Write A Program That Converts A String...

Another exercise attempt. I almost have a headache after writing this. I'm going out for a drink. When I saw the question, the first thing that came to my mind was a switch case (I'm assuming the author does not want us to use the atoi ASCII to Integer function), so I wrote it using it switch case although there probably are other ways to do this, I can't be bothered now. To increase the number the user can input, I used unsigned integers. If you want the program to accept only smaller numbers:
1) Change unsigned to int if needed
    If you do this also replace %u with a %i in:
    printf("Here is your input as an (Unsigned) integer: %i",integer);
2) Change prompt the "upto 9 digits" part.
3) if(str_length<=9) replace 9 with desired limit.
4) Delete or comment out the code starting from case 9: to right before case:desired limit.

Let Us C, Chapter 9, Exercise D-d:
Write a program that converts a string like "124" to an integer 124.

#include <stdio.h>
#include <strings.h>

main()
{
    unsigned str_length,temp,integer=0;
    int i=0;
    char input[9];

    printf("Enter a string of numbers (upto 9 digits): ");
    scanf(" %s",input);

    str_length=strlen(input);
    if(str_length<=9)
    {
        switch(str_length)
        {
        case 9:     //if it is a 9 digit number
            /*Without the -48, temp will be assigned the ASCII value, 48 for 0, 49 for 1, 50 for 2 etc.
            so we subtract 48 to cut it down to the correct value*/
            temp=input[i]-48;
            integer=temp*100000000;
            i++;

        case 8:
            temp=input[i]-48;
            integer=temp*10000000+integer;
            i++;

        case 7:
            temp=input[i]-48;
            integer=temp*1000000+integer;
            i++;

        case 6:
            temp=input[i]-48;
            integer=temp*100000+integer;
            i++;

        case 5:
            temp=input[i]-48;
            integer=(temp*10000)+integer;
            i++;

        case 4:
            temp=input[i]-48;
            integer=(temp*1000)+integer;
            i++;

        case 3:
            temp=input[i]-48;
            integer=(temp*100)+integer;
            i++;

        case 2:
            temp=input[i]-48;
            integer=(temp*10)+integer;
            i++;

        case 1:
            temp=input[i]-48;
            integer=temp+integer;
            i++;
        }
        printf("Here is your input as an (Unsigned) integer: %u",integer);
    }

    else
    {
        printf("Invalid Input!");
    }

}