Builidng a calculator in c language can use a previous result - c

Question

i got problem with this calculator which i need to use the previous result of previous operartion (if i want to ) but when i do it it gives correct if only give it the same previous operation
here is the code :
#include<stdio.h>
#include<math.h>
double addition (double a, double b)
{
double add;
add=a+b;
return add;
}
double soustraction (double a, double b)
{
double sous;
sous=a-b;
return sous;
}
double multiplication (double a,double b)
{
double multi;
multi=a*b;
return multi;
}
double division (double a, double b)
{
double div;
div=a/b;
return div;
}
double reste_de_division (int a,int b)
{
double rest;
rest=a%b;
return rest;
}
double puissance( double a,double b)
{
int i;
double r;
i = 0;
r = 1;
while (i < b){
r = r * a;
i ++;
}
return r;
}
double factorielle (double a)
{
int i;
double facto;
facto=1;
for(i=0;i<a;i++)
{facto=(a-i)*facto;
}
return facto;
}
double PGCD (int a,int b)
{
int i;
double pgcd;
for(i=1; i <= a && i <= b; ++i)
{
if(a%i==0 && b%i==0)
pgcd = i;
}
return pgcd;
}
double PPCM (int a,int b)
{
int ppcm;
ppcm = (a > b) ? a : b;
while(ppcm%a !=0 || ppcm%b !=0)
ppcm=ppcm+1;
return ppcm;
}
double inverse (double a)
{
double inv;
inv=1/a;
return inv;
}
int main(){
double a,b,r;
char ch;
printf("\n***************************************\n");
printf (" \n + Addition \t \t - Subtraction \n * Multiplication \t / Division \n Reste de division \t c Puissance \n ! Factorielle \t \t g PGCD \n p PPCM \t \t i Inverse \n q Quitter \n");
printf("\n***************************************\n");
printf("Donner les deux nombres et l'operation:\n");
do
{b=r;
scanf("%lf %lf %c",&a,&b,&ch);
switch (ch)
{case '+':
r=addition(a,b);
printf("%lf\n",r);
break;
case '-':
r=soustraction(a,b);
printf("%lf\n",r);
break;
case '*':
r=multiplication(a,b);
printf("%lf\n",r);
break;
case '/':
r=division(a,b);
printf("%lf\n",r);
break;
case '%':
printf("%lf\n",reste_de_division(a,b));
break;
case 'c':
printf("%lf\n",puissance(a,b));
break;
case '!':
printf("%lf\n",factorielle(a));
break;
case 'g':
printf("%lf\n",PGCD(a,b));
break;
case 'p':
printf("%lf\n",PPCM(a,b));
break;
case 'i':
printf("%lf\n",inverse(a));
break;
}
}while(ch !='q');
return 0;
}
Sorry for my bad english
example:
5 6 +
11
6 +
17
3 -
20 //this is error, it's supposed to give 14 not 20

Answer 1

Your problem is here:
scanf("%lf %lf %c",&a,&b,&ch);
You expect the user to always input two doubles and one character. If the user only gives one double and one character, the value of ch (and b) is unchanged and the program will do the same operation as it did the last time.
Example:
First input:
5 6 +
This will make a equal 6, b equal 5 and ch equal +. Consequently the program will add the two numbers 5 and 6 and produce the result 11 which is stored in b.
Next input:
1 -
This will make a equal one but leave both b and ch unchanged, i.e. b is still equal 11 and ch is still equal +. Consequently the program will add the two numbers 1 and 11 and produce the result 12 which is stored in b.
This is just one example of things that can go wrong when doing scanf("%lf %lf %c",&a,&b,&ch); There are other input sequences that will lead to (even more) bizar output.
You need a much more strict parsing of user input. The scanf functions isn't good for that. Consider using fgets to read a whole line and the parse it afterwards. With care that can be done using sscanf.