Question

Yesterday I was able to make a program (ASCII converter etc etc) that had the same problem [ Every function had a missing prototype error when I build the program ] I was able to fix it through random trial and error having no idea how I did it. Here’s my arithmetic quiz practice program. I also tried putting int initialize(),clear(),exit(),additionquiz(),subtractionquiz(),divisionquiz(),multiplicationquiz(); and it still gave me a missing prototype.

 #include <stdio.h>
    /* Main Menu */
    int numbers[10];
    int main()
    {
        while(1==1)
        {
            int choice;
            initialize();
            printf("Arithmetic Quiz Program\n");
            printf("1 - Addition\n2 - Subtraction\n3 - Multiplication\n4 - Division\n5 - Exit\n");
            scanf("%d",&choice);
            if(choice==1)
            {
                clear();
                additionquiz();
            }
            else if(choice==2)
            {
                clear();
                subtractionquiz();
            }
            else if(choice==3)
            {
                clear();
                multiplicationquiz();
            }
            else if(choice==4)
            {
                clear();
                divisionquiz();
            }
            else if(choice==5)
            {
                exit();
            }
            else
            {
                printf("%cPlease choose a number from 1 - 5",7);
                clear();
                continue;
            }
        }
        return 0;
    }
    /* For clearing the page */
    int clear()
    {
        int i;
        for(i=0;i<25;i++)
        {
            printf("\n");
        }
    }
    /* Assigns the array */
    int initialize()
    {
        numbers[0]=6;
        numbers[1]=0;
        numbers[2]=2;
        numbers[3]=5;
        numbers[4]=3;
        numbers[5]=1;
        numbers[6]=9;
        numbers[7]=4;
        numbers[8]=7;
        numbers[9]=8;
        return 0;
    }
    /* addition quiz */
    int addition()
    {
        int a,diff,b,answer,choice;
        a=0;
        diff=1;
        b=a+diff;
        while(1==1)
        {
            if(a>9)
            {
                a=0;
                diff++;
            }
            if(b>9)
            {
                b=0;
            }
            if(diff>9)
            {
                diff=0;
            }
            printf("%d + %d = ",number[a],number[b]);
            scanf("%d",&answer);
            if(answer==number[a]+number[b])
            {
                printf("\nCORRECT!!!\n");
                a++;
            }
            else
            {
                printf("\nWRONG!!!\n");
                clear();
                additionquiz();
            }
            printf("\nWhat do you want to do next?\n1 - Answer another addition Question\n2 - Go back to main menu\n3 - Exit program\n");
            scanf("%d",&choice);
            if(choice==1)
            {
                clear();
                additionquiz();
            }
            else if(choice==2)
            {
                clear();
                main();
            }
            else if(choice==3)
            {
                exit();
            }
            else
            {
                printf("%cPlease choose a number from 1 to 3",7);
            }
        }
        return 0;
    }
    /* The subtraction quiz */
    int subtraction()
    {
    int a,diff,b,answer,choice;
        a=0;
        diff=1;
        b=a+diff;
        while(1==1)
        {
            if(a>9)
            {
                a=0;
                diff++;
            }
            if(b>9)
            {
                b=0;
            }
            if(diff>9)
            {
                diff=0;
            }
            if(numbers[a]-numbers[b]<0)
            {
                a++;
                subtraction();
            }
            printf("%d - %d = ",numbers[a],numbers[b]);
            scanf("%d",&answer);
            if(answer==numbers[a]-numbers[b])
            {
                printf("CORRECT!!!\n\n");
            }
            else
            {
                printf("WRONG!!!\n\n");
                clear();
                subtractionquiz();
            }
            printf("\nWhat do you want to do next?\n1 - Answer another subtraction Question\n2 - Go back to main menu\n3 - Exit program\n");
            scanf("%d",&choice);
            if(choice==1)
            {
                clear();
                subtractionquiz();
            }
            else if(choice==2)
            {
                clear();
                main();
            }
            else if(choice==3)
            {
                exit();
            }
            else
            {
                printf("%cPlease choose a number from 1 to 3",7);
            }
        }
        return 0;
    }
    /* multiplication quiz */
    int multiplicationquiz()
    {
    int a,diff,b,answer,choice;
        a=0;
        diff=1;
        b=a+diff;
        while(1==1)
        {
            if(a>9)
            {
                a=0;
                diff++;
            }
            if(b>9)
            {
                b=0;
            }
            if(diff>9)
            {
                diff=0;
            }
            printf("%d * %d = ",number[a],number[b]);
            scanf("%d",&answer);
            if(answer==number[a]*number[b])
            {
                printf("\nCORRECT!!!\n");
                a++;
            }
            else
            {
                printf("\nWRONG!!!\n");
                clear();
                multiplicationquiz();
            }
            printf("\nWhat do you want to do next?\n1 - Answer another multiplication Question\n2 - Go back to main menu\n3 - Exit program\n");
            scanf("%d",&choice);
            if(choice==1)
            {
                clear();
                multiplicationquiz();
            }
            else if(choice==2)
            {
                clear();
                main();
            }
            else if(choice==3)
            {
                exit();
            }
            else
            {
                printf("%cPlease choose a number from 1 to 3",7);
            }
        }
        return 0;
    }
    /* Division quiz */
    int divisionquiz()
    {
        int a,diff,b,answer,choice,remain;
        a=0;
        diff=1;
        b=a+diff;
        while(1==1)
        {
            if((numbers[a]<numbers[b])||numbers[b]==0)
            {
                a++;
                clear();
                divisionquiz();
            }
            if(a>9)
            {
                a=0;
                diff++;
            }
            if(b>9)
            {
                b=0;
            }
            if(diff>9)
            {
                diff=0;
            }
            printf("%d % %d = \n",numbers[a],numbers[b]);
            printf("What is the whole number?\n");
            scanf("%d",&answer);
            printf("What is the remainder? (0 if none\n)");
            scanf("%d",&remain);
            if(answer==numbers[a]/numbers[b] && remain==numbers[a]%numbers[b])
            {
                printf("\nCORRECT!!!");
                a++;
            }
            else
            {
                printf("\nWRONG!!!");
                clear();
                divisionquiz();
            }
            printf("\nWhat do you want to do next?\n1 - Answer another division Question\n2 - Go back to main menu\n3 - Exit program\n");
            scanf("%d",&choice);
            if(choice==1)
            {
                clear();
                divisionquiz();
            }
            else if(choice==2)
            {
                clear();
                main();
            }
            else if(choice==3)
            {
                exit();
            }
            else
            {
                printf("%cPlease choose a number from 1 to 3",7);
            }
        }
        return 0;
    }

Answer 1

exit is an external function and you need to include its header at the top of your source code:

#include <stdlib.h>   // exit

Please notice that in the function call the addition function is called addition and in the function definition additionquiz. Same for the substraction.

For the other functions, you should declare them before you call them: that is before the main function definition.

int initialize(void);
int clear(void);
int additionquiz(void);
int subtractionquiz(void);
int divisionquiz(void);
int multiplicationquiz(void);

int main(void)
{
    /* ... */

Note that declaring all the functions in one go like this:

int initialize(void), clear(void), additionquiz(void),
    subtractionquiz(void), divisionquiz(void), multiplicationquiz(void);

is permitted but it is not very readable and may surprise the reader.

Finally, if these functions are not called from another source code, you should tell the reader (and the compiler) by adding a static specifier at the beginning of the declaration like this:

static int clear(void);  // the function is only called in this source code