How do I set a value to a variable by passing the variales name in C? - c

Question

char whatVar[] = "";
float phValue, ecValue, tempValue;
I am not sure if this is possible or not in C. Each of the float values will be set randomly. I would like to set their values depending on what the char whatVar corresponds to. For example, lets say char whatVar[] = "phValue"; How do I point to the variable name inside whatVar to then set the value? I would like to achieve this without using if conditionals or a switch.

Answer 1

Don't; use an array. Tying your program logic to the names of your variables is a horrible idea. Variable names exist to help you as a programmer reason about what the code is doing. Beyond that the do not (and should not) have any significance.
Instead, use a collection where the index/bucket/mapping/whatever has logical meaning. It is very common for beginners to do exactly as you want to do, but it is almost certainly a bad idea. Sounds to me like you want a string -> float map or hash table.

Answer 2

You will have to implement the translation of whatVar onto the actual variable value manually. C language has no such immediate feature.
It can be done by if/switch ladder. Or it can be done by pre-building a table that "projects" (maps) variable names to variable addresses (and, if necessary, types). You will have to manually search through the table. It will still have if's though, unless you develop some sort of [minimal] perfect hashing function for your names (see Steve Jessop's comment).

Answer 3

This is outside of the language specification and I think if this is a feature that you wish to have in the language another language would be of more use to you that has this kind of functionality in it.
The only way I can think of implementing this is by doing some preprocessing tricks, which just seems like a bad idea. With this method you will not have if's in the code it self but some very unmaintainable preprocessing.

Answer 4

why not ?
if(strcmp(whatVar, "phValue") == 0)
{
phvalue = 3; //some value
}
and so on and so forth.
Edit : if the ifs and the switch is not desired then we can use some sort of stl map ing between the string representations as the keys and their variables as the values.

Answer 5

If you really really want to do this, you can use the dlsym function (as long as the variables you are accessing are global). Here's a quick example:
#include <dlfcn.h>
#include <stdlib.h>
#include <stdio.h>
float phValue, ecValue, tempValue;
int main(int argc, char *argv[]) {
int i;
phValue = 0;
ecValue = 0;
tempValue = 0;
printf("Before:\n");
printf("\tphValue = %f\n", phValue);
printf("\tecValue = %f\n", ecValue);
printf("\ttempValue = %f\n", tempValue);
for(i = 1; i < argc; i += 2) {
float *varptr = (float *)dlsym(RTLD_MAIN_ONLY, argv[i]);
*varptr = atof(argv[i+1]);
}
printf("\nAfter:\n");
printf("\tphValue = %f\n", phValue);
printf("\tecValue = %f\n", ecValue);
printf("\ttempValue = %f\n", tempValue);
}
And a demo:
% gcc varname.c -ldl
% ./a.out phValue 12.1 tempValue 4.3 ecValue 2.2
Before:
phValue = 0.000000
ecValue = 0.000000
tempValue = 0.000000
After:
phValue = 12.100000
ecValue = 2.200000
tempValue = 4.300000