function convert string to int - C - c

Question

I want to scan in a string that can take at least 200 characters and then I want to convert the string to an int, so that I can print it with e.g. printf("%d", digit).
How can I write a function kinda like this I've written here
(this one does not work!):
int main()
{
char car[200];
int number;
int i,x;
int sum = 0;
printf("Write in number: \n");
scanf("%c", &car);
for (i=0; i<200; i++) {
if (car[i] != '\0') {
x = car[i]-'0';
sum = sum + x;
if (i != 0) {
sum = sum*10;
}
}
}
return 0;
}

Answer 1

first:
scanf("%c", &car);
from man scanf:
c
Matches a sequence of characters whose length is specified by the maximum field width (default 1); the next pointer
must be a pointer to char, and there must be enough
room for all the characters (no terminating null byte is added). The usual skip of leading white space is suppressed. To
skip white space first, use an explicit space
in the format.
So you're reading exactly one character, not a whole string.
Then, you rely on a terminating null byte being added, which isn't happening. Use %199s instead, leaving enough room for the terminating null byte.
Then, considering no int in this world should have enough space for numbers with 199 decimal digits, you should think about your 200 character buffer.

Answer 2

If your goal is not to write such a function for educational purposes, but because you need one:
int number;
scanf("%d", &number);
does exactly that: Read one number from the input, and place it in number.

Answer 3

scanf("%s", car);
You need to read a string %s not a single char %c. Also char array will decay to a pointer when passed as an argument, so you shouldn't use &char.

Answer 4

Here is the solution,Start from the end of the string because units place is from right to left then increment the units place to ten's place then 100 and so on.
#include<stdio.h>
#include<string.h>
int main()
{
char input[9];
int digit,number=0,i=1;
printf("Enter Number:\n");
scanf("%s",input);
digit=strlen(input)-1;
while(digit>=0)
{
number=number + i*(input[digit]-'0');
digit=digit-1;
i=i*10;
}
printf("%d",number);
return 0;
}

Answer 5

Close. Suggested changes below.
Use char car[200+1] to "take at least 200 characters".
Use "%200s" rather than "%c" to read a string rather than a char and to limit its input.
//int main()
int main(void) {
// char car[200];
char car[200+1];
int number;
int i,x;
int sum = 0;
printf("Write in number: \n");
// scanf("%c", &car);
scanf("%200s", car);
// for (i=0; i<200; i++) {
for (i=0; car[i]; i++) {
// if (car[i] != '\0'){
x = car[i]-'0';
// sum = sum + x;
sum = 10*sum + x;
// if (i != 0) { sum = sum*10; }
}
printf("%d\n", sum)
return 0;
}
Unless input is like "000000000000000000000000000123", 200 digits will certainly overflow sum.
To detect that
x = car[i]-'0';
if (sum >= INT_MAX/10 && (sum > INT_MAX/10 || x > INT_MAX%10)) {
x = INT_MAX;
// maybe set an error flag
break;
}
sum = 10*sum + x;
An int has an maximum value of INT_MAX from #include < limits.h>. It is at least 32767. Some platforms use 16-bit, 32-bit or 64-bit int Other ranges are possible. Let us assume a worst case of 128-bit. That would need about 39 decimal digits. Leaving room for a sign and terminating null character, suggest
char car[39+1+1];