Thank you @Riva! I fixed those mistakes and slightly changed the code:
#include<Arduino.h>
int a;
int b;
int c;
int d;
int e;
int f;
int g;
void sevensegpins(int z, int y, int x, int w, int v, int u, int t){
a=z;
b=y;
c=x;
d=w;
e=v;
f=u;
g=t;
pinMode(0, INPUT);
pinMode(a, OUTPUT);
pinMode(b, OUTPUT);
pinMode(c, OUTPUT);
pinMode(d, OUTPUT);
pinMode(e, OUTPUT);
pinMode(f, OUTPUT);
pinMode(g, OUTPUT);
}
void clearall(){
digitalWrite(a, LOW);
digitalWrite(b, LOW);
digitalWrite(c, LOW);
digitalWrite(d, LOW);
digitalWrite(e, LOW);
digitalWrite(f, LOW);
digitalWrite(g, LOW);
}
void zero(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(d, HIGH);
digitalWrite(f, HIGH);
digitalWrite(e, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
}
void one(){
clearall();
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
}
void two(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(g, HIGH);
digitalWrite(e, HIGH);
digitalWrite(d, HIGH);
}
void three(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(g, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
}
void four(){
clearall();
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
}
void five(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
}
void six(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(f, HIGH);
digitalWrite(e, HIGH);
digitalWrite(g, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
}
void seven(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
}
void eight(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
}
void nine(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
}
void lettera(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
}
void letterb(){
clearall();
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(d, HIGH);
digitalWrite(c, HIGH);
digitalWrite(g, HIGH);
}
void letterc(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(d, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
}
void letterd(){
clearall();
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
}
void lettere(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(d, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
}
void letterf(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(g, HIGH);
}
void letterh(){
clearall();
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(g, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
}
void letterl(){
clearall();
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(d, HIGH);
}
void letterp(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
}
void letterq(){
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
}
void letters(){
clearall();
digitalWrite(a, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
}
void letteru(){
clearall();
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
}
void sevenseg(int val){
if(val==0){
zero();
}
if(val==1){
one();
}
if(val==2){
two();
}
if(val==3){
three();
}
if(val==4){
four();
}
if(val==5){
five();
}
if(val==6){
six();
}
if(val==7){
seven();
}
if(val==8){
eight();
}
if(val==9){
nine();
}
}