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###int vs size_t Accordingly to the c language specification, it's highly recommendable to use size_t as the type for index variables. So in your case...

int i = 0, k = 0;

should be:

size_t i = 0, k = 0;

###if condition repeats while condition I was a bit confused by this piece of code

while (input[j] != '0円' && input[j] == ' ') {
 if (input[j] == ' ') {
 i++; // keep indexing input-index
 } 
 j++; // keep incrementing temporary indexer.
} 

Specifically the if condition - it will always be true due to the while condition logic. So, as result, variable j just duplicates i with an invariant j = i+1.

###Lack of null character If you test your code on any string input, you will notice that the program prints some strange characters after your result. It happens because you didn't copy the 0円 character to the output array.

Taking all this into an account (as well as @pacmaninbw's and @holroy's recommendations)

#include <stdio.h>
#include <ctype.h>
#define MAX 100
int
main ()
{
 char input [MAX], output[MAX];
 fgets(input, MAX, stdin);
 size_t inputIndex = 0, outputIndex = 0;
 while (input[inputIndex] != '0円')
 { 
 output[outputIndex++] = input[inputIndex];
 if (input[inputIndex] == ' ')
 while(input[++inputIndex] == ' ')
 ; 
 else
 inputIndex++;
 } 
 output[outputIndex] = '0円';
 printf("%s", output);
}

int vs size_t

Accordingly to the c language specification, it's highly recommendable to use size_t as the type for index variables. So in your case...

int i = 0, k = 0;

should be:

size_t i = 0, k = 0;

if condition repeats while condition

I was a bit confused by this piece of code

while (input[j] != '0円' && input[j] == ' ') {
 if (input[j] == ' ') {
 i++; // keep indexing input-index
 } 
 j++; // keep incrementing temporary indexer.
} 

Specifically the if condition - it will always be true due to the while condition logic. So, as result, variable j just duplicates i with an invariant j = i+1.

Lack of null character

If you test your code on any string input, you will notice that the program prints some strange characters after your result. It happens because you didn't copy the 0円 character to the output array.

Taking all this into an account (as well as @pacmaninbw's and @holroy's recommendations)

#include <stdio.h>
#include <ctype.h>
#define MAX 100
int
main ()
{
 char input [MAX], output[MAX];
 fgets(input, MAX, stdin);
 size_t inputIndex = 0, outputIndex = 0;
 while (input[inputIndex] != '0円')
 { 
 output[outputIndex++] = input[inputIndex];
 if (input[inputIndex] == ' ')
 while(input[++inputIndex] == ' ')
 ; 
 else
 inputIndex++;
 } 
 output[outputIndex] = '0円';
 printf("%s", output);
}

###int vs size_t Accordingly to the c language specification, it's highly recommendable to use size_t as the type for index variables. So in your case...

int i = 0, k = 0;

should be:

size_t i = 0, k = 0;

###if condition repeats while condition I was a bit confused by this piece of code

while (input[j] != '0円' && input[j] == ' ') {
 if (input[j] == ' ') {
 i++; // keep indexing input-index
 } 
 j++; // keep incrementing temporary indexer.
} 

Specifically the if condition - it will always be true due to the while condition logic. So, as result, variable j just duplicates i with an invariant j = i+1.

###Lack of NULL termination If you test your code on any string input, you will notice that the program prints some strange characters after your result. It happens because you didn't copy the NULL symbol to the output array.

Taking all this into an account (as well as @pacmaninbw's and @holroy's recommendations)

#include <stdio.h>
#include <ctype.h>
#define MAX 100
int
main ()
{
 char input [MAX], output[MAX];
 fgets(input, MAX, stdin);
 size_t inputIndex = 0, outputIndex = 0;
 while (input[inputIndex] != '0円')
 { 
 output[outputIndex++] = input[inputIndex];
 if (input[inputIndex] == ' ')
 while(input[++inputIndex] == ' ')
 ; 
 else
 inputIndex++;
 } 
 output[outputIndex] = '0円';
 printf("%s", output);
}

###int vs size_t Accordingly to the c language specification, it's highly recommendable to use size_t as the type for index variables. So in your case...

int i = 0, k = 0;

should be:

size_t i = 0, k = 0;

###if condition repeats while condition I was a bit confused by this piece of code

while (input[j] != '0円' && input[j] == ' ') {
 if (input[j] == ' ') {
 i++; // keep indexing input-index
 } 
 j++; // keep incrementing temporary indexer.
} 

Specifically the if condition - it will always be true due to the while condition logic. So, as result, variable j just duplicates i with an invariant j = i+1.

###Lack of null character If you test your code on any string input, you will notice that the program prints some strange characters after your result. It happens because you didn't copy the 0円 character to the output array.

Taking all this into an account (as well as @pacmaninbw's and @holroy's recommendations)

#include <stdio.h>
#include <ctype.h>
#define MAX 100
int
main ()
{
 char input [MAX], output[MAX];
 fgets(input, MAX, stdin);
 size_t inputIndex = 0, outputIndex = 0;
 while (input[inputIndex] != '0円')
 { 
 output[outputIndex++] = input[inputIndex];
 if (input[inputIndex] == ' ')
 while(input[++inputIndex] == ' ')
 ; 
 else
 inputIndex++;
 } 
 output[outputIndex] = '0円';
 printf("%s", output);
}

###int vs size_t Accordingly to the c language specification, it's highly recommendable to use size_t as the type for index variables. So in your case...

int i = 0, k = 0;

should be:

size_t i = 0, k = 0;

###if condition repeats while condition I was a bit confused by this piece of code

while (input[j] != '0円' && input[j] == ' ') {
 if (input[j] == ' ') {
 i++; // keep indexing input-index
 } 
 j++; // keep incrementing temporary indexer.
} 

Specifically the if condition - it will always be true due to the while condition logic. So, as result, variable j just duplicates i with an invariant j = i+1.

###Lack of NULL termination If you test your code on any string input, you will notice that the program prints some strange characters after your result. It happens because you didn't copy the NULL symbol to the output array.

Taking all this into an account (as well as @pacmaninbw's and @holroy's recomndations)

#include <stdio.h>
#include <ctype.h>
#define MAX 100
int
main ()
{
 char input [MAX], output[MAX];
 fgets(input, MAX, stdin);
 size_t inputIndex = 0, outputIndex = 0;
 while (input[inputIndex] != '0円')
 { 
 output[outputIndex++] = input[inputIndex];
 if (input[inputIndex] == ' ')
 while(input[++inputIndex] == ' ')
 ; 
 else
 inputIndex++;
 } 
 output[outputIndex] = '0円';
 printf("%s", output);
}

###int vs size_t Accordingly to the c language specification, it's highly recommendable to use size_t as the type for index variables. So in your case...

int i = 0, k = 0;

should be:

size_t i = 0, k = 0;

###if condition repeats while condition I was a bit confused by this piece of code

while (input[j] != '0円' && input[j] == ' ') {
 if (input[j] == ' ') {
 i++; // keep indexing input-index
 } 
 j++; // keep incrementing temporary indexer.
} 

Specifically the if condition - it will always be true due to the while condition logic. So, as result, variable j just duplicates i with an invariant j = i+1.

###Lack of NULL termination If you test your code on any string input, you will notice that the program prints some strange characters after your result. It happens because you didn't copy the NULL symbol to the output array.

Taking all this into an account (as well as @pacmaninbw's and @holroy's recommendations)

#include <stdio.h>
#include <ctype.h>
#define MAX 100
int
main ()
{
 char input [MAX], output[MAX];
 fgets(input, MAX, stdin);
 size_t inputIndex = 0, outputIndex = 0;
 while (input[inputIndex] != '0円')
 { 
 output[outputIndex++] = input[inputIndex];
 if (input[inputIndex] == ' ')
 while(input[++inputIndex] == ' ')
 ; 
 else
 inputIndex++;
 } 
 output[outputIndex] = '0円';
 printf("%s", output);
}