[move + (j - 1)] = PS[move];
}
for(move = 0; move < j; move++) //This loop adds all the characters of string S into string PS at the relevant place.
{
PS[k + move] = S[move];
}
i = i + (j - 1); // 'i' now holds the new value of size of string PS after adding all the characters of string S.
}
k++;
}
puts(PS);
return 0;
}
Warning: -
The above code is very unoptimised and unreadable. Do not use it for
long term projects. It just "works".
Any suggestions for improvements of the above code are welcomed in
the comments.
Further necessary reading material recommended if you face any issue regarding C buffer in the future: -
Read 1
Read 2