In C How to use leftmost digit from an integer - c

Question

I was wondering how to reverse my output to match entered number.
Example if user entered 543210, I want the output to be: Five Four Three Two One Zero. But instead it's reversed and I can't figure out how to reverse it.
I can't use loops or anything else.
Code:
int main(void){
int value;
int digit;
printf("enter:");
scanf("%i", &value);
while(value)
{
digit = value % 10;
value = value / 10;
if(digit != 0)
{
switch(digit)
{
case 0:
printf("zero ");
break;
case 1:
printf("one ");
break;
case 2:
printf("two ");
break;
case 3:
printf("three ");
break;
case 4:
printf("four ");
break;
case 5:
printf("five ");
break;
case 6:
printf("six ");
break;
case 7:
printf("seven ");
break;
case 8:
printf("eight ");
break;
case 9:
printf("nine ");
break;
}
}
}
return 0;
}
Exmaple: If user entered 1234
Output would be: four three two one.
How would I fix it to be: One Two Three Four.

Answer 1

Since you've said that you aren't allowed to use loops, then recursion really is the thing that you are probably being expected to use. I personally am not sure if it would be right to not consider a recursion as a loop, but whatever.
You are using a while there, which also is a loop. If you are allowed to use loops, then you could just do the following, easy-to-understand modification in your code, and get the output you desire:
...
int input; // <-- added this
int value;
int digit;
printf( "enter:" );
scanf( "%i", &input ); // <-- notice the change in variable usage
value = 0;
while ( input ) {
value = 10 * value + input % 10; // adds the last digit of input to value from right
input /= 10;
}
while ( value ) { ... }
...
If you aren't allowed to use loops, then you probably are expected to use a special function, a function which outputs a specific value for a single case, and returns back to itself in any other case. You need a recursive function. Examine this simple example:
// This is in maths, not C
f(x) = 2x + 1 for all integer x >= 0
Out of many ways, this one way to describe the function which maps 0 to 1, then 1 to 3, then n to 2n + 1. If we wanted to define the exact same function recursively:
// In maths
f(x = 0) = 1 for x = 0
f(x > 0) = f(x-1) + 2 for integer x > 0
You see what's going on in there? It's saying that each subsequent f(x) is 2 greater than the previous one f(x-1). But more importantly, the function is calling itself! If you look closer, the called function f(x-1) will also call itself:
f(x) = f(x-1) + 2
f(x) = f(x-2) + 2 + 2
f(x) = f(x-3) + 2 + 2 + 2
...
// all these are the same
All this calling deeper and deeper has to end somewhere, and that somewhere is when f(x-...) is f(0), which has been explicitly defined to be 1.
This is what recursion is all about. Let me write out the examples I gave above in C:
// non-recursive version
int fnonrec( int x ){
return 2 * x + 1;
}
// recursive version
int frec( int x ){
if ( x == 0 )
return 1; // explicit return value for f(0)
else // redundant else, hehe
return frec( x - 1 ) + 2;
}
Definitions of the functions really look similar to how they were defined in maths, don't they? Yeah, well, I don't think giving you the answer for your question would be nice of me. All I can say is that you can print things in reverse really nicely with recursive functions.

Answer 2

//store user input to int variable "value"
char str[15];
sprintf(str, "%d", value);
You can then use the strrev function to reverse the string array. Manipulate it from there.

Answer 3

#include <stdio.h>
void print(int v){
static char *numbers[] = {
"zero","one","two","three","four",
"five","six","seven","eight","nine"
};
int digit = v % 10;
int value = v / 10;
if(value){
print(value);
printf(" %s", numbers[digit]);
} else
printf("%s", numbers[digit]);
}
int main(void){
int value;
printf("enter:");
scanf("%i", &value);
print(value);
return 0;
}

Answer 4

Example using recursive function and numbers from the parameters :
#include <stdio.h>
void display(char c)
{
char *numbers[] = {
"zero","one","two","three","four",
"five","six","seven","eight","nine "
};
printf("%s ", numbers[c]);
}
int aff_num(char *c)
{
if (*c == '\0')
return (0);
display(*c-48);
aff_num(++c);
return (1);
}
int main(int argc, char **argv)
{
if (argc < 2)
{
printf("Need numbers\n");
return (-1);
}
aff_num(argv[1]);
return (0);
}

Answer 5

I'm a python hacker and I almost never program in C. that being said:
#include <stdlib.h>
#include <stdio.h>
int highest_power_of_ten(int value){
int exponent = 0;
int tens = 1;
while(value > tens){
tens *= 10;
exponent += 1;
}
return exponent-1;
}
int pow(int base, int exponent){
if (exponent == 0)
return 1;
int temp = base;
while(exponent > 1){
base *= temp;
exponent -= 1;
}
return base;
}
int main(int argc, char** argv){
char* digits[] =
{"zero","one","two","three","four","five","six","seven","eight","nine"};
int value, n, exp, x;
scanf("%i", &value);
while(highest_power_of_ten(value)>0){
exp = highest_power_of_ten(value);
x = pow(10, exp);
n = value/x;
printf("%s ",digits[n]);
value -= n*x;
}
printf("%s\n", digits[value]);
//system("PAUSE"); for windows i guess
return 0;
}

Answer 6

Another method to get the digits in the right order:
E.g. To get the digit at 1st position in 123 divide 123 by 100, to get 2nd - 123 / 10, to get 3rd 123 / 1. That equals: value / 10^(index of desired digit)
So what we have to do is
Get the length of the (remaining) number by calculating log10(value).
Then get the (remaining) first (most significant) digit by dividing value by 10^length (length of 1.)
calculate value := value - 10^length and start from 1, unless the result is 0 (mind handeling numbers that end on 0).
while (value)
{
len = log10(value);
digit = (int) value / pow(10, len);
value -= pow(10, len);
}
And your code does never enter case 0. To fix that just leave the if(digit != 0) - that's what I meant when I wrote "mind the 0").
if(digit != 0) // enters if digit is not 0
{
switch(digit)
{
case 0: // enters if digit is 0
...
}
}