Mathematics 101 using C - c

Question

Can you help me with the code below? I am trying to make a program where users need to choose the following options:
1 - Multiplication table
2- Test
When the user chooses either one and it's finished running, the program will prompt users to choose the following:
1 - Another multiplication table
2 - Another test
3 - Exit
Problem occurs when I choose the 2nd option. Whenever I try to exit (3rd option), it goes back to 1st option. Everything is normal when I choose the 1st option and then the 3rd option. How do I solve this?
Here's my code:
int c1;
int num;
int multi;
int num1, num2;
int answer;
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("Choice: ");
scanf("%d", &c1);
while (c1 != 1 && c1 != 2)
{
printf("\nInvalid selection. Please choose again.\n");
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("Choice: ");
scanf("%d", &c1);
}
if (c1 == 1)
{
do
{
printf("\nChoose a number ranging from 1 to 12.\n");
printf("Choice: ");
scanf("%d", &num);
while (num > 12)
{
printf("\nInvalid selection. Please choose again.\n");
printf("Choose a number ranging from 1 to 12.\n");
printf("Choice: ");
scanf("%d", &num);
}
printf("\nMultiply of %d\n", num);
for (multi = 1; multi <= 12; multi++)
{
printf("%d x %d = %d\n", num, multi, num * multi);
}
printf("\nNext program?\n");
printf("1 - Another multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d", &c1);
while (c1 != 1 && c1 != 2 && c1 != 3)
{
printf("\nInvalid selection. Please choose again.\n");
printf("1 - Another multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d", &c1);
}
} while (c1 == 1);
}
if (c1 == 2)
{
do
{
printf("\nEnter 1st number: ");
scanf("%d", &num1);
printf("Enter 2nd number: ");
scanf("%d", &num2);
printf("Your answer is: ");
scanf("%d", &answer);
if (answer == num1 * num2)
{
printf("\nYou are correct. The answer for %d times %d is %d.", num1, num2, num1 * num2);
}
else if (answer != num1 * num2)
{
printf("\nYou are incorrect. The answer for %d times %d is %d, not %d.", num1, num2,
num1 * num2, answer);
}
printf("\nNext program?\n");
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d", &c1);
while (c1 != 1 && c1 != 2 && c1 != 3)
{
printf("\nInvalid selection. Please choose again.\n");
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d", &c1);
}
do
{
printf("\nChoose a number ranging from 1 to 12.\n");
printf("Choice: ");
scanf("%d", &num);
while (num > 12)
{
printf("\nInvalid selection. Please choose again.\n");
printf("Choose a number ranging from 1 to 12.\n");
printf("Choice: ");
scanf("%d", &num);
}
printf("\nMultiply of %d\n", num);
for (multi = 1; multi <= 12; multi++)
{
printf("%d x %d = %d\n", num, multi, num * multi);
}
printf("\nNext program?\n");
printf("1 - Another multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d", &c1);
while (c1 != 1 && c1 != 2 && c1 != 3)
{
printf("\nInvalid selection. Please choose again.\n");
printf("1 - Another multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d", &c1);
}
} while (c1 == 1);
} while (c1 == 2);
}

Answer 1

The do..while loop in C executes one time before checking. In the second if: "if (c1 == 2)" your code first performs the test then it asks again to choose. Even if your answer is "3" it executes one time the code in the following do..while section which is actually the code for the "multiplication table".
You can use the while loop instead of the do..while. However the code seems overcomplicated and some sections are repeated I'll suggest you to review the logic (also consider to use the switch statement).

Answer 2

Found the solution. Apparently, do while loop does not work well. Had to change to while loop. Here's the code:
int c1;
int num;
int multi;
int num1,num2;
int answer;
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("Choice: ");
scanf("%d",&c1);
while(c1!=1 && c1!=2){
printf("\nInvalid selection. Please choose again.\n");
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("Choice: ");
scanf("%d",&c1);
}
while(c1==1 || c1==2){
if(c1==1){
printf("\nChoose a number ranging from 1 to 12.\n");
printf("Choice: ");
scanf("%d",&num);
while(num>12){
printf("\nInvalid selection. Please choose again.\n");
printf("Choose a number ranging from 1 to 12.\n");
printf("Choice: ");
scanf("%d",&num);
}
printf("\nMultiply of %d\n",num);
for(multi=1;multi<=12;multi++){
printf("%d x %d = %d\n",num,multi,num*multi);
}
}else if(c1==2){
printf("\nEnter 1st number: ");
scanf("%d",&num1);
printf("Enter 2nd number: ");
scanf("%d",&num2);
printf("%d times %d equals to: ",num1,num2);
scanf("%d",&answer);
if(answer==num1*num2){
printf("Your answer is correct! %d times %d equals to %d.",num1,num2,num1*num2);
}else if(answer!=num1*num2){
printf("Your answer is incorrect! %d times %d equals to %d, not %d.",num1,num2,num1*num2,answer);
}
}
printf("\n\nNext program?\n");
printf("1 - Another multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d",&c1);
while(c1!=1 && c1!=2 && c1!=3){
printf("\nInvalid selection. Please choose again.\n");
printf("1 - Another multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
scanf("%d",&c1);
}
}

Answer 3

You shouldn't rely on getting the correct input from scanf(), the safer option is to read the input using fgets() and convert the value to an int using atoi().
int getValidNumberInput(void)
{
char buf[BUFSIZ];
int value = 0;
if (fgets(buf, sizeof(buf), stdin) != NULL)
value = atoi(buf);
return value;
}
The atoi() function returns 0 for a non numeric value so it's perfectly suitable for you needs.
As mentioned in the comments, the logic of the program is easily simplified by using a while() loop and a switch() statement.
int main(void)
{
bool hasLooped = false;
bool invalidChoice = false;
int choice;
while (1)
{
/* Display the relevant menu */
displayProgramMenu(hasLooped, invalidChoice);
/* Get the selected menu choice */
choice = getValidNumberInput();
if (choice < 1 || choice > 3)
invalidChoice = true;
else
invalidChoice = false;
/* Perform the chosen action */
switch (choice)
{
case 1:
doFirstOption();
hasLooped = true;
break;
case 2:
doSecondOption();
hasLooped = true;
break;
case 3:
exit(0);
break;
default:
break;
}
}
}
The use of the boolean variables allows the complexity to be moved into functions where it is easy to reason about.
void displayMenu(bool hasLooped, bool showInvalidMsg)
{
if (showInvalidMsg)
printf("\nInvalid selection. Please choose again.\n");
else if (hasLooped)
printf("\nNext program?\n");
if (hasLooped)
printf("1 - Another multiplication table\n");
else
printf("1 - Multiplication table\n");
printf("2 - Test\n");
printf("3 - Exit\n");
printf("Choice: ");
}