Conversion from any base to decimal and case sensitivity

I am using the following portal to verify the results of my code (again sourced from the web):

RapidTables BaseConverter

My code is as shown below:

// C program to convert a number from any base 
// to decimal 
#include <stdio.h> 
#include <string.h> 
  
// To return value of a char. For example, 2 is 
// returned for '2'.  10 is returned for 'A', 11 
// for 'B' 
int val(char c) 
{ 
    if (c >= '0' && c <= '9') 
        return (int)c - '0'; 
    else
        return (int)c - 'A' + 10; 
} 
  
// Function to convert a number from given base 'b' 
// to decimal 
int toDeci(char *str, int base) 
{ 
    int len = strlen(str); 
    int power = 1; // Initialize power of base 
    int num = 0;  // Initialize result 
    int i; 
  
    // Decimal equivalent is str[len-1]*1 + 
    // str[len-1]*base + str[len-1]*(base^2) + ... 
    for (i = len - 1; i >= 0; i--) 
    { 
        // A digit in input number must be 
        // less than number's base 
        if (val(str[i]) >= base) 
        { 
           printf("Invalid Number\n"); 
           return -1; 
        } 
  
        num += val(str[i]) * power; 
        power = power * base; 
    } 
  
    return num; 
} 
  
// Driver code 
int main() 
{ 
    char str[] = "25a"; 
    int base = 36; 
    printf("Decimal equivalent of %s in base %d is " " %d ", str, base, toDeci(str, base)); 
    return 0; 
}

As shown inside main(), if use the value "25a" for str[], I get the following output:

Invalid Number
Decimal equivalent of 25a in base 36 is  -1

But if I change the value of str[] to "25A", I get the following output:

Decimal equivalent of 25A in base 36 is  2782

Using the portal link that I have provided above for result verification, I get the last result for both values.

So my question is, how can I modify my code to make the corresponding change?

TIA

Vinod

If you can't or don't want to change the function signatures or use the proper library functions to perform the conversion, a simple change to the val function would allow the code to also manage the lower cases.

int val(char c) 
{ 
    if ( '0' <= c  &&  c <= '9') 
        return c - '0'; 
    else if ( 'A' <= c  &&  c <= 'Z' )
        return c - 'A' + 10;
    else if ( 'a' <= c  &&  c <= 'z' )
        return c - 'a' + 10;
    else
        return -1;
} 

A little change to the other function (using the Horner method to evaluate the polynomial) and we can also avoid a call to strlen.

int toDeci(char *str, int base) 
{ 
    unsigned long long int num = 0;
    
    while( *str != '\0' )
    {
        int digit = val(*str);
        if ( digit == -1  ||  digit >= base )
        {
            printf("Invalid digit.\n");
            return -1;
        }
        unsigned long long int n = num * base + digit;
        if ( n < num )
        {
            printf("The number is too big.\n");
            return -1;
        }
        num = n;
        ++str;
    }
    if ( num > INT_MAX )  // You'll have to include <limits.h>
    {
        printf("The number is too big to fit into an int.\n");
        return -1;
    }
    return num; 
}

Note that this code still can't process negative numbers or prefixes like 0 for octals and 0x for hexadecimal numbers. The error checking also follows OP's choice (returning -1 when something goes wrong) and needs to be rethinked.