#include <stdio.h>
#define MAXN 20


int readmarks(FILE* fin, int* id, float* mark);
int changemarks(int* id,float*mark, int n, int studentid, float* addr_revisedmark);
void writemarks(FILE* fin, int* id, float* mark, int n);
int main(void){

	FILE* fin = fopen("marksin.txt","r");

	FILE* fout = fopen("marksout.txt","w");

	if(fin==NULL){
		printf("The input file is missing!");
		return -1;
	}


	float revisedmark,mark[MAXN];
	int n = 0,studentid,id[MAXN],r; // initialize counter to zero

	n = readmarks(fin, id, mark);
	printf("Would you like to enter a revised mark? If yes, enter a student id and the new mark. If no, enter a student id of 0.");
	//Once the arrays have been read, the main then prompts the user to enter a student ID and the revised mark
	printf("Enter student ID: ");

	scanf("%d",studentid);
	if(studentid==0){


	}else{
		printf("Enter revised mark: ");
		scanf("%f",revisedmark);
		r=changemarks(id,mark,n,studentid,&revisedmark);
		if(r==0){
			printf("That student is not on the marks list.");

		}
		printf("Enter student ID and revised mark: ");
	}



	writemarks(fout, id, mark, n);


	fclose(fin);
	fclose(fout);
	return 0;
}

int readmarks(FILE* fin, int* id, float* mark){
	int n;
		while (fscanf(fin, "%d%f", &id[n],&mark[n]) != EOF) {
		n++; // update n
	}


	return n;
}
int changemarks(int* id,float*mark, int n, int studentid, float* addr_revisedmark){



	if(studentid==id[0]){
		mark[0]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[1]){
		mark[1]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[2]){
		mark[2]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[3]){
		mark[3]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[4]){
		mark[4]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[5]){
		mark[5]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[6]){
		mark[6]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[7]){
		mark[7]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[8]){
		mark[8]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[9]){
		mark[9]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[9]){
		mark[9]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[9]){
		mark[9]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[10]){
		mark[10]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[11]){
		mark[11]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[12]){
		mark[12]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[13]){
		mark[13]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[14]){
		mark[14]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[15]){
		mark[15]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[16]){
		mark[16]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[17]){
		mark[17]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[18]){
		mark[18]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[19]){
		mark[19]=*addr_revisedmark;
		return 1;
	}else if(studentid==id[20]){
		mark[20]=*addr_revisedmark;
		return 1;
	}else{
		return 0;
	}
}

void writemarks(FILE* fout, int* id, float* mark, int n){
	int i;

	fprintf(fout,"There are %d marks.",n);
	for(i=0;i<n;i++){
		fprintf(fout,"%d %f",id[i],mark[i]);
	}

	return;
}