Thread: Problem with input validation(no alphabet allowed) and loop

Requirement:
1. ask user to input value for x,y,z
2. check the value is not alphabet
3. loop when user wish to continue

Problem:
1. When the input is all numeric it still will display the error message in the alphabet checking code and stop.
Example:
Enter real number X: 1
Enter real number Y: 1
Enter real number Z: 1
Entered value contained alphabet/symbol

2. If the above successful, i wish to allow user to decide want to continue or not. I tried and it not looping.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>


main ()
{


    float X,Y,Z;
    float rsltA = 0;
    float rsltB = 0;
    float rsltC= 0;
    int i=0;
    char str[10];
    char choice;
    
    
    do
    {
            printf("Enter real number X: ");
            scanf("%f",&X);
        
            printf("Enter real number Y: ");
            scanf("%f",&Y);
            
            printf("Enter real number Z: ");
            scanf("%f",&Z);
        
        //while (scanf("%f%f%f", &X,&Y,&Z) != 1)
        //{
    
            fgets(str,sizeof(str),stdin);
    
            while(i<strlen(str))
                {
        
                    if(str[i]<48||str[i]>57)
                    {
                        printf("Entered value contained alphabet/symbol\n");
                        getchar();
                        return 0;
                        
                    }
                
                    else
                    {


                    //display the value for X,Y and Z
                        printf("X=%.4f ; Y=%.4f ; Z=%.4f\n", X, Y, Z );
                        
                        
                        //mathematic equations
                        rsltA= (5*powf(X, 2)+(3*Y))*((4*X)-(5*Y));
                        rsltB= logf(Z)/powf((2-logf(X)),2);
                        rsltC= ((4*rsltA)+(3*rsltB))/(powf(rsltB,2));
                        
                        
                        //condition for X,Y and Z
                        if (X==0 && Z==0)
                        {
                            printf("Function g(x,z) is not defined at x=0.0000, z=0.0000\n");
                            printf("f(x,y): %.4f; ",rsltA);
                            printf("g(x,z):#INF; ");
                            printf("g(x,y,z):#UNDEF\n");
                            printf("Do you want to continue Y/N: ");
                            scanf(" %c", &choice);
                        }
                        
                        if (X<0 && Z<0)
                        {
                            printf("Function g(x,z) is not defined at x=%.4f, z=%.4f\n",X,Z);
                            printf("f(x,y): %.4f; ",rsltA);
                            printf("g(x,z):#INF; ");
                            printf("g(x,y,z):#UNDEF\n");
                            printf("Do you want to continue Y/N: ");
                            scanf(" %c", &choice);
                        }
                        
                        if ((X < 0 )||(X == 0))
                        {
                            printf("Function g(x,z) is not defined at x=%.4f\n",X);
                            printf("f(x,y): %.4f; ",rsltA);
                            printf("g(x,z):#INF; ");
                            printf("g(x,y,z):#UNDEF\n");
                            printf("Do you want to continue Y/N: ");
                            scanf(" %c", &choice);
                        }
                        /*
                         if (X+=0.01)
                         {
                         printf("Function g(x,z) is not defined at x=0.0100\n");
                         
                         }
                         
                         
                         if ((X += 0.01) && (Z <= 0))
                         {
                         printf("Function g(x,z) is not defined at x=0.0100, z=%.4f\n",Z);
                         
                         }
                         
                         if ((X += 0.01) && Z > 0)
                         {
                         
                         printf("Function g(x,z) is not defined at x=0.0100\n");
                         
                         
                         }*/
                        
                        if ((Z < 0 )||(Z == 0))
                        {
                            printf("Function g(x,z) is not defined at z=%.4f\n",Z);
                            printf("f(x,y): %.4f; ",rsltA);
                            printf("g(x,z):#INF; ");
                            printf("g(x,y,z):#UNDEF\n");
                            printf("Do you want to continue Y/N: ");
                            scanf(" %c", &choice);
                        }
                        else
                        {
                            
                            printf("f(x,y): %.4f; ", rsltA);
                            printf("g(x,z): %.4f; ", rsltB);
                            printf("h(x,y,z): %.4f \n", rsltC);
                            printf("Do you want to continue Y/N: ");
                            scanf(" %c", &choice);
                        }
                    }i++;
                //}
        }
    }while(choice=='Y'|| choice=='y');
                
        
    
    printf("Thanks you");
    


}

scanf() (line 29) leaves the ENTER in the input buffer.
fgets() (line 34) reads that ENTER to the first position in str.
The subsequent test fails.

Do not mix scanf() and fgets().

Ignore the "check the value is not alphabet". scanf() already does that on its own. If the input cannot be interpreted as a float, scanf() returns a number less than the expected assignments.

Code:

    if (scanf("%f", &X) != 1) /* error */;
    while (scanf("%f%f%f", &X, &Y, &Z) == 3) { /* loop */ }

The best way to deal with user input (in respect to error checking, validation, recovery) is to read full lines with fgets(), then parse those lines with sscanf().
This requires a bit more work than doing it all with scanf() (see example below).

Code:

    while (1) {
        if (scanf("%f%f%f", &x, &y, &z) != 3) /* error */;
        // deal with x, y, and z
        printf("Continue (Y/N)? ");
        scanf(" %c", &choice);
        if ((choice != 'Y') && (choice != 'y')) break; // exit infinite while loop
    }