code removed for easier scrolling

int selection = 0;

Code removed for easier scrolling

int selection = 0;

do {
   /* menu code here */
} while ( selection != 4 );

#include <stdio.h>
#include <string.h> /* for the string data type */
#include <stdlib.h> /* for the conversion function */

string input;
int     selection = 0;

printf("Enter an integer value : ");
scanf("%s", &input);

selection = atoi( input );

if ( input == 0 )
{
   printf("That was not an integer value");
}
else
{
   printf("Integer entered was : %d", selection);
}

#include<stdio.h>
void equation();
void calculate_current();
void time_conversion();
void main()
{
int selection = 0;
char input;
do
{
 printf("1.Equation Evaluation\n");
 printf("2.Current Calculation\n");
 printf("3.Time Conversion\n");
 printf("4.Quit\n\n");
 printf("Enter Selection: ");
 scanf("%d", &selection);
 scanf("%c", &input);
 printf("\n----------------------------------\n\n");

 switch(selection)
 {
 case 1:
  equation();
  break;
 case 2:
  calculate_current();
  break;
 case 3:
  time_conversion();
  break;
 case 4:
  break;
 default:
  printf("Try Again\n\n");
 }
}
while (selection !=4);
}

void equation()
{
int selection = 0;
float a, b, c, d, x1, x2, b1, b2;
printf("Enter value of A11 :");
scanf("%f", &a);
printf("Enter value of A12 :");
scanf("%f", &b);
printf("Enter value of B1  :");
scanf("%f", &b1);
printf("Enter value of A21 :");
scanf("%f", &c);
printf("Enter value of A22 :");
scanf("%f", &d);
printf("Enter value of B2  :");
scanf("%f", &b2);

printf("\nThe equations are :\n");
printf("%.fX1 + %.fX2 = %.f\n", a, b, b1);
printf("%.fX1 + %.fX2 = %.f\n\n", c, d, b2);
x1=((b1*d)-(b2*b))/((a*d)-(c*b));
x2=((a*b2)-(b1*c))/((a*d)-(c*b));
printf("X1=%.2f, X2=%.2f\n\n", x1, x2);
printf("----------------------------------\n\n");
 
}
void calculate_current()
{
float resistance, total, gtotal;

total=0;
printf("Enter value of parallel resistances, Enter 0 to end :\n");
 scanf("%f", &resistance);
 while(resistance !=0)
 {
  total +=1/resistance;
  scanf("%f", &resistance);
 }
 gtotal=1/total;

 printf("\nTotal Resistance = %.2f Ohms\n\n", gtotal);
 printf("----------------------------------\n\n");

}

void time_conversion()
{
int sec, min, hr;
printf("Please enter time in seconds :");
scanf("%d", &sec);
hr=sec/3600;
min=(sec%3600)/60;
sec=(sec%3600)%60;
printf("%d hours %d minutes %d seconds\n\n", hr, min, sec);
printf("----------------------------------\n\n");
}

char input[100]; /* String of 100 characters */