How to convert decimal numbers stored as characters in a string to doubles?

Question

New to programming, I'm looping through a postfix string and trying to retrieve the numbers individually, I only need one at a time.

Initially I wrote this for integers and just did "- '0'", however, I now need to try and make it compatible for decimal numbers.

An example of my integer conversion is below, how could I adapt this?

int i;
char postfix[] = "4 3 +";

for (i=0; i<strlen(postfix); i++) {
    if (isalnum(postfix[i])) {
        int value=(postfix[i]-'0');
        printf("%d\n", value);
    }
}

4
3

e.g. how to evaluate when

char postfix[] = "1.2 3.4 +"

storing the value as a double

If you only want to solve "a b op", you can use `sscanf(postfix, "%f %f %s", &a, &b, c)`, and you get the values in floats `a`, `b`, and in `c[0]` the operator. For a more complicated expression, you will need a stack data structure I think, and more complicated parsing — Eduardo Pascual Aseff, Nov 18 '20 at 06:02
The [`strtod()`](https://linux.die.net/man/3/strtod) function would probably be my choice, unless I could rely on exactly the "*x* *y* *op*" form for every case (for which I would probably choose [`s`]`scanf`). — John Bollinger, Nov 18 '20 at 06:14
Note also, by the way, that your approach for integers works only for non-negative, single-digit numbers, so I hesitate to call it a complete solution for the integer case. There is, however, `strtod()`'s integer-oriented sibling [`strtol()`](https://linux.die.net/man/3/strtol) that you might consider instead. — John Bollinger, Nov 18 '20 at 06:20

score 0 · Answer 1 · answered Nov 18 '20 at 06:54

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{

    int i;
    double res;
    char *ptr1;
    char *ptr2;
    char postfix[] = "4.5 3.4 4.5 7.6 8.9 3.6 +";

    ptr1 = postfix;
    res = strtod(ptr1, &ptr2);
    
    
    while (res!= 0.0f )  // strtod() return 0.0 if double value is not found.
    {
        printf("Value in decimal is %lf\n",res);
        ptr1 = ptr2;
        res = strtod(ptr1, &ptr2);
    }
    return 0;
}

The output is :

Value in decimal is 4.500000
Value in decimal is 3.400000
Value in decimal is 4.500000
Value in decimal is 7.600000
Value in decimal is 8.900000
Value in decimal is 3.600000

Please see this link to know about strtod() function :https://linux.die.net/man/3/strtod

score 0 · Answer 2 · answered Nov 19 '20 at 11:59

Use strtod() to parse the string for a valid double.

current == endptr implies conversion failed.

char postfix[] = "4 3 +";
char *current = postfix;
char *endptr;

double v1 = strtod(current, &endptr);
if (current == endptr) TBD_code_handle_failure();
else current = endptr; 

double v2 = strtod(current, &endptr);
if (current == endptr) TBD_code_handle_failure();
else current = endptr; 

while (isspace((unsigned char) *current)) {  // skip white-space
  current++;
}

if (strchr("+-*/", *current) == NULL) {
  TBD_code_handle_failure(); // unexpected operator
}

printf("%g %g %c\n", v1, v2, *current);

Or ....

double v1, v2;
char oper;
if (sscanf(postfix, "%lf %lf %c", &v1, &v2, &oper) == 3) {
  ; // Success!
}

How to convert decimal numbers stored as characters in a string to doubles?

2 Answers2