C - Reading just one character with getchar() - c

Question

I'm having some trouble with getchar(), in particular I have a char which, in a while loop, get the value returned from getchar() but I want to take just the first char and, if I insert a longer string (like "aaawssdawa"), I still want just the first char. My code, instead, processes the entire string.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#define ROWS 20
#define COLUMNS 65
char grid[ROWS][COLUMNS];
int score = 0;
void fillGridInitializer();
void printGrid();
void start();
void printScore();
void printAll();
int main(int argc, char *argv[]) {
start();
return 0;
}
void start() {
char movement;
int riga = 0, colonna = 0;
fillGridInitializer();
grid[riga][colonna] = '#';
system("clear");
printAll();
while (1) {
movement = getchar();
switch (movement) {
case 'w':
if ((riga - 1) >= 0) {
grid[riga][colonna] = '-';
riga = riga - 1;
}
break;
case 's':
if ((riga + 1) < ROWS) {
grid[riga][colonna] = '-';
riga = riga + 1;
}
break;
case 'a':
if ((colonna - 1) >= 0) {
grid[riga][colonna] = '-';
colonna = (colonna - 1) % COLUMNS;
}
break;
case 'd':
if ((colonna + 1) < COLUMNS) {
grid[riga][colonna] = '-';
colonna = (colonna + 1) % COLUMNS;
}
break;
default:
break;
}
if (movement == 'p') {
printf("+++++Game Over+++++\n\n");
break;
}
system("clear");
grid[riga][colonna] = '#';
printAll();
}
}
void printAll() {
printScore();
printGrid();
}
void fillGridInitializer() {
int i = 0, j = 0;
for (i = 0; i < ROWS; i++) {
for (j = 0; j < COLUMNS; j++) {
grid[i][j] = '-';
}
}
}
void printScore() { printf("\t SCORE: %d\n", score); }
void printGrid() {
int i = 0, j = 0;
for (i = 0; i < ROWS; i++) {
printf("\t");
for (j = 0; j < COLUMNS; j++) {
printf("%c", grid[i][j]);
}
printf("\n");
}
}

Answer 1

After you process the first character, call getchar() in a loop until you get a newline or EOF.
Also, getchar() returns int, you should declare the variable accordingly, so you can compare with EOF properly.
void start() {
int movement;
int riga = 0, colonna = 0;
fillGridInitializer();
grid[riga][colonna] = '#';
system("clear");
printAll();
while (1) {
movement = getchar();
if (movement == EOF) {
break;
}
switch (movement) {
case 'w':
if ((riga - 1) >= 0) {
grid[riga][colonna] = '-';
riga = riga - 1;
}
break;
case 's':
if ((riga + 1) < ROWS) {
grid[riga][colonna] = '-';
riga = riga + 1;
}
break;
case 'a':
if ((colonna - 1) >= 0) {
grid[riga][colonna] = '-';
colonna = (colonna - 1) % COLUMNS;
}
break;
case 'd':
if ((colonna + 1) < COLUMNS) {
grid[riga][colonna] = '-';
colonna = (colonna + 1) % COLUMNS;
}
break;
default:
break;
}
if (movement == 'p') {
printf("+++++Game Over+++++\n\n");
break;
}
int ch;
while ((ch = getchar()) != '\n' && ch != EOF) {} # ignore the rest of the line
if (ch == EOF) {
break;
}
system("clear");
grid[riga][colonna] = '#';
printAll();
}
}

Answer 2

Assuming you just want to read a char and discard everything else in the same line, you can do the following:
Just read a line and only save the first char.
But, you shouldn't use this as it it vulnerable to buffer overflows(read what comes after it).
char read(){
char buf[BUF_SIZE];
gets(buf);
return buf[0];
}
But you shouldn't use gets because of possible buffer overflows.
In order to prevent that, you could use fgets(see this):
char read(){
char buf[BUF_SIZE];
fgets(buf,BUF_SIZE,stdin);
return buf[0];
}
In both cases, BUF_SIZE is a constant defining the maximum length of the string. The second example is buffer overflow protected but it will only work if the line ends before the buffer ends. You can just call fgets multiple times in order to bypass this issue:
char read(){
char buf[BUF_SIZE];
do{
fgets(buf, BUF_SIZE,stdin);
}while(buf[strlen(buf)]!='\n');
return buf[0];
}
Note that the last method needs <string.h>.
Integrated in your code, it would be something like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#define ROWS 20
#define COLUMNS 65
#define BUF_SIZE 255
char grid[ROWS][COLUMNS];
int score = 0;
void fillGridInitializer();
void printGrid();
void start();
void printScore();
void printAll();
int main(int argc, char *argv[]) {
start();
return 0;
}
void start() {
char movement;
int riga = 0, colonna = 0;
fillGridInitializer();
grid[riga][colonna] = '#';
system("clear");
printAll();
while (1) {
movement = read();
switch (movement) {
case 'w':
if ((riga - 1) >= 0) {
grid[riga][colonna] = '-';
riga = riga - 1;
}
break;
case 's':
if ((riga + 1) < ROWS) {
grid[riga][colonna] = '-';
riga = riga + 1;
}
break;
case 'a':
if ((colonna - 1) >= 0) {
grid[riga][colonna] = '-';
colonna = (colonna - 1) % COLUMNS;
}
break;
case 'd':
if ((colonna + 1) < COLUMNS) {
grid[riga][colonna] = '-';
colonna = (colonna + 1) % COLUMNS;
}
break;
default:
break;
}
if (movement == 'p') {
printf("+++++Game Over+++++\n\n");
break;
}
system("clear");
grid[riga][colonna] = '#';
printAll();
}
}
void printAll() {
printScore();
printGrid();
}
void fillGridInitializer() {
int i = 0, j = 0;
for (i = 0; i < ROWS; i++) {
for (j = 0; j < COLUMNS; j++) {
grid[i][j] = '-';
}
}
}
void printScore() { printf("\t SCORE: %d\n", score); }
void printGrid() {
int i = 0, j = 0;
for (i = 0; i < ROWS; i++) {
printf("\t");
for (j = 0; j < COLUMNS; j++) {
printf("%c", grid[i][j]);
}
printf("\n");
}
}
char read(){
char buf[BUF_SIZE];
do{
fgets(buf, BUF_SIZE,stdin);
}while(buf[strlen(buf)]!='\n');
return buf[0];
}