Learning C, problem with an exercise.

[medeshago]

I have to do the following:
Read a number and calculate the following sequence:
1-1/2+1/3-1/4+1/5...1/n

This is the way I'm doing it, but it doesn't work:


#include <stdio.h>

int main()
{
int a=0, n, suma=0;
float den;

printf ("\n Ingrese el número : ");
scanf ("%i", &n);

while (a<n)
{
if (a%2==0){
a++;
den=1/a;
suma-=den;}

else{
a++;
den=1/a;
suma+=den;}
}


printf ("\n Resultado secuencia = %i", suma);

return 0;
}

[dmm1285]

if (a%2==0){
a++;
den=1/a;
suma-=den;}

else{
a++;
den=1/a;
suma+=den;}
}


}

Try switching your addition and subtraction like this.

[medeshago]

Just did it, still not working.

[Can+~]

Problem is, you're doing integer division:

1/2 = [0.5] = 0
1/3 = [0.33333..] = 0
1/4 = [0.25] = 0

To circumvent this issue, work with floats.

[medeshago]

This is how it looks now:


#include <stdio.h>

int main()
{
int a=0, n;
float den, suma=0.0;

printf ("\n Ingrese el número : ");
scanf ("%i", &n);

while (a<n)
{
if (a%2==0){
a++;
den=1.0/a;
suma-=den;}

else{
a++;
den=1.0/a;
suma+=den;}
}

printf("%d", den);
printf ("\n Resultado secuencia = %d", suma);

return 0;
}

But this is what I get:

Ingrese el número : 5
Resultado secuencia = 536870912

[Can+~]

Now you got a printing problem:


printf("%d", den);
printf ("\n Resultado secuencia = %d", suma);

Should be


printf("%f", den);
printf ("\n Resultado secuencia = %f", suma);

btw, here's my implementation (with for, and inverting sign)


int main(int argc, char** argv)
{
float total=0;
int count, sign=1, max=20;

for (count=1; count < max; count++)
{
total += sign*(1.0/count);
printf("%d : %.8f\n", count, total);

sign = -sign;
}

printf("%.8f", total);

return 0;
}

sign inverts each iteration, meaning that you don't need that "if" in between.

[medeshago]

Thanks, man, now it works great.