Error in implementing a basic string to integer converter - c

Question

I am writing a basic C program which will take the input of a string, then convert it to an integer. I am aware of the standard atoi() function, this is purely an exercise I set myself. The code for the program is as follows:
#include <stdio.h>
#include <string.h>
int main (void) {
char input[20];
int counter, temp, magnitude = 0, value = 0;
scanf("%s", input); //takes user input for string to convert
for (counter = strlen(input); counter >= 0; counter--) { //from last to first char of input
temp = input[counter] - '0'; //converts from ASCII to actual value and stores in int
value += temp * magnitude * 10; //temp to correct magnitude and adds to total
magnitude++; //increments magnitude counter
}
printf("%d\n", value); //prints result
return 0;
}
I am trying to iterate backwards from the end of the string, then convert each character to it's actual value from ASCII, then convert it to the correct magnitude, before finally adding it to the final value. It is giving me values which are generally close although wrong.
For example 256 gives 220 and 90 gives 180.

Answer 1

You have two problems. Firstly, you're also trying to convert the terminating NUL byte, which doesn't make sense. Start iterating at strlen(input) - 1.
Second, for the magnitude you want to exponentiate, not multiply, i.e. 10, 100, 1000 rather than 10, 20, 30. For this, you can use pow(), for example:
for (counter = strlen(input) - 1; counter >= 0; counter--) { //from last to first char of input
temp = input[counter] - '0'; //converts from ASCII to actual value and stores in int
value += temp * pow(10, magnitude); //temp to correct magnitude and adds to total
magnitude++; //increments magnitude counter
}

Answer 2

counter = strlen(input)
should be
counter = strlen(input) - 1
so you're using the last valid character of the string.

Answer 3

Two problems here.
1) You should ignore the string null-terminator, so going from strlen(input)-1 will do
2) Your calculation is wrong. You should not multiply by magnitude, but raise 10 to it's power. Or simpler, it itself the required 10's power:
#include <stdio.h>
#include <string.h>
int main (void) {
char input[20];
int counter, temp, magnitude = 1, value = 0;
scanf("%s", input); //takes user input for string to convert
for (counter = strlen(input)-1; counter >= 0; counter--) { //from last to first char of input
temp = input[counter] - '0'; //converts from ASCII to actual value and stores in int
value += temp * magnitude ; //temp to correct magnitude and adds to total
magnitude*=10; //increments magnitude counter
}
printf("%d\n", value); //prints result
return 0;
}

Answer 4

strlen gives the total length of the string. So you have to use
counter = strlen(input)-1
In your for loop. This is because arrays in C start from 0 and end at strlen -1.

Answer 5

While your approach should work (after the issues noted in other answers have been fixed, anyway), it's probably simpler to take this approach:
value = 0;
for (char *p = input; *p; ++p)
value = value * 10 + (*p - '0');
That eliminates a couple unnecessary extra variables, and is probably a bit more efficient. It might not hurt to change the for loop termination condition to *p && isdigit(*p) - that would terminate the conversion on the first non-digit character seen, which might be a desirable approach for dealing with invalid input...

Answer 6

here is a complete answer,
that includes good programming paradigm practices
and defensive coding against user input problems
and results in the right answer
#include <stdio.h> // fgets(), exit(), printf()
#include <stdlib.h> // EXIT_FAILURE, perror()
#include <string.h> // strlen()
#include <ctype.h> // isdigit()
int main () // <-- do not use void
{
char input[20] = {'\0'};
int counter; // index into input array
int value = 0; // initialize the converted int from string
if( NULL != fgets(input, sizeof(input), stdin) )
{ // fgets failed
perror( "fgets failed for string input");
exit( EXIT_FAILURE );
}
// implied else, fgets successful
// note no need to replace trailing '\n' in following code block
for( counter=0; isdigit(input[counter]); counter++)
{
value = value*10 + (input[counter]-'0');
} // end for
printf("the converted value from %s is: %d\n", input, value); //prints result
return 0;
} // end function: main